Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 295–303
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Insights into the new Th (IV) sulfate fluoride complex: Synthesis, crystal structures, and temperature dependent spectroscopic properties Yanyan Zhao a, Chunxiang Wang b,c, Jing Su d, Yaxing Wang b,c, Yanlong Wang b,c, Shuao Wang b,c, Juan Diwu b,c,⇑, Zhihong Liu a,⇑ a
School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710062, China School for Radiological and Interdisciplinary Sciences (RAD-X), Soochow University, Jiangsu 215123, China Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Jiangsu 215123, China d Department of Chemistry & Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Tsinghua University, Beijing 100084, China b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A novel thorium sulfate fluoride
Under hydrothermal condition, the decomposition of methanesulfonic acid to sulfate anion is observed, resulting in the formation of a novel thorium sulfate fluoride compound ThF2(SO4)(H2O) (1). The temperature dependent UV–Vis–NIR absorption spectra and fluorescence spectra were collected from 77 K to 300 K, where the intensities of the peaks varied as a function of temperature. The Raman vibrational spectrum of the samples collected from 100 to 2000 cm1 shows identical SO42 vibration modes.
compound has been synthesized and characterized. The temperature dependent UV–Vis– NIR absorption and fluorescence spectra were collected. The Raman vibrational spectrum of the samples shows identical SO42 vibration modes.
a r t i c l e
i n f o
Article history: Received 1 February 2015 Received in revised form 17 April 2015 Accepted 19 April 2015 Available online 23 April 2015 Keywords: Thorium(IV) sulfate fluoride Crystal structure In situ ligand decomposition Temperature dependent spectrum
a b s t r a c t Under hydrothermal condition, the decomposition of methanesulfonic acid to sulfate anion is observed, resulting in the formation of a novel thorium sulfate fluoride compound ThF2(SO4)(H2O) (1). This complex is structurally characterized by single crystal X-ray diffraction, revealing a three-dimensional structure crystallized in the monoclinic space-group P21/n, where thorium cation is nine coordinated by four oxygen atoms, four bridging F, and one H2O molecule. The crystal lattice parameters are SO2 4 a = 6.9065(7) Å, b = 6.9256(7) Å, c = 10.5892(11) Å, b = 96.755(2)°, V = 502.98(9) Å3, Z = 4. The temperature dependent UV–Vis–NIR absorption spectra and fluorescence spectra were collected from 77 K to 300 K, where the intensities of the peaks varied as a function of temperature. The Raman vibrational spectrum of the samples collected from 100 to 2000 cm1 shows identical SO2 4 vibration modes. Ó 2015 Elsevier B.V. All rights reserved.
Introduction ⇑ Corresponding authors at: School for Radiological and Interdisciplinary Sciences (RAD-X), Soochow University, Jiangsu 215123, China (J. Diwu). http://dx.doi.org/10.1016/j.saa.2015.04.037 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.
Basic research on the chemical properties of thorium has been an important part of the nuclear energy development, for its usage
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Fig. 1. The photograph of the products from the two reactions, showing (left) the colorless prism crystal of 1 as well as a few microcrystalline products and (right) the colorless block crystal of 2 together with the yellow block crystal, which is the known Th(VO3)4 phase [23]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 Crystallographic data for ThF2(SO4)(H2O) (1) and Th4(SO4)7(OH)2(H2O)62H2O (2).
a b *
Compound
ThF2(SO4)(H2O) (1)
Th4(SO4)7(OH)2(H2O)62H2O (2)*
Formula mass Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z T (K) k (Å) Max 2h (°) q calcd (g cm3) l (Mo Ka) R1(F) for F2o > 2r(F2o)a wR2(F2O)b
378.10 Monoclinic P21/n 6.9065(7) 6.9256(7) 10.5892(11) 90.00 96.755(2) 90.00 502.98(9) 4 273(2) 0.71073 27.62 5.072 300.60 0.0140 0.0351
1778.72 Orthorhombic Pnma 18.1912(9) 18.1760(9) 14.4256(8) 90.00 90.00 90.00 2932.8(3) 4 273(2) 0.71073 27.53 4.028 208.63 0.0293 0.0749
R1 = Fo Fc/Fo. wR2 = [w(F2o F2c )2/w(F2o)2]1/2. For detailed structural information, please refer to Ref. [17].
as nuclear fuel and a good surrogate for tetravalent transuranium elements to predict their chemical and environmental behaviors [1]. According to China’s current plan, the nuclear energy capacity will be increased to about 58 GWe by the year 2020. However, influenced by the Fukushima nuclear reactor accident, the concerns regarding the nuclear reaction safety is one of the key issues of the development of nuclear industry [2,3]. Thorium molten salt reactor is believed to be one of the alternatives, which bears many advantages such as low reaction temperature, controllable fission which can stop the chain reaction quickly, and non-applicable proliferation for nuclear weapons [4–6]. Although there is no thorium sulfate mineral in nature, human activities have artificially concentrate both together. For example, sulfonic acid is an important functional group of the diphonix resin widely used in nuclear waste remediation processes [7,8]. On the other hand, a detailed analysis of the components of underground water in Beishan, the most promising nuclear waste repository in China so far, shows that the sulfate anion is the most abundant anion species [9]. As a consequence, research regarding the basic chemistry of thorium sulfates is critical for nuclear waste remediation and storage in China [10–12]. The crystal structure characterization is an accurate way to analyze the interaction between thorium cation and sulfate anion, which is critical for predicting the environmental behavior of thorium. Despite the
aforementioned importance, the study of thorium sulfates is significantly less than its cerium analogue, which could be attributed to the radioactivity of thorium that highly restricted its research. Up to now, there are only 11 thorium sulfate crystal structures reported [13–19], whose structural topology varies from finite clusters, one dimensional chains, two dimensional layers, to three dimensional networks. The Th3(SO4)6(H2O)6H2O contains nanoscale voids as large as 11.5 Å in diameter [19]. Yet extremely rare research was conducted to study the spectroscopic chemistry of thorium sulfate, and no temperature dependent spectra data were reported [20,21]. This statement is even true for the solid state chemistry of thorium in general, indicating a significant research blank in this area. In order to investigate the structural diversity of thorium sulfates, a new route of synthesizing thorium sulfate via organo ligand decomposition was utilized, as reported in the borate system [22]. Under hydrothermal condition, a thorium sulfate fluoride compound was successfully isolated, whose structure was determined by single crystal X-ray diffraction for the first time, expanding the basic knowledge of the coordination chemistry of thorium sulfates. The temperature dependent solid state UV–Vis–NIR absorption spectra and fluorescence spectra were collected, revealing that the absorption and fluorescence intensities of thorium sulfates exhibit inconsistent trend as a function of temperature. The Raman spectrum was also recorded and analyzed to show the characteristic S–O vibrations.
Experimental Materials and synthesis Th(NO3)46H2O was obtained commercially and used without further purification, methanesulfonic acid (99%, Adamas-beta), hydrofluoric acid (AR, Adamas-beta), tripropyl amine (98%, Adamas-beta), sodium nitrate (AR, Adamas-beta), vanadium oxide (95%, Aifa Aesar) were used as received. Caution! Since the Th-232 isotope used in this experiment is radioactive, standard precautions for handling radioactive substances should be followed. ThF2(SO4)(H2O) (1): thorium nitrate (Th(NO3)46H2O) (500 lL, 0.4 M), methanesulfonic acid (13 lL), hydrofluoric acid (40%) (10 lL), tripropyl amine (20 lL), sodium nitrate (100 lL, 1 M) and H2O (400 lL) were loaded into a 20 mL Teflon-lined autoclave. The autoclave was sealed and heated to 220 °C in an oven for 72 h and then cooled down to room temperature at a rate of 5 °C/h. The products were washed with distilled water and ethanol. Colorless transparent prism crystals of 1 were obtained together with some white microcrystalline powder as shown in Fig. 1(left).
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Fig. 2. (a) The building unit of ThF2(SO4)(H2O) (1) with atomic labeling scheme at 50% probability, showing the local coordination environment of Th(IV) ions. (b) The ball and stick mode of ThO5F4 polyhedron. Color code: Th-green, O (SO42)-red, F-blue, H2O-purple. (c) The illustration of ThO5F4 polyhedra chain which is edge-shared via two F extending along the a axis. (d) Depiction of the three-dimensional network structure of ThF2(SO4)(H2O) (1) down the a axis, showing the connection of the thorium chains via 2 bridging sulfate anions. The ThO5F4 is shown as green polyhedron, SO2 4 as yellow polyhedron, F as blue sphere, whereas oxygen atoms from SO4 and H2O are shown in red and purple, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Th4(SO4)7(OH)2(H2O)62H2O (2): thorium nitrate (Th(NO3)46H2O) (0.1478 g), vanadium oxide (0.0375 g), H2SO4 (500 lL, 1 M) and H2O (2 mL) were loaded into a 10 mL Teflon-lined autoclave. The autoclave was sealed and heated to 200 °C in a muffle furnace for 72 h and then cooled down to room temperature at a rate of 3.5 °C/h. The products were washed with distilled water and ethanol. The colorless transparent block crystals were obtained as 2. The structure of 2 was previously reported by Burns et al. [17]. The yellow block crystalline phase yielded from this reaction was Th(VO3)4, a known compound reported before [23], as shown in Fig. 1(right). The spectroscopic properties of 2 were studied for comparison with 1. Crystallographic studies Single crystals of ThF2(SO4)(H2O) (1) and Th4(SO4)7(OH)2(H2O)62H2O (2) were chosen using an Olympus polarized microscope, then mounted on CryoLoops with Krytox oil and optically aligned on Bruker D8-Venture diffractometer single crystal X-ray diffractometer using a Turbo X-ray Source (Mo-Ka radiation, k = 0.71073 Å) adopted with the direct-drive rotating anode technique and a CMOS detector at room temperature. The
Table 2 Selected bond distances (Å) for ThF2(SO4)(H2O) (1). Bond distances (Å) Th(1)–F(1) Th(1)–F(1)0 Th(1)–F(2) Th(1)–F(2)0 Th(1)–O1W Th(1)–O(4) Th(1)–O(5) Th(1)–O(6) Th(1)–O(7)
2.371(2) 2.382(2) 2.352(2) 2.353(2) 2.544(3) 2.446(3) 2.485(3) 2.450(3) 2.481(3)
S(1)–O(4) S(1)–O(5) S(1)–O(6) S(1)–O(7)
1.475(3) 1.468(3) 1.471(3) 1.475(3)
data frames were collected using the program APEX II and processed using the program SAINT routine within APEX II. For 1, a total of 1158 reflections in the range of 3.52 < 2h < 27.62, and 8 < h < 9, 9 < k < 9, 13 < l < 13 were collected, of which 1115 reflections were accepted. The final R1 is 0.0140. For 2, a total of 3529 reflections in the range of 2.24 < 2h < 27.53, and 23 < h < 16, 14 < k < 14, 16 < l < 18 were collected, of which 3025 reflections were accepted. The final R1 is 0.0293. The structures were all solved by direct methods and refined by the full-matrix least squares on F2 using the SHELXTL-97 program. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were assigned by searching the q peaks located in the electron density map and refined normally. Selected crystallographic data and refinement details of the compounds are given in Table 1. Atomic coordinates and additional structural information are provided in Supporting information (CIF files).
Scanning electron microscopy–energy-dispersive spectroscopy (SEM– EDS) Scanning electron microscopy images and energy-dispersive spectroscopy data were recorded on a FEI Quanta 200FEG Scanning Electron Microscope with the energy of the electron beam being 30 keV. Samples were mounted directly on the carbon conductive tape with Au coating.
Raman spectra The Raman spectra were recorded by placing single crystals of the two compounds on a quartz slide under a CRAIC Technologies microspectrophotometer with a CRAIC Apollo™ 785
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Table 3 The bond valence sum calculation results of ThF2(SO4)(H2O) (1). Bond distances (Å)
F1 F2 O1W O4 O5 O6 O7
Bond valence (v.u.)
Th1
Th1
2.371 2.352 2.544 2.446 2.485 2.450 2.481
2.382 2.353
SUM (v.u.)
S1
Th1
Th1 0.430313 0.465397
1.475 1.468 1.471 1.475
0.443298 0.466656 0.360985 0.470455 0.423391 0.465397 0.427993
S1
1.495862 1.524431 1.512121 1.495862
0.873611 0.932053 0.360985 1.966317 1.947822 1.977518 1.923855
Fig. 3. (a) The building unit of Th4(SO4)7(OH)2(H2O)62H2O (2) with atomic labeling scheme at 50% probability, showing the local coordination environment of three Th(IV) ions. (b) The ball and stick mode of ThO9 polyhedron of Th(2) and Th(3). Color code: Th(2)-blue, Th(3)-magenta, O(SO2 4 )-red, OH -blue, H2O-purple. (c) The illustration of Th(1)O9 pseudo-dimer which is face-shared via two OH groups and one water molecule along the b axis. (d) Depiction of the three-dimensional network structure of Th4(SO4)7(OH)2(H2O)62H2O (2) down the b axis, showing the connection of the thorium centers via bridging sulfate anions. The ThO9 is shown as green, blue, and magenta 2 polyhedron, SO2 4 as yellow polyhedron, and oxygen atoms from SO4 , OH , and H2O are shown in red, blue, and purple, respectively. The protons are all omitted for clarity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
coherent laser (k = 785 nm). The data integration time is 2000 ms for both samples.
Table 4 Selected bond distances (Å) for Th4(SO4)7(OH)2(H2O)62H2O (2). Bond distances (Å) Th(1)–O(3) Th(1)–O(4) Th(1)–O(6) Th(1)–O(8) Th(1)–O(11) Th(1)–O(19) Th(1)–O(21) Th(1)–O(23) Th(1)–O(24) Th(2)–O(2) Th(2)–O(10) Th(2)–O(13) Th(2)–O(16) Th(2)–O(16)0 Th(2)–O(20) Th(2)–O(20)0 Th(2)–O(25) Th(2)–O(25)0 Th(3)–O(1) Th(3)–O(5) Th(3)–O(5)0 Th(3)–O(9)
2.456(6) 2.433(5) 2.382(6) 2.398(5) 2.463(6) 2.412(6) 2.487(5) 2.366(6) 2.686(10) 2.394(8) 2.529(8) 2.599(8) 2.408(6) 2.408(6) 2.369(6) 2.369(6) 2.436(5) 2.436(5) 2.473(8) 2.362(6) 2.362(6) 2.425(5)
Th(3)–O(9)0 Th(3)–O(17) Th(3)–O(18) Th(3)–O(22) Th(3)–O(22)0 S(1)–O(1) S(1)–O(2) S(1)–O(3) S(1)–O(3)0 S(2)–O(6) S(2)–O(20) S(2)–O(21) S(2)–O(22) S(3)–O(4) S(3)–O(9) S(3)–O(14) S(3)–O(25) S(4)–O(5) S(4)–O(7) S(4)–O(16) S(4)–O(19)
2.425(5) 2.596(10) 2.604(7) 2.388(6) 2.388(6) 1.485(9) 1.476(8) 1.476(6) 1.476(6) 1.475(6) 1.463(6) 1.468(5) 1.471(6) 1.466(5) 1.474(6) 1.459(6) 1.483(6) 1.462(6) 1.473(6) 1.461(6) 1.464(6)
Temperature dependent solid-state UV–Vis–NIR and fluorescence spectra Variable temperature solid-state UV–Vis–NIR and fluorescence spectra analysis of the single crystals were collected using a CRAIC Technologies microspectrophotometer. Crystals were placed on quartz slides under Krytox oil, and data was collected after the optimization of microspectro-photometer. Measurements were carried out between 77 K and 300 K with a 30 K step at a rate of 6 K min1. The UV–Vis–NIR absorption spectra were collected with an integration time of 13 ms and 12 ms for 1 and 2, respectively. The fluorescence spectra were excited at 365 nm and emission spectra were recorded from 300 nm to 900 nm with an integration time of 1000 ms for both samples. Bond-valence sum analysis The bond-valence sum (BVS) calculations were performed for all donor atoms in 1 and 2 to verify the oxidation states, and the
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Y. Zhao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 295–303 Table 5 The bond valence sum calculation results of Th4(SO4)7(OH)2(H2O)62H2O (2). Bond distances (Å) Th1 O3 O4 O6 O8 O11 O19 O21 O23 O24
2.456 2.433 2.382 2.398 2.463 2.412 2.487 2.366 2.686
Bond valence (v.u.) Th1
S
Th1
1.476 1.466 1.475
0.457911 0.487279 0.559293 0.535623 0.449329 0.515735 0.421108 0.584010 0.245931
2.398 1.464 1.468 2.366 2.686
Bond distances (Å) Th2 O2 O10 O13 O16 O20 O25 O1 O5 O9 O17 O18 O22
SUM (v.u.) Th1
S 1.491825 1.532694 1.495862
0.535623 1.541001 1.524432 0.584010 0.245931
Bond valence (v.u.) Th3
2.394 2.529 2.599 2.408 2.369 2.436 2.473 2.362 2.425 2.596 2.604 2.388
S
Th2
1.476
0.541445 0.375920 0.311123 0.521341 0.579294 0.483344
1.461 1.463 1.483 1.485 1.462 1.474
1.471
1.949736 2.019973 2.055155 1.071246 0.449329 2.056736 1.94554 1.16802 0.491862 SUM (v.u.)
Th3
S 1.491825
0.437348 0.590357 0.497929 0.313656 0.306947 0.550297
1.553547 1.545172 1.463866 1.455975 1.549353 1.499910
1.512121
2.03327 0.37592 0.311123 2.074888 2.124466 1.94721 1.893323 2.13971 1.997839 0.313656 0.306947 2.062418
Fig. 4. SEM images of single crystals of ThF2(SO4)(H2O) (1) (left), Th4(SO4)7(OH)2(H2O)62H2O (2) (right) and their respective Energy-dispersive X-ray Spectroscopy (EDS) of the selected areas.
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results are consistent with their expected valences. The bond-valence parameters used for analyzing Th(IV)–O, Th(IV)–F and S(VI)–O bonds were from Brese and O’Keeffe et al. [24]. Results and discussion Synthesis The compound 1 was synthesized under hydrothermal condition with the existence of HF. HF has been demonstrated to be a good mineralizing agent to assist formation of large porosity structure such as TOF-2 by O’Hare et al. [25], and uranyl phosphonate nanotube by Albrecht-Schmitt et al. [26]. During the process of the reaction, the methanesulfonic acid decomposed to sulfate and afforded the thorium sulfate fluoride. It is noticed that the HF played a critical role in the formation of this complex, without which, no crystalline phase was yielded under identical conditions. The compound 2 is reported by Burns et al. before, synthesized via reacting thorium nitrate aqueous solution directly with sulfuric acid (0.2 M) under hydrothermal condition. However, an alternate synthesis route is reported here, where thorium nitrate, vanadium oxide, sulfuric acid, and H2O were mixed and reacted under hydrothermal condition. The addition of vanadium oxide would yield both thorium sulfate and vanadate, indicating that sulfate and vanadate are both strong thorium complexing ligands. However, the reaction yield was very low, so there was not enough sufficient amount of sample for powder X-ray diffraction. Thus, the single crystal X-ray diffraction data were only collected for compound 2 to verify the structure. Photos of the products from two reactions are shown in Fig. 1, showing the morphology of the crystals. Crystallography ThF2(SO4)(H2O) (1): this compound crystallizes in the central symmetric space group, P21/n. The asymmetric unit contains one Th(IV) cation, two F, one SO2 anion, and one H2O molecule. 4 The atomic labeling scheme of 2 is shown in Fig. 2a, indicating that the Th cation is coordinated to five oxygen atoms and four fluorine atoms. Th(1) is bonded to O(4), O(5A), O(6A), and O(7A), each from different sulfate anions S(1), S(1A), S(1B) and S(1C), respectively, and their bond lengths range from 2.446(3) to 2.485(3) Å. O1 W is the water molecule bonded to Th(1), with a distance of
Fig. 5. The UV–Vis–NIR absorption spectra of the reaction products 1, 2 and Th(VO3)4, as well as the starting materials (Th(NO3)46H2O and Na2SO4) from 200 nm to 800 nm were recorded.
2.544(3) Å. Th(1) is linked to the symmetry equivalent Th(1A) via F(1) and F(1A) on the upper side shown in the figure, while F(2) and F(2A) are bridging between Th(1) and Th(1B) on the lower side. Th–F(1) and Th–F(2) bond distances are in the range of 2.352(2)– 2.382(2) Å. It is clear from the depiction that the thorium centers are edge-sharing with each other via F(1) and F(2) atoms to form a chain extended along the a axis as shown in Fig. 2c. The sulfate anion bridges thorium centers using O(4) and O(7), while the other two oxygen atoms O(5) and O(6) connected to the thorium centers from adjacent chains, leading to a three-dimensional network (Fig. 2d). The S–O bond lengths are all within the normal region from 1.468(3) to 1.475(3) Å. These distances are in agreement with those found in other thorium(IV) sulfate compounds [13–19]. The selected bond distances of Th–O bonds, Th–F bonds and S–O bonds are listed in Table 2. Since the electron density of the proton is too low, it is difficult to locate the protons in the structure from single crystal X-ray diffraction data. Thus bond valence sum calculation results were therefore used to define the protonation of oxygen atoms, whose results are listed in Table 3. The BVS of O(4), O(5), O(6) and O(7) that bonds to thorium and sulfur atoms are all close to 2 v.u. (from 1.92 to 1.98 v.u.), indicating that these are fully deprotonated O2. The BVS of the bridging F(1) and F(2) is 0.87 and 0.93 v.u., whereas O1W is only 0.36 v.u. Thus, those sites were assigned to be F and H2O accordingly. This result is consistent with the EDS results, where fair amount of fluorine was found in the compound 1 with larger molar percentage than thorium. The thorium center is bonded to nine donor atoms, being four sulfate oxygen, four fluorines, and one water molecule, as shown in Fig. 2b, forming the ThO5F4 polyhedron, which is best described as the tricapped trigonal prism. Th4(SO4)7(OH)2(H2O)62H2O (2) [17]: this compound has been previously reported by Burns et al., thus only a brief description is provided here in comparison to compound 1. Compound 2 crystallizes in the centrosymmetric space group, Pnma. The asymmetric unit contains three Th(IV) cations, four SO2 anions, two 4 hydroxyl groups, six coordinated water molecules and one free water molecule. The atomic labeling scheme is shown in Fig. 3a. There are some disorders of the terminal oxygen atoms, which would probably cause the level B alert of ‘‘large Non-solvent O Ueqmax/Ueqmin’’. However, since the structure is already reported by Burns et al., the single crystal data here is only utilized to confirm the structure of the compound 2. So the major concerns were put in the structure deviation between compounds 1 and 2. Unlike compound 1, compound 2 adopts three distinct thorium centers. Th(1) is face sharing two hydroxyl groups and one coordinated water molecule with adjacent symmetry equivalent Th(1) centers, forming a pseudo-dimer topology. The dimers are further linked by the sulfate anions along b and c axis to form discrete chains as shown in Fig. 3c. Both Th(2) and Th(3) centers present local coordination environment of nine, where seven oxygen atoms are from seven unique SO2 4 groups, and the rest two coordinate sites are occupied by the water molecules (Fig. 3b). The Th–O(S) bond length ranges from 2.362 to 2.487 Å. The Th–OH bonds are 2.398 and 2.366 Å. The Th–OH2 distances are from 2.463 to 2.686 Å. The sulfate anions in this compound act as only the bridging ligand, leading to the three dimensional framework. The S–O bond distances are all in the normal range 1.459–1.485 Å, which are all in agreement with the previously reported thorium sulfate data [13–19]. The selected bond distances of Th–O bonds and S–O bonds are listed in Table 4. The BVS calculations as listed in Table 4 were performed using the crystal data obtained in this work to confirm the oxygen speciation. The BVS results of the O(8) and O(23) are 1.07 v.u. and 1.17 v.u., respectively, indicating that these are hydroxyl groups, bridging the two Th(1) centers together as a pseudo-dimer. The
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Fig. 6. Temperature-dependent UV–Vis–NIR absorption spectra of single crystals of ThF2(SO4)(H2O) (1) and Th4(SO4)7(OH)2(H2O)62H2O (2) from 300 nm to 800 nm recorded with a temperature range of 77–300 K. The photograph of the crystal used for spectra collection is shown on the upper right side of each figure.
O(11),O(24), O(10), O(13), O(17), and O(18) are all below 0.5 v.u., therefore they are assigned as coordinated water molecules (see Table 5). Scanning electron microscopy–energy-dispersive spectroscopy (SEM– EDS) analysis The single crystals of 1 and 2 were mounted on the carbon conductive tape with Au coating for SEM imaging and EDS analysis. The spike of the Si Ka edge peak around 1.74 keV is observed, where the Si is most likely from the Krytox oil used for crystal sample storage. Although the exact quantity ratio of F: Th cannot be determined accurately, since the F Ka edge peak at 0.677 keV is greatly affected by the O Ka peak at 0.525 keV, in comparison to compound 2, it is evident that there are a fair amount of fluorine in compound 1 (see Fig. 4). Solid state UV–Vis–NIR absorbance spectroscopy The absorption spectra of the reaction products 1, 2 and Th(VO3)4, as well as the starting materials (Th(NO3)46H2O and Na2SO4) are shown in Fig. 5. All thorium complexes show absorption features between 250 nm and 350 nm. Thus the major absorption peaks of 1 and 2 observed at 300 nm and 320 nm, respectively, could be attributed from the thorium centers. These absorption features for thorium compounds are likely originated from charge transfer electronic transitions from O 2p orbitals to the empty Th 6d/5f orbitals similar with the cases of UO2+ 2 compounds, where change transfer features from O 2p orbitals to uranium 5f/6d orbitals are always observed [27]. To understand the electronic absorption mechanism of these two Th(IV) compounds, time-dependent density functional theory (TDDFT) [28] implemented in ADF 2013.01 [29–31] was employed to calculate the vertical excitation energies of a simplified model compound Th(H2O)4+ 9 at its optimal ground-sate geometry at the scalar relativistic level. The Perdew– Burke–Ernzerhof (PBE) exchange–correlation functional [32] was used in the geometry optimization, and the statistically averaged orbital potentials (SAOP) [33] with correct asymptotic 1/r behavior were employed in the TDDFT calculations. Uncontracted Slater basis sets of triple-f plus one polarization (TZP) quality for all atoms with frozen atomic core approximation applied to [1s2] of O and to [1s2–4f14] of Th [34]. The average Th–OH2 bond distance is 2.536 Å, in good agreement with the experimental result 2.544 Å. The lowest absorption energy is around 174 nm,
corresponding to electron transition from occupied O-2p orbitals to the virtually occupied Th–5f orbitals, i.e., ligand to metal charge transfer transition. The calculated lowest excitation energy is lower than the experimental result, which is likely due to the simplified model. Further investigation of the relationship between the peak position and intensity with temperature were conducted. Utilizing a temperature control sample stage, the spectra can be recorded from 77 K to 300 K. However, the additional quartz window on the stage would introduce intense noise below 300 nm. As a consequence, The UV–Vis–NIR absorption spectra were collected in the range from 300 nm to 800 nm, which covered the UV, Visible, and a small part of NIR regions, as shown in Fig. 6. The figures represent the raw data without normalization. The absorption intensities of the two compounds changed dramatically as a function of temperature. For compound 1, from 77 K to 180 K, the absorption intensity is decreased as the temperature increased. However, from 210 K, the trend was reversed, where the peak intensity is increased as the increase of temperature. Such phenomenon is associated with a blue shift into the UV region. Compound 2 present a similar phenomenon. From 77 K to 180 K, the peak intensity dropped constantly, while from 210 K to 300 K, the peak intensity gradually increased. Such a reversed correlation between the intensity of the absorption peaks and temperature is likely originated from temperature induced structural changes. Fluorescence spectroscopy It is noteworthy that the fluorescence characters of the thorium complexes were not well studied, since Th(IV) ion does not possess any intrinsic photoluminescence properties. Thus, most of the emission spectra reported for thorium compounds are originated from luminophores other than the thorium centers. For instance, Gorden et al. have reported the luminescence properties of the thorium tetracyanoplatinates, where the emission is assigned from the TCP portion of the structures [35]. On the other hand, Brittain et al., have studied the luminescence of the hydrolysis products of Th(IV), showing that the thorium hydroxide complexes emits between 500 nm and 600 nm [36]. They concluded that such luminescence emission is from the cooperative phenomena between Th(IV) centers, so the distance between Th(IV) centers could greatly affect the intensity of luminescence peaks of the complexes. Such hypothesis is confirmed by the experimental
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equally intense emission peaks around 420 nm and 700 nm, respectively. The temperature dependent luminescence spectra indicated that the two peaks showed the same relation of intensity change versus temperature, where the intensity first increased from 90 K to 180 K and then decreased from 210 K to 300 K as the increase of temperature. However, the relative peak intensity between the two peaks varied at different temperature. For example, the 450 nm peak showed the minimum intensity at 90 K, whereas the 700 nm peak showed the minimum intensity at 300 K. The spectra of 1 all show a broad emission region from 400 nm to 800 nm. At low temperature zone, the intensities of the emission were the weakest, where from 77 K to 120 K, raising the temperature would result in a slight increase of the intensities of the emission. Then the intensities suddenly jumped to the highest at 150 K, after which, keep increasing the temperature would cause a constant decrease of intensity from 630 nm to 800 nm region, whereas the 400 nm to 630 nm did not show a clear relation of the decrease in intensity versus temperature. The spectra of 2 all appear to have one strong peak at 720 nm and a weak shoulder around 580 nm, where monotonic decrease in intensity is observed with the increase of temperature. Owing to the fact that the non-radiative emission of phonon is promoted at elevated temperature, it is common the fluorescence intensity increases while temperature decreases. However, the inconsistent trends observed for three compounds clearly indicate that the emission of thorium compounds is a complicated and sensitive process. Given that the thorium nitrate and sulfates all absorb around 350 nm and emit above 420 nm, these emission phenomenon cannot be simply explained by the reserved charge transfer process. Therefore, multiple energy transfer and decay mechanism via possible vibrational relaxation and dissipation reorganization must have been involved. In addition, referring to the explanation provided by Brittain et al., whose work demonstrated that the distance between thorium centers greatly affected the intensity of fluorescence, one reasonable extrapolation is that the local coordination environment of thorium can greatly affect its corresponding fluorescence property. Comparing the structure of compound 1 and 2, the most significant deviation is that fluorine is coordinated to thorium centers in 1, whereas in 2 more water molecules are bonded to thorium centers, which could possibly generate the deviation in the emission energy range and the temperature dependent intensity alteration of their
Fig. 7. Temperature-dependent fluorescence spectra of single crystals of Th(NO3)46H2O, ThF2(SO4)(H2O) (1) and Th4(SO4)7(OH)2(H2O)62H2O (2) with excitation wavelength of 365 nm recorded from 77 K to 300 K.
results that by increasing the incorporation ratio of trivalent lanthanide ions into the thorium hydrolysis products, which brought in better separation between thorium centers, the fluorescence intensity was decreased [36]. We herein further investigated the solid state fluorescence properties of the thorium complexes. The fluorescence spectra of 1, 2 and thorium nitrate (Th(NO3)46H2O) were excited at 365 nm and recorded from 77 K to 300 K, showing emissions from 420 nm to approximately 800 nm (Fig. 7). The fluorescence spectra of thorium nitrate hexahydrate Th(NO3)46H2O showed two
Fig. 8. The Raman spectra collected from single crystal samples of ThF2(SO4)(H2O) (1) and Th4(SO4)7(OH)2(H2O)62H2O (2) at room temperature.
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emission spectra. It is also noteworthy that the fluorescence quenching behavior of 2 is much more pronounced, which could be attributed from the higher numbers of coordinated water in 2 compared to 1. In the study of the fluorescence lifetime of lanthanide complexes, the hydration is one of the key features [37]. To fully understand the mechanism of the thorium fluorescence, detailed comparison experiment together with theoretical calculation must be involved before pronounced conclusion can be reached.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2015.04.037. References [1] [2] [3] [4]
Raman spectroscopy The Raman spectra of compound 1 and 2 both show identical vibration modes of the S–O bond [38,39] (see Fig. 8). The vibrations between 300 and 700 cm1 (452 and 625 cm1 peaks for 1, and 439, 467, 610 cm1 peaks for 2) could be assigned to the m2 and m4 bending modes of the S–O bond, whereas the peaks around 1000 cm1 (1055, 1096, and 1250 cm1 peaks for 1, and 1037, 1083, 1108, and 1200 cm1 peaks for 2) are from the m1 and m3 stretching of the S–O bond. Conclusion In summary, the first thorium sulfate fluoride compound ThF2(SO4)(H2O) (1) has been successfully isolated from hydrothermal reactions via the decomposition of methanesulfonic acid. A known thorium sulfate Th4(SO4)7(OH)2(H2O)62H2O (2) obtained alternatively compared to the previous reported route was studied as well. The single crystal X-ray diffraction structural analysis provided insights of the local coordinated environment of thorium centers of the two compounds. The Energy Dispersive X-ray Spectroscopy and Raman spectroscopy analysis were applied to validate the composition of the compounds. The temperature dependent UV–Vis–NIR absorption spectra and fluorescence spectra collected from 200 nm to 800 nm with a temperature range of 77 K to 300 K showed that the intensities of the peaks for compound 1 and 2 varied as a function of temperature. Although all thorium complexes adopt weak fluorescence properties, the mechanism is not fully understood yet. It is proposed that such phenomenon cannot be simple explained as a charge transfer process, but rather a complicated system involving multiple energy transfer and multiple decay mechanism via possible vibrational relaxation, dissipation reorganization, and interaction between thorium centers. Further investigation regarding the mechanism study of this uncommon fluorescence is under the way in our lab. Acknowledgments We are grateful for funding supported by National Science Foundation of China (21173143, 91326112, 21422704, 21471107, 21201106), the Science Foundation of Jiangsu Province (BK20140007, BK20140303), a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and ‘‘Young Thousand Talented Program’’ in China.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Insights into the new Th (IV) sulfate fluoride complex: Synthesis, crystal structures, and temperature dependent spectroscopic properties Yanyan Zhao a, Chunxiang Wang b,c, Jing Su d, Yaxing Wang b,c, Yanlong Wang b,c, Shuao Wang b,c, Juan Diwu b,c,⇑, Zhihong Liu a,⇑ a
School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710062, China School for Radiological and Interdisciplinary Sciences (RAD-X), Soochow University, Jiangsu 215123, China Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Jiangsu 215123, China d Department of Chemistry & Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Tsinghua University, Beijing 100084, China b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A novel thorium sulfate fluoride
Under hydrothermal condition, the decomposition of methanesulfonic acid to sulfate anion is observed, resulting in the formation of a novel thorium sulfate fluoride compound ThF2(SO4)(H2O) (1). The temperature dependent UV–Vis–NIR absorption spectra and fluorescence spectra were collected from 77 K to 300 K, where the intensities of the peaks varied as a function of temperature. The Raman vibrational spectrum of the samples collected from 100 to 2000 cm1 shows identical SO42 vibration modes.
compound has been synthesized and characterized. The temperature dependent UV–Vis– NIR absorption and fluorescence spectra were collected. The Raman vibrational spectrum of the samples shows identical SO42 vibration modes.
a r t i c l e
i n f o
Article history: Received 1 February 2015 Received in revised form 17 April 2015 Accepted 19 April 2015 Available online 23 April 2015 Keywords: Thorium(IV) sulfate fluoride Crystal structure In situ ligand decomposition Temperature dependent spectrum
a b s t r a c t Under hydrothermal condition, the decomposition of methanesulfonic acid to sulfate anion is observed, resulting in the formation of a novel thorium sulfate fluoride compound ThF2(SO4)(H2O) (1). This complex is structurally characterized by single crystal X-ray diffraction, revealing a three-dimensional structure crystallized in the monoclinic space-group P21/n, where thorium cation is nine coordinated by four oxygen atoms, four bridging F, and one H2O molecule. The crystal lattice parameters are SO2 4 a = 6.9065(7) Å, b = 6.9256(7) Å, c = 10.5892(11) Å, b = 96.755(2)°, V = 502.98(9) Å3, Z = 4. The temperature dependent UV–Vis–NIR absorption spectra and fluorescence spectra were collected from 77 K to 300 K, where the intensities of the peaks varied as a function of temperature. The Raman vibrational spectrum of the samples collected from 100 to 2000 cm1 shows identical SO2 4 vibration modes. Ó 2015 Elsevier B.V. All rights reserved.
Introduction ⇑ Corresponding authors at: School for Radiological and Interdisciplinary Sciences (RAD-X), Soochow University, Jiangsu 215123, China (J. Diwu). http://dx.doi.org/10.1016/j.saa.2015.04.037 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.
Basic research on the chemical properties of thorium has been an important part of the nuclear energy development, for its usage
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Fig. 1. The photograph of the products from the two reactions, showing (left) the colorless prism crystal of 1 as well as a few microcrystalline products and (right) the colorless block crystal of 2 together with the yellow block crystal, which is the known Th(VO3)4 phase [23]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 Crystallographic data for ThF2(SO4)(H2O) (1) and Th4(SO4)7(OH)2(H2O)62H2O (2).
a b *
Compound
ThF2(SO4)(H2O) (1)
Th4(SO4)7(OH)2(H2O)62H2O (2)*
Formula mass Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z T (K) k (Å) Max 2h (°) q calcd (g cm3) l (Mo Ka) R1(F) for F2o > 2r(F2o)a wR2(F2O)b
378.10 Monoclinic P21/n 6.9065(7) 6.9256(7) 10.5892(11) 90.00 96.755(2) 90.00 502.98(9) 4 273(2) 0.71073 27.62 5.072 300.60 0.0140 0.0351
1778.72 Orthorhombic Pnma 18.1912(9) 18.1760(9) 14.4256(8) 90.00 90.00 90.00 2932.8(3) 4 273(2) 0.71073 27.53 4.028 208.63 0.0293 0.0749
R1 = Fo Fc/Fo. wR2 = [w(F2o F2c )2/w(F2o)2]1/2. For detailed structural information, please refer to Ref. [17].
as nuclear fuel and a good surrogate for tetravalent transuranium elements to predict their chemical and environmental behaviors [1]. According to China’s current plan, the nuclear energy capacity will be increased to about 58 GWe by the year 2020. However, influenced by the Fukushima nuclear reactor accident, the concerns regarding the nuclear reaction safety is one of the key issues of the development of nuclear industry [2,3]. Thorium molten salt reactor is believed to be one of the alternatives, which bears many advantages such as low reaction temperature, controllable fission which can stop the chain reaction quickly, and non-applicable proliferation for nuclear weapons [4–6]. Although there is no thorium sulfate mineral in nature, human activities have artificially concentrate both together. For example, sulfonic acid is an important functional group of the diphonix resin widely used in nuclear waste remediation processes [7,8]. On the other hand, a detailed analysis of the components of underground water in Beishan, the most promising nuclear waste repository in China so far, shows that the sulfate anion is the most abundant anion species [9]. As a consequence, research regarding the basic chemistry of thorium sulfates is critical for nuclear waste remediation and storage in China [10–12]. The crystal structure characterization is an accurate way to analyze the interaction between thorium cation and sulfate anion, which is critical for predicting the environmental behavior of thorium. Despite the
aforementioned importance, the study of thorium sulfates is significantly less than its cerium analogue, which could be attributed to the radioactivity of thorium that highly restricted its research. Up to now, there are only 11 thorium sulfate crystal structures reported [13–19], whose structural topology varies from finite clusters, one dimensional chains, two dimensional layers, to three dimensional networks. The Th3(SO4)6(H2O)6H2O contains nanoscale voids as large as 11.5 Å in diameter [19]. Yet extremely rare research was conducted to study the spectroscopic chemistry of thorium sulfate, and no temperature dependent spectra data were reported [20,21]. This statement is even true for the solid state chemistry of thorium in general, indicating a significant research blank in this area. In order to investigate the structural diversity of thorium sulfates, a new route of synthesizing thorium sulfate via organo ligand decomposition was utilized, as reported in the borate system [22]. Under hydrothermal condition, a thorium sulfate fluoride compound was successfully isolated, whose structure was determined by single crystal X-ray diffraction for the first time, expanding the basic knowledge of the coordination chemistry of thorium sulfates. The temperature dependent solid state UV–Vis–NIR absorption spectra and fluorescence spectra were collected, revealing that the absorption and fluorescence intensities of thorium sulfates exhibit inconsistent trend as a function of temperature. The Raman spectrum was also recorded and analyzed to show the characteristic S–O vibrations.
Experimental Materials and synthesis Th(NO3)46H2O was obtained commercially and used without further purification, methanesulfonic acid (99%, Adamas-beta), hydrofluoric acid (AR, Adamas-beta), tripropyl amine (98%, Adamas-beta), sodium nitrate (AR, Adamas-beta), vanadium oxide (95%, Aifa Aesar) were used as received. Caution! Since the Th-232 isotope used in this experiment is radioactive, standard precautions for handling radioactive substances should be followed. ThF2(SO4)(H2O) (1): thorium nitrate (Th(NO3)46H2O) (500 lL, 0.4 M), methanesulfonic acid (13 lL), hydrofluoric acid (40%) (10 lL), tripropyl amine (20 lL), sodium nitrate (100 lL, 1 M) and H2O (400 lL) were loaded into a 20 mL Teflon-lined autoclave. The autoclave was sealed and heated to 220 °C in an oven for 72 h and then cooled down to room temperature at a rate of 5 °C/h. The products were washed with distilled water and ethanol. Colorless transparent prism crystals of 1 were obtained together with some white microcrystalline powder as shown in Fig. 1(left).
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Fig. 2. (a) The building unit of ThF2(SO4)(H2O) (1) with atomic labeling scheme at 50% probability, showing the local coordination environment of Th(IV) ions. (b) The ball and stick mode of ThO5F4 polyhedron. Color code: Th-green, O (SO42)-red, F-blue, H2O-purple. (c) The illustration of ThO5F4 polyhedra chain which is edge-shared via two F extending along the a axis. (d) Depiction of the three-dimensional network structure of ThF2(SO4)(H2O) (1) down the a axis, showing the connection of the thorium chains via 2 bridging sulfate anions. The ThO5F4 is shown as green polyhedron, SO2 4 as yellow polyhedron, F as blue sphere, whereas oxygen atoms from SO4 and H2O are shown in red and purple, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Th4(SO4)7(OH)2(H2O)62H2O (2): thorium nitrate (Th(NO3)46H2O) (0.1478 g), vanadium oxide (0.0375 g), H2SO4 (500 lL, 1 M) and H2O (2 mL) were loaded into a 10 mL Teflon-lined autoclave. The autoclave was sealed and heated to 200 °C in a muffle furnace for 72 h and then cooled down to room temperature at a rate of 3.5 °C/h. The products were washed with distilled water and ethanol. The colorless transparent block crystals were obtained as 2. The structure of 2 was previously reported by Burns et al. [17]. The yellow block crystalline phase yielded from this reaction was Th(VO3)4, a known compound reported before [23], as shown in Fig. 1(right). The spectroscopic properties of 2 were studied for comparison with 1. Crystallographic studies Single crystals of ThF2(SO4)(H2O) (1) and Th4(SO4)7(OH)2(H2O)62H2O (2) were chosen using an Olympus polarized microscope, then mounted on CryoLoops with Krytox oil and optically aligned on Bruker D8-Venture diffractometer single crystal X-ray diffractometer using a Turbo X-ray Source (Mo-Ka radiation, k = 0.71073 Å) adopted with the direct-drive rotating anode technique and a CMOS detector at room temperature. The
Table 2 Selected bond distances (Å) for ThF2(SO4)(H2O) (1). Bond distances (Å) Th(1)–F(1) Th(1)–F(1)0 Th(1)–F(2) Th(1)–F(2)0 Th(1)–O1W Th(1)–O(4) Th(1)–O(5) Th(1)–O(6) Th(1)–O(7)
2.371(2) 2.382(2) 2.352(2) 2.353(2) 2.544(3) 2.446(3) 2.485(3) 2.450(3) 2.481(3)
S(1)–O(4) S(1)–O(5) S(1)–O(6) S(1)–O(7)
1.475(3) 1.468(3) 1.471(3) 1.475(3)
data frames were collected using the program APEX II and processed using the program SAINT routine within APEX II. For 1, a total of 1158 reflections in the range of 3.52 < 2h < 27.62, and 8 < h < 9, 9 < k < 9, 13 < l < 13 were collected, of which 1115 reflections were accepted. The final R1 is 0.0140. For 2, a total of 3529 reflections in the range of 2.24 < 2h < 27.53, and 23 < h < 16, 14 < k < 14, 16 < l < 18 were collected, of which 3025 reflections were accepted. The final R1 is 0.0293. The structures were all solved by direct methods and refined by the full-matrix least squares on F2 using the SHELXTL-97 program. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were assigned by searching the q peaks located in the electron density map and refined normally. Selected crystallographic data and refinement details of the compounds are given in Table 1. Atomic coordinates and additional structural information are provided in Supporting information (CIF files).
Scanning electron microscopy–energy-dispersive spectroscopy (SEM– EDS) Scanning electron microscopy images and energy-dispersive spectroscopy data were recorded on a FEI Quanta 200FEG Scanning Electron Microscope with the energy of the electron beam being 30 keV. Samples were mounted directly on the carbon conductive tape with Au coating.
Raman spectra The Raman spectra were recorded by placing single crystals of the two compounds on a quartz slide under a CRAIC Technologies microspectrophotometer with a CRAIC Apollo™ 785
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Table 3 The bond valence sum calculation results of ThF2(SO4)(H2O) (1). Bond distances (Å)
F1 F2 O1W O4 O5 O6 O7
Bond valence (v.u.)
Th1
Th1
2.371 2.352 2.544 2.446 2.485 2.450 2.481
2.382 2.353
SUM (v.u.)
S1
Th1
Th1 0.430313 0.465397
1.475 1.468 1.471 1.475
0.443298 0.466656 0.360985 0.470455 0.423391 0.465397 0.427993
S1
1.495862 1.524431 1.512121 1.495862
0.873611 0.932053 0.360985 1.966317 1.947822 1.977518 1.923855
Fig. 3. (a) The building unit of Th4(SO4)7(OH)2(H2O)62H2O (2) with atomic labeling scheme at 50% probability, showing the local coordination environment of three Th(IV) ions. (b) The ball and stick mode of ThO9 polyhedron of Th(2) and Th(3). Color code: Th(2)-blue, Th(3)-magenta, O(SO2 4 )-red, OH -blue, H2O-purple. (c) The illustration of Th(1)O9 pseudo-dimer which is face-shared via two OH groups and one water molecule along the b axis. (d) Depiction of the three-dimensional network structure of Th4(SO4)7(OH)2(H2O)62H2O (2) down the b axis, showing the connection of the thorium centers via bridging sulfate anions. The ThO9 is shown as green, blue, and magenta 2 polyhedron, SO2 4 as yellow polyhedron, and oxygen atoms from SO4 , OH , and H2O are shown in red, blue, and purple, respectively. The protons are all omitted for clarity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
coherent laser (k = 785 nm). The data integration time is 2000 ms for both samples.
Table 4 Selected bond distances (Å) for Th4(SO4)7(OH)2(H2O)62H2O (2). Bond distances (Å) Th(1)–O(3) Th(1)–O(4) Th(1)–O(6) Th(1)–O(8) Th(1)–O(11) Th(1)–O(19) Th(1)–O(21) Th(1)–O(23) Th(1)–O(24) Th(2)–O(2) Th(2)–O(10) Th(2)–O(13) Th(2)–O(16) Th(2)–O(16)0 Th(2)–O(20) Th(2)–O(20)0 Th(2)–O(25) Th(2)–O(25)0 Th(3)–O(1) Th(3)–O(5) Th(3)–O(5)0 Th(3)–O(9)
2.456(6) 2.433(5) 2.382(6) 2.398(5) 2.463(6) 2.412(6) 2.487(5) 2.366(6) 2.686(10) 2.394(8) 2.529(8) 2.599(8) 2.408(6) 2.408(6) 2.369(6) 2.369(6) 2.436(5) 2.436(5) 2.473(8) 2.362(6) 2.362(6) 2.425(5)
Th(3)–O(9)0 Th(3)–O(17) Th(3)–O(18) Th(3)–O(22) Th(3)–O(22)0 S(1)–O(1) S(1)–O(2) S(1)–O(3) S(1)–O(3)0 S(2)–O(6) S(2)–O(20) S(2)–O(21) S(2)–O(22) S(3)–O(4) S(3)–O(9) S(3)–O(14) S(3)–O(25) S(4)–O(5) S(4)–O(7) S(4)–O(16) S(4)–O(19)
2.425(5) 2.596(10) 2.604(7) 2.388(6) 2.388(6) 1.485(9) 1.476(8) 1.476(6) 1.476(6) 1.475(6) 1.463(6) 1.468(5) 1.471(6) 1.466(5) 1.474(6) 1.459(6) 1.483(6) 1.462(6) 1.473(6) 1.461(6) 1.464(6)
Temperature dependent solid-state UV–Vis–NIR and fluorescence spectra Variable temperature solid-state UV–Vis–NIR and fluorescence spectra analysis of the single crystals were collected using a CRAIC Technologies microspectrophotometer. Crystals were placed on quartz slides under Krytox oil, and data was collected after the optimization of microspectro-photometer. Measurements were carried out between 77 K and 300 K with a 30 K step at a rate of 6 K min1. The UV–Vis–NIR absorption spectra were collected with an integration time of 13 ms and 12 ms for 1 and 2, respectively. The fluorescence spectra were excited at 365 nm and emission spectra were recorded from 300 nm to 900 nm with an integration time of 1000 ms for both samples. Bond-valence sum analysis The bond-valence sum (BVS) calculations were performed for all donor atoms in 1 and 2 to verify the oxidation states, and the
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Y. Zhao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 295–303 Table 5 The bond valence sum calculation results of Th4(SO4)7(OH)2(H2O)62H2O (2). Bond distances (Å) Th1 O3 O4 O6 O8 O11 O19 O21 O23 O24
2.456 2.433 2.382 2.398 2.463 2.412 2.487 2.366 2.686
Bond valence (v.u.) Th1
S
Th1
1.476 1.466 1.475
0.457911 0.487279 0.559293 0.535623 0.449329 0.515735 0.421108 0.584010 0.245931
2.398 1.464 1.468 2.366 2.686
Bond distances (Å) Th2 O2 O10 O13 O16 O20 O25 O1 O5 O9 O17 O18 O22
SUM (v.u.) Th1
S 1.491825 1.532694 1.495862
0.535623 1.541001 1.524432 0.584010 0.245931
Bond valence (v.u.) Th3
2.394 2.529 2.599 2.408 2.369 2.436 2.473 2.362 2.425 2.596 2.604 2.388
S
Th2
1.476
0.541445 0.375920 0.311123 0.521341 0.579294 0.483344
1.461 1.463 1.483 1.485 1.462 1.474
1.471
1.949736 2.019973 2.055155 1.071246 0.449329 2.056736 1.94554 1.16802 0.491862 SUM (v.u.)
Th3
S 1.491825
0.437348 0.590357 0.497929 0.313656 0.306947 0.550297
1.553547 1.545172 1.463866 1.455975 1.549353 1.499910
1.512121
2.03327 0.37592 0.311123 2.074888 2.124466 1.94721 1.893323 2.13971 1.997839 0.313656 0.306947 2.062418
Fig. 4. SEM images of single crystals of ThF2(SO4)(H2O) (1) (left), Th4(SO4)7(OH)2(H2O)62H2O (2) (right) and their respective Energy-dispersive X-ray Spectroscopy (EDS) of the selected areas.
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results are consistent with their expected valences. The bond-valence parameters used for analyzing Th(IV)–O, Th(IV)–F and S(VI)–O bonds were from Brese and O’Keeffe et al. [24]. Results and discussion Synthesis The compound 1 was synthesized under hydrothermal condition with the existence of HF. HF has been demonstrated to be a good mineralizing agent to assist formation of large porosity structure such as TOF-2 by O’Hare et al. [25], and uranyl phosphonate nanotube by Albrecht-Schmitt et al. [26]. During the process of the reaction, the methanesulfonic acid decomposed to sulfate and afforded the thorium sulfate fluoride. It is noticed that the HF played a critical role in the formation of this complex, without which, no crystalline phase was yielded under identical conditions. The compound 2 is reported by Burns et al. before, synthesized via reacting thorium nitrate aqueous solution directly with sulfuric acid (0.2 M) under hydrothermal condition. However, an alternate synthesis route is reported here, where thorium nitrate, vanadium oxide, sulfuric acid, and H2O were mixed and reacted under hydrothermal condition. The addition of vanadium oxide would yield both thorium sulfate and vanadate, indicating that sulfate and vanadate are both strong thorium complexing ligands. However, the reaction yield was very low, so there was not enough sufficient amount of sample for powder X-ray diffraction. Thus, the single crystal X-ray diffraction data were only collected for compound 2 to verify the structure. Photos of the products from two reactions are shown in Fig. 1, showing the morphology of the crystals. Crystallography ThF2(SO4)(H2O) (1): this compound crystallizes in the central symmetric space group, P21/n. The asymmetric unit contains one Th(IV) cation, two F, one SO2 anion, and one H2O molecule. 4 The atomic labeling scheme of 2 is shown in Fig. 2a, indicating that the Th cation is coordinated to five oxygen atoms and four fluorine atoms. Th(1) is bonded to O(4), O(5A), O(6A), and O(7A), each from different sulfate anions S(1), S(1A), S(1B) and S(1C), respectively, and their bond lengths range from 2.446(3) to 2.485(3) Å. O1 W is the water molecule bonded to Th(1), with a distance of
Fig. 5. The UV–Vis–NIR absorption spectra of the reaction products 1, 2 and Th(VO3)4, as well as the starting materials (Th(NO3)46H2O and Na2SO4) from 200 nm to 800 nm were recorded.
2.544(3) Å. Th(1) is linked to the symmetry equivalent Th(1A) via F(1) and F(1A) on the upper side shown in the figure, while F(2) and F(2A) are bridging between Th(1) and Th(1B) on the lower side. Th–F(1) and Th–F(2) bond distances are in the range of 2.352(2)– 2.382(2) Å. It is clear from the depiction that the thorium centers are edge-sharing with each other via F(1) and F(2) atoms to form a chain extended along the a axis as shown in Fig. 2c. The sulfate anion bridges thorium centers using O(4) and O(7), while the other two oxygen atoms O(5) and O(6) connected to the thorium centers from adjacent chains, leading to a three-dimensional network (Fig. 2d). The S–O bond lengths are all within the normal region from 1.468(3) to 1.475(3) Å. These distances are in agreement with those found in other thorium(IV) sulfate compounds [13–19]. The selected bond distances of Th–O bonds, Th–F bonds and S–O bonds are listed in Table 2. Since the electron density of the proton is too low, it is difficult to locate the protons in the structure from single crystal X-ray diffraction data. Thus bond valence sum calculation results were therefore used to define the protonation of oxygen atoms, whose results are listed in Table 3. The BVS of O(4), O(5), O(6) and O(7) that bonds to thorium and sulfur atoms are all close to 2 v.u. (from 1.92 to 1.98 v.u.), indicating that these are fully deprotonated O2. The BVS of the bridging F(1) and F(2) is 0.87 and 0.93 v.u., whereas O1W is only 0.36 v.u. Thus, those sites were assigned to be F and H2O accordingly. This result is consistent with the EDS results, where fair amount of fluorine was found in the compound 1 with larger molar percentage than thorium. The thorium center is bonded to nine donor atoms, being four sulfate oxygen, four fluorines, and one water molecule, as shown in Fig. 2b, forming the ThO5F4 polyhedron, which is best described as the tricapped trigonal prism. Th4(SO4)7(OH)2(H2O)62H2O (2) [17]: this compound has been previously reported by Burns et al., thus only a brief description is provided here in comparison to compound 1. Compound 2 crystallizes in the centrosymmetric space group, Pnma. The asymmetric unit contains three Th(IV) cations, four SO2 anions, two 4 hydroxyl groups, six coordinated water molecules and one free water molecule. The atomic labeling scheme is shown in Fig. 3a. There are some disorders of the terminal oxygen atoms, which would probably cause the level B alert of ‘‘large Non-solvent O Ueqmax/Ueqmin’’. However, since the structure is already reported by Burns et al., the single crystal data here is only utilized to confirm the structure of the compound 2. So the major concerns were put in the structure deviation between compounds 1 and 2. Unlike compound 1, compound 2 adopts three distinct thorium centers. Th(1) is face sharing two hydroxyl groups and one coordinated water molecule with adjacent symmetry equivalent Th(1) centers, forming a pseudo-dimer topology. The dimers are further linked by the sulfate anions along b and c axis to form discrete chains as shown in Fig. 3c. Both Th(2) and Th(3) centers present local coordination environment of nine, where seven oxygen atoms are from seven unique SO2 4 groups, and the rest two coordinate sites are occupied by the water molecules (Fig. 3b). The Th–O(S) bond length ranges from 2.362 to 2.487 Å. The Th–OH bonds are 2.398 and 2.366 Å. The Th–OH2 distances are from 2.463 to 2.686 Å. The sulfate anions in this compound act as only the bridging ligand, leading to the three dimensional framework. The S–O bond distances are all in the normal range 1.459–1.485 Å, which are all in agreement with the previously reported thorium sulfate data [13–19]. The selected bond distances of Th–O bonds and S–O bonds are listed in Table 4. The BVS calculations as listed in Table 4 were performed using the crystal data obtained in this work to confirm the oxygen speciation. The BVS results of the O(8) and O(23) are 1.07 v.u. and 1.17 v.u., respectively, indicating that these are hydroxyl groups, bridging the two Th(1) centers together as a pseudo-dimer. The
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Fig. 6. Temperature-dependent UV–Vis–NIR absorption spectra of single crystals of ThF2(SO4)(H2O) (1) and Th4(SO4)7(OH)2(H2O)62H2O (2) from 300 nm to 800 nm recorded with a temperature range of 77–300 K. The photograph of the crystal used for spectra collection is shown on the upper right side of each figure.
O(11),O(24), O(10), O(13), O(17), and O(18) are all below 0.5 v.u., therefore they are assigned as coordinated water molecules (see Table 5). Scanning electron microscopy–energy-dispersive spectroscopy (SEM– EDS) analysis The single crystals of 1 and 2 were mounted on the carbon conductive tape with Au coating for SEM imaging and EDS analysis. The spike of the Si Ka edge peak around 1.74 keV is observed, where the Si is most likely from the Krytox oil used for crystal sample storage. Although the exact quantity ratio of F: Th cannot be determined accurately, since the F Ka edge peak at 0.677 keV is greatly affected by the O Ka peak at 0.525 keV, in comparison to compound 2, it is evident that there are a fair amount of fluorine in compound 1 (see Fig. 4). Solid state UV–Vis–NIR absorbance spectroscopy The absorption spectra of the reaction products 1, 2 and Th(VO3)4, as well as the starting materials (Th(NO3)46H2O and Na2SO4) are shown in Fig. 5. All thorium complexes show absorption features between 250 nm and 350 nm. Thus the major absorption peaks of 1 and 2 observed at 300 nm and 320 nm, respectively, could be attributed from the thorium centers. These absorption features for thorium compounds are likely originated from charge transfer electronic transitions from O 2p orbitals to the empty Th 6d/5f orbitals similar with the cases of UO2+ 2 compounds, where change transfer features from O 2p orbitals to uranium 5f/6d orbitals are always observed [27]. To understand the electronic absorption mechanism of these two Th(IV) compounds, time-dependent density functional theory (TDDFT) [28] implemented in ADF 2013.01 [29–31] was employed to calculate the vertical excitation energies of a simplified model compound Th(H2O)4+ 9 at its optimal ground-sate geometry at the scalar relativistic level. The Perdew– Burke–Ernzerhof (PBE) exchange–correlation functional [32] was used in the geometry optimization, and the statistically averaged orbital potentials (SAOP) [33] with correct asymptotic 1/r behavior were employed in the TDDFT calculations. Uncontracted Slater basis sets of triple-f plus one polarization (TZP) quality for all atoms with frozen atomic core approximation applied to [1s2] of O and to [1s2–4f14] of Th [34]. The average Th–OH2 bond distance is 2.536 Å, in good agreement with the experimental result 2.544 Å. The lowest absorption energy is around 174 nm,
corresponding to electron transition from occupied O-2p orbitals to the virtually occupied Th–5f orbitals, i.e., ligand to metal charge transfer transition. The calculated lowest excitation energy is lower than the experimental result, which is likely due to the simplified model. Further investigation of the relationship between the peak position and intensity with temperature were conducted. Utilizing a temperature control sample stage, the spectra can be recorded from 77 K to 300 K. However, the additional quartz window on the stage would introduce intense noise below 300 nm. As a consequence, The UV–Vis–NIR absorption spectra were collected in the range from 300 nm to 800 nm, which covered the UV, Visible, and a small part of NIR regions, as shown in Fig. 6. The figures represent the raw data without normalization. The absorption intensities of the two compounds changed dramatically as a function of temperature. For compound 1, from 77 K to 180 K, the absorption intensity is decreased as the temperature increased. However, from 210 K, the trend was reversed, where the peak intensity is increased as the increase of temperature. Such phenomenon is associated with a blue shift into the UV region. Compound 2 present a similar phenomenon. From 77 K to 180 K, the peak intensity dropped constantly, while from 210 K to 300 K, the peak intensity gradually increased. Such a reversed correlation between the intensity of the absorption peaks and temperature is likely originated from temperature induced structural changes. Fluorescence spectroscopy It is noteworthy that the fluorescence characters of the thorium complexes were not well studied, since Th(IV) ion does not possess any intrinsic photoluminescence properties. Thus, most of the emission spectra reported for thorium compounds are originated from luminophores other than the thorium centers. For instance, Gorden et al. have reported the luminescence properties of the thorium tetracyanoplatinates, where the emission is assigned from the TCP portion of the structures [35]. On the other hand, Brittain et al., have studied the luminescence of the hydrolysis products of Th(IV), showing that the thorium hydroxide complexes emits between 500 nm and 600 nm [36]. They concluded that such luminescence emission is from the cooperative phenomena between Th(IV) centers, so the distance between Th(IV) centers could greatly affect the intensity of luminescence peaks of the complexes. Such hypothesis is confirmed by the experimental
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equally intense emission peaks around 420 nm and 700 nm, respectively. The temperature dependent luminescence spectra indicated that the two peaks showed the same relation of intensity change versus temperature, where the intensity first increased from 90 K to 180 K and then decreased from 210 K to 300 K as the increase of temperature. However, the relative peak intensity between the two peaks varied at different temperature. For example, the 450 nm peak showed the minimum intensity at 90 K, whereas the 700 nm peak showed the minimum intensity at 300 K. The spectra of 1 all show a broad emission region from 400 nm to 800 nm. At low temperature zone, the intensities of the emission were the weakest, where from 77 K to 120 K, raising the temperature would result in a slight increase of the intensities of the emission. Then the intensities suddenly jumped to the highest at 150 K, after which, keep increasing the temperature would cause a constant decrease of intensity from 630 nm to 800 nm region, whereas the 400 nm to 630 nm did not show a clear relation of the decrease in intensity versus temperature. The spectra of 2 all appear to have one strong peak at 720 nm and a weak shoulder around 580 nm, where monotonic decrease in intensity is observed with the increase of temperature. Owing to the fact that the non-radiative emission of phonon is promoted at elevated temperature, it is common the fluorescence intensity increases while temperature decreases. However, the inconsistent trends observed for three compounds clearly indicate that the emission of thorium compounds is a complicated and sensitive process. Given that the thorium nitrate and sulfates all absorb around 350 nm and emit above 420 nm, these emission phenomenon cannot be simply explained by the reserved charge transfer process. Therefore, multiple energy transfer and decay mechanism via possible vibrational relaxation and dissipation reorganization must have been involved. In addition, referring to the explanation provided by Brittain et al., whose work demonstrated that the distance between thorium centers greatly affected the intensity of fluorescence, one reasonable extrapolation is that the local coordination environment of thorium can greatly affect its corresponding fluorescence property. Comparing the structure of compound 1 and 2, the most significant deviation is that fluorine is coordinated to thorium centers in 1, whereas in 2 more water molecules are bonded to thorium centers, which could possibly generate the deviation in the emission energy range and the temperature dependent intensity alteration of their
Fig. 7. Temperature-dependent fluorescence spectra of single crystals of Th(NO3)46H2O, ThF2(SO4)(H2O) (1) and Th4(SO4)7(OH)2(H2O)62H2O (2) with excitation wavelength of 365 nm recorded from 77 K to 300 K.
results that by increasing the incorporation ratio of trivalent lanthanide ions into the thorium hydrolysis products, which brought in better separation between thorium centers, the fluorescence intensity was decreased [36]. We herein further investigated the solid state fluorescence properties of the thorium complexes. The fluorescence spectra of 1, 2 and thorium nitrate (Th(NO3)46H2O) were excited at 365 nm and recorded from 77 K to 300 K, showing emissions from 420 nm to approximately 800 nm (Fig. 7). The fluorescence spectra of thorium nitrate hexahydrate Th(NO3)46H2O showed two
Fig. 8. The Raman spectra collected from single crystal samples of ThF2(SO4)(H2O) (1) and Th4(SO4)7(OH)2(H2O)62H2O (2) at room temperature.
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emission spectra. It is also noteworthy that the fluorescence quenching behavior of 2 is much more pronounced, which could be attributed from the higher numbers of coordinated water in 2 compared to 1. In the study of the fluorescence lifetime of lanthanide complexes, the hydration is one of the key features [37]. To fully understand the mechanism of the thorium fluorescence, detailed comparison experiment together with theoretical calculation must be involved before pronounced conclusion can be reached.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2015.04.037. References [1] [2] [3] [4]
Raman spectroscopy The Raman spectra of compound 1 and 2 both show identical vibration modes of the S–O bond [38,39] (see Fig. 8). The vibrations between 300 and 700 cm1 (452 and 625 cm1 peaks for 1, and 439, 467, 610 cm1 peaks for 2) could be assigned to the m2 and m4 bending modes of the S–O bond, whereas the peaks around 1000 cm1 (1055, 1096, and 1250 cm1 peaks for 1, and 1037, 1083, 1108, and 1200 cm1 peaks for 2) are from the m1 and m3 stretching of the S–O bond. Conclusion In summary, the first thorium sulfate fluoride compound ThF2(SO4)(H2O) (1) has been successfully isolated from hydrothermal reactions via the decomposition of methanesulfonic acid. A known thorium sulfate Th4(SO4)7(OH)2(H2O)62H2O (2) obtained alternatively compared to the previous reported route was studied as well. The single crystal X-ray diffraction structural analysis provided insights of the local coordinated environment of thorium centers of the two compounds. The Energy Dispersive X-ray Spectroscopy and Raman spectroscopy analysis were applied to validate the composition of the compounds. The temperature dependent UV–Vis–NIR absorption spectra and fluorescence spectra collected from 200 nm to 800 nm with a temperature range of 77 K to 300 K showed that the intensities of the peaks for compound 1 and 2 varied as a function of temperature. Although all thorium complexes adopt weak fluorescence properties, the mechanism is not fully understood yet. It is proposed that such phenomenon cannot be simple explained as a charge transfer process, but rather a complicated system involving multiple energy transfer and multiple decay mechanism via possible vibrational relaxation, dissipation reorganization, and interaction between thorium centers. Further investigation regarding the mechanism study of this uncommon fluorescence is under the way in our lab. Acknowledgments We are grateful for funding supported by National Science Foundation of China (21173143, 91326112, 21422704, 21471107, 21201106), the Science Foundation of Jiangsu Province (BK20140007, BK20140303), a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and ‘‘Young Thousand Talented Program’’ in China.
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