Experimental verification of the high pressure crystal structures in NH3BH3 Yanping Huang, Xiaoli Huang, Zhonglong Zhao, Wenbo Li, Shuqing Jiang, Defang Duan, Kuo Bao, Qiang Zhou, Bingbing Liu, and Tian Cui Citation: The Journal of Chemical Physics 140, 244507 (2014); doi: 10.1063/1.4884819 View online: http://dx.doi.org/10.1063/1.4884819 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/140/24?ver=pdfcov Published by the AIP Publishing Articles you may be interested in First-principles high-pressure unreacted equation of state and heat of formation of crystal 2,6-diamino-3, 5dinitropyrazine-1-oxide (LLM-105) J. Chem. Phys. 141, 064702 (2014); 10.1063/1.4891933 High pressure phase transition study of B–C–N compound J. Appl. Phys. 107, 073508 (2010); 10.1063/1.3369279 An extended high pressure-temperature phase diagram of NaBH 4 J. Chem. Phys. 131, 074505 (2009); 10.1063/1.3211081 High pressure investigation of α -form and CH 4 -loaded β -form of hydroquinone compounds J. Chem. Phys. 130, 124511 (2009); 10.1063/1.3097763 High-pressure structures of methane hydrate observed up to 8 GPa at room temperature J. Chem. Phys. 115, 7066 (2001); 10.1063/1.1403690
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THE JOURNAL OF CHEMICAL PHYSICS 140, 244507 (2014)
Experimental verification of the high pressure crystal structures in NH3 BH3 Yanping Huang, Xiaoli Huang, Zhonglong Zhao, Wenbo Li, Shuqing Jiang, Defang Duan, Kuo Bao, Qiang Zhou, Bingbing Liu, and Tian Cuia) State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, People’s Republic of China
(Received 26 February 2014; accepted 30 May 2014; published online 26 June 2014) A detailed high-pressure study on NH3 BH3 has been carried out using in situ synchrotron X-ray diffraction (XRD) and Raman scattering with a diamond anvil cell up to 20 and 33 GPa, respectively. The Rietveld refinement based on the XRD pattern and analysis of Raman data indicate two first-order phase transitions from the ambient pressure I4 mm structure (α-NH3 BH3 ) to a high pressure Cmc21 phase (β-NH3 BH3 ) at 2.14 GPa, and further into a monoclinic P21 (Z = 2) phase (γ NH3 BH3 ) at 9.67 GPa. Fitting the measured volumetric compression data to the third order BirchMurnaghan equation of state reveals a bulk modulus of B0 = 9.9 ± 0.5 and 17.0 ± 3.0 GPa (with fixed B0 = 4) for the β-NH3 BH3 below and above 5 GPa, respectively. Still, with the splitting of the NBH rock mode in Raman experiment, it is concluded that a second-order isostructural phase transition occurs at 5 GPa. By analyzing the dihydrogen bonding framework, the origin of the isostructural phase transition is attributed to the number of dihydrogen bondings per molecule in the Cmc21 phase increasing from 12 to 14 at 5 GPa. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4884819] I. INTRODUCTION
In recent years, there has been a vast and growing interest in light hydrides due to their potential use as hydrogen storage materials.1, 2 As a promising candidate material for hydrogen storage, ammonia borane (NH3 BH3 ) has attracted significant interest. It has a high gravimetric (19.6 wt. %) and volumetric (145 gH/L) hydrogen density.3, 4 Hydrogen is irreversibly released from ammonia borane through thermolysis at moderate temperatures5–7 and acid- or transition metal-catalyzed8–11 dehydrogenation. While, thermal decomposition of ammonia borane proceeds in three steps and the release of all the hydrogen atoms is only accomplished at 500 ◦ C. At ambient conditions, ammonia borane crystallizes in a tetragonal space group I4mm (α-NH3 BH3 ) with a unit cell containing two molecules.12, 13 This α phase is highly disordered, as the BH3 and NH3 groups undergo a 3-fold rotation about the B–N bond, so that each H atom occupies three available equilibrium positions with equal probability. This α phase transforms to an orthorhombic structure when cooled to 225 K, resulting in a significant change in the lattice dynamics. The new orthorhombic structure was shown to have the space group Pmn21 (Z = 2) with the lattice parameters a = 5.517 Å, b = 4.742 Å, and c = 5.020 Å.14 Although temperature plays an important role in controlling the stability of materials, as an independent thermodynamic parameter, pressure can significantly alter the crystal structures and properties of materials. The effect of pressure on the behavior of NH3 BH3 expands the fundamental understanding of this system and sheds further light onto its potential application as a hydrogen-storage material. The a) Author to whom correspondence should be addressed. Electronic mail:
[email protected]. Tel./Fax: +86-431-85168825.
0021-9606/2014/140(24)/244507/9/$30.00
high-pressure behavior and properties of NH3 BH3 have been studied by both experimental and theoretical study.15–25 Although numerous investigations were made to examine high pressure behavior of NH3 BH3 , the results were still controversial. The earliest in situ Raman spectra study has found that ammonia borane may undergo several phase transitions under high pressure. And they have observed a disorder-order phase transition around 8 kbar.15 At very similar time, Trudel and Gilson reported two phase transitions at 5 and 14 kbar, respectively.24 In some of later studies, Lin et al. identified three phase transitions at approximately 2, 5, and 12 GPa on the basis of vibration bands splitting and changes of the pressure coefficient.16 Xie et al. then discovered several solid-tosolid transformations with phase boundaries roughly at 2.4, 5.5, 8.5, and 10.4 GPa, characterized by distinctive profiles and the pressure dependences of characteristic modes.20 Very recently, Kupenko and Dubrovinsky reported another transition at 27 GPa.25 They identified the phase transition at 2 GPa as a disorder-order phase transition from the α phase to the β phase by means of XRD and thermal conductivity measurements. However, there is no consensus about the behavior of NH3 BH3 under higher pressure. In Ref. 17 no other transitions were observed up to 12 GPa apart from the one at 2 GPa, while one more transition to the triclinic P1 structure was suggested around 8 GPa in Ref. 21 with the highest pressure of 27 GPa. In Ref. 22, a phase transition at 5 GPa was reported and no further transformations were detected upon compression up to 23 GPa. In Ref. 23, a second-order isostructural phase transition and a first-order transition were discovered around 5 GPa and 13 GPa, respectively. Besides, several previous theoretical calculations were done concerning high pressure phases of NH3 BH3 but again with no consensus.26, 27 In order to shed more light on the structural behavior of NH3 BH3 under high pressure and to verify the high
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pressure structures reported in the literatures, we have performed synchrotron angle-dispersive XRD and Raman spectra experiments up to 30 GPa. Our experimental results confirm the pressure induced structural phase transition from I4mm (α-NH3 BH3 ) to orthorhombic Cmc21 structure (βNH3 BH3 ) at 2.14 GPa and we have identified the high pressure monoclinic P21 (Z = 2) phase (γ -NH3 BH3 ) at 9.67 GPa in experiment supplementary with the theoretical method. The structure of Cmc21 phase (β-NH3 BH3 ) was further optimized using density functional theoretical (DFT) calculations. The dihydrogen bond, which plays a significant role in determining the behavior of NH3 BH3 , is discussed extensively. For the first time, the high pressure crystal structures of NH3 BH3 have been identified by the joint of the experimental and theoretical results. II. EXPERIMENTAL AND THEORETICAL METHODS
Commercially available crystalline NH3 BH3 powder (>97% purity) was purchased from Sigma Aldrich. The purity was checked at ambient conditions by the Raman spectroscopy and XRD pattern. All observed Raman modes of the starting material were found to be associated with the NH3 BH3 , so it was used without further purification. A symmetric diamond anvil cell (DAC) with 400 μm diamond culets was used for these experiments. Finely ground samples, together with a ruby chip for pressure calibration, were loaded into the sample chamber created by drilling a 100 μm hole in a 50 μm thick stainless steel gasket. No pressure transmitting medium was used because pure NH3 BH3 is quite soft and provides good quasi-hydrostatic conditions. The Raman spectra were measured by an Acton SpctraPro 500i spectrograph with a liquid-nitrogen-cooled CCD detector (Princeton Instruments, 1340 × 100). A solidstate, diode-pumped, frequency-doubled Nd: vanadate laser (λ = 532 nm) was used as the excitation laser. Raman spectra were collected in a back scattering geometry with an 1800 gr/mm holographic grating, and the slit width was selected as 80 μm, corresponding to a resolution of ∼2 cm−1 . The sample image can be collected through an achromatic
lens and then focused onto a CCD detector for visual monitoring during experiments. The synchrotron ADXRD experiments were performed at the wiggler beamline X17C of the National Synchrotron Light Source in Brookhaven National Laboratory (BNL). And a MAR3450 image plate detector was used to record the diffracted X-rays. The wavelength was 0.4075 Å. Twodimensional patterns were radially integrated using the software FIT2D.28 All experiments were carried out at room temperature. The XRD patterns were fitted by Rietveld profile matching using the MATERIALS STUDIO program.29 During each refinement cycle, the fractional coordinates, scale factor, background parameter, isotropic thermal parameters, profile function, and cell parameters were optimized. Using the experimentally determined structures as input, we have performed DFT calculations to further optimize for experimentally obtained structures. Each structure was fully relaxed to an energy minimum at different pressure with the Vienna ab initio simulation package (VASP).30, 31 The allelectron projector augmented waves (PAW) method32 within the generalized gradient approximation (GGA) of Perdew, Burke, and Ernzerhof (PBE) was used.33 During the calculations, the uniform cutoff energy of 520 eV for the wave function expansion into plane waves and Monkhorst-Pack k points meshes were chosen to ensure that all the structures are converged to better than 1 meV/f.u. III. RESULTS AND DISCUSSION A. High pressure Raman spectroscopic measurements on NH3 BH3
The structural phase transformations of NH3 BH3 have been studied by using in situ Raman scattering at high pressures. It provides valuable information on local and cooperative changes during pressure-related transformation between phases. Raman vibrational spectra of NH3 BH3 were measured as a function of pressure up to 33 GPa. The Raman spectrum of the powder sample outside the DAC at ambient temperature is shown in Fig. 1 and listed in Table I. All active Raman
FIG. 1. Raman spectrum collected at ambient conditions.
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TABLE I. Assignments and vibrational frequencies (cm−1 ) of observed Raman modes of NH3 BH3 at ambient condition. Frequency(cm−1 ) This work 728 783 798 1069 1161 1187 1377 1597 2280 2373 3251 3316
Ref. 10
Ref. 11
Ref. 13
Ref. 26
Ref. 27
Assignment
Mode
726 782 798 1066 1158 1185 1375 1596 2280 2375 3255 3316
726 795 815 1066 1171 1192 1373 1600 2293 2397 3247 3318
728 783 798 1067 1161 1188 1377 1595 2280 2376 3253 3319
728 784 799 1067 1158 1189 n. r. 1596 2278 2375 3250 3314
738 803 817 1093 1164 1192 1376 1596 2291 2382 3251 3312
ν1 ν3 ν4 ν2 ν5 ν6 ν7 ν8 ν9 ν 10 ν 11 ν 12
NBH rock stretch 10 B–N stretch NBH rock BH3 deformation BH3 deformation NH3 deformation NH3 deformation Sym. B–H stretch Asym. B–H stretch Sym. N–H stretch Asym. N–H stretch
modes are measured in five distinct spectral regions. The two weak modes that are assigned as NBH rock ν 1 and ν 2 are observed at 728 and 1069 cm−1 , respectively. The B–N stretching manifests itself in two modes, 11 B–N, ν 3 , at 783 cm−1 and less intensive 10 B–N, ν 4 , at 798 cm−1 . Two BH3 deformation modes ν 5 and ν 6 center at 1161 cm−1 and 1187 cm−1 , respectively. Two NH3 deformation modes ν 7 is at 1377 cm−1 and ν 8 is at 1597 cm−1 . The two peaks at 2280 cm−1 and 2373 cm−1 correspond to the B–H stretching symmetric mode (ν 9 ) and asymmetric mode (ν 10 ), respectively. The N–H stretching vibrations are seen at 3251 cm−1 for symmetric ν 11 and at 3316 cm−1 for asymmetric ν 12 modes. In addition, a weak peak at 3173 cm−1 was observed. Earlier work left this peak unassigned,9, 10 while Hess et al.14 indicated that this mode may be the result of Fermi resonance of the overtone of the 1597 cm−1 and the N–H symmetric stretch fundamental, and disappeared around 6 GPa.
11 B–N
In our experiment, the Raman spectra were recorded from the sample during pressurizing up to 33 GPa at room temperature. The Raman spectra of NH3 BH3 and the Raman frequency shift as a function of pressure for the lattice vibration region (100–700 cm−1 ) are shown in Fig. 2. Dramatic spectral changes arising from phase transition are observed in the lattice region. As can be seen in Fig. 2(a), the lattice mode is not obvious at ambient pressure. Upon compression to 1.76 GPa, there are two clearly visible lattice modes: a high intensity lattice mode 1 at 176 cm−1 and a weak lattice mode 2 at 219 cm−1 . The sharp lattice modes strongly suggest a phase transition from the disordered tetragonal structure at ambient pressure to an ordered structure at about 2 GPa. Upon further compression to 8.43 GPa, a small peak begins to appear at 234 cm−1 (assigned as lattice mode 3) in the high frequency side of the lattice mode 1, indicating a possible beginning structural change in NH3 BH3 . This shoulder peak grows
FIG. 2. (a) Lattice vibration modes of solid NH3 BH3 measured to 33 GPa at 298 K and (b) pressure dependence of the corresponding Raman shift of NH3 BH3 on compression for the lattice mode.
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FIG. 3. Representative Raman spectra of NH3 BH3 of internal vibrational modes and the Raman shift versus pressure in the frequency range 600–3400 cm−1 .
rapidly with a slight increase in pressure to 33 GPa. The relative intensity of all the lattice modes increases with pressure until 18.17 GPa, and all of them move slowly to high frequency in the whole pressure range. Figure 3 shows representative Raman spectra of NH3 BH3 of internal vibrational modes and the Raman shift versus pressure in the frequency range 600–3400 cm−1 as the pressure increased up to 33 GPa. Upon compression from ambient pressure to 1.17 GPa, the Raman spectra change dramatically, indicating substantial changes in the crystal structures. For instance, the low frequency NBH rock mode (728 cm−1 ) splits
into a doublet very close each other, and exhibits blue shift up to 13.1 GPa, where this doublet cannot be resolved directly, and the high frequency mode (1069 cm−1 ) that looks like a broad weak peak at ambient pressure shows slight red-shift and appears as a part of a doublet at 1.17 GPa. Meanwhile, the two adjacent BH3 deformation modes at ambient pressure become shaper and more intense. In addition, the splitting of the asymmetric N–H stretching mode was observed at 1.17 GPa. The splitting as well as the significant narrowing of these modes indicates the onset of the structural phase transition. Up to 2.97 GPa, the high frequency BH3 deformation
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mode centered at 1187 cm−1 at ambient pressure split into two modes (1191 cm−1 and 1207 cm−1 ). The low frequency NH3 deformation (1377 cm−1 ) mode coalesces into the diamond first order peak, only the 1597 cm−1 mode is shown. The doubly degenerate mode ν 8 (NH3 deformation) split into two modes with similar intense. At ambient pressure, two dominant peaks at 2280 and 2373 cm−1 are observed in the B–H stretching region. The intermediate peak at 2328 cm−1 was assigned as the overtone of the symmetric B–H bending mode. It can be seen that the asymmetric B–H stretching mode (2373 cm−1 ) split at 2.97 GPa, and remained separated up to the highest pressure. The phase transition from the ambient phase to high pressure phase proceeds sluggishly, starting at 1.17 GPa and completing at 2.97 GPa. The transition pressure is hence determined to be about 2 GPa by taking the midpoint pressure. This result is consistent with our XRD results. Nevertheless, the splitting at B–N stretching mode is due to an isotopic effect rather than the removal of degeneracy. The 11 B and 10 B contributions to the B–N symmetric stretch are readily apparent up to 13.1 GPa. Both 11 B–N and 10 B–N stretching modes are sharp in the pressure region under 8.43 GPa, whereas 10 B–N stretching mode is much weaker than 11 B–N stretching mode because of the lower abundance of 10 B. Also, we did not observe the splitting of the 11 B–N stretching mode reported in Ref. 11. With further compression to 4.88 GPa, the high frequency NBH rock mode splits into a triplet, nevertheless, no remarkable changes are seen in other regions at this pressure, this simple change is likely due to a change in atomic bonding of the structure. With the pressure increasing, the high frequency BH3 deformation mode split into three peaks at 11.55 GPa, and remained split to the highest pressure. Together with changes in the lattice region, we can conclude that a new transition occurred at about 10 GPa. With continuous compression, there is no obvious change in the number of peaks in all five regions, in addition to weakening and broadening of Raman modes. This implies that the compound has become compact at this pressure range. The pressure dependences of Raman modes can also be seen in Fig. 3. In general, most Raman modes exhibit pressure-induced blue shift, consistent with the finding that the bonds stiffen upon compression. However, in contrast to the other internal Raman modes, both the symmetric and asymmetric N–H stretching mode shifted to low frequency with increasing pressure, indicating the weakening of N–H bond. This is to be believed a result of the lengthening of the N–H bond, owning to the formation and strengthening of dihydrogen bonding (N–H· · ·H–B) as pressure increasing.
J. Chem. Phys. 140, 244507 (2014)
FIG. 4. XRD patterns collected at various pressures for NH3 BH3 with incident wavelength λ = 0.4075 Å. The arrow in the diagram indicates changes in the peak shapes indicating a phase transition.
1. Structural determination of α and β phase at low pressure
Below 2.14 GPa, all diffraction patterns are attributed to the α phase with the lattice parameters a = 5.255 (8) Å and c = 5.055 (1) Å. This is in good agreement with the values determined from the previous study with a = 5.2630 (4) Å and c = 5.0504 (8) Å,13 as shown in Fig. 5(a). At 2.14 GPa the spectrum showed very different features from the lowpressure spectra. We observed that two strongest peaks around 2θ = 6.8◦ merged together. Several new diffraction lines emerged at the higher angles around 2θ = 7.29◦ , 7.78◦ , and 8.28◦ were indicative of a transformation to a high-pressure phase with lower symmetry. After the first transition, the
B. High pressure XRD on NH3 BH3
To further explore the crystal structure of NH3 BH3 , we have carried out in situ high pressure XRD experiment from ambient pressure to 20 GPa. XRD patterns collected at different pressures during compression are shown in Fig. 4. Over the whole pressure range, the sample remains transparent and pressure-induced metallization is so ruled out.
FIG. 5. Background-subtracted Rietveld refinement profile for the α phase at 0.26 GPa (a) and β phase at 2.14 GPa (b) and γ phase at 9.67 GPa (c).
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FIG. 6. Schematic crystal structures of polymorphs: (a) α-NH3 BH3 ; (b) β-NH3 BH3 .
diffraction patterns are identified as the β phase yields an orthorhombic unit cell dimensions with the lattice parameters a = 5.9970 Å, b = 6.392 (1) Å, and c = 5.9200 Å (Fig. 5(b)), in consistent with Filinchuk et al.’s observation.17 We have further performed a high-pressure Raman experiment that is also consistent with a transition at this pressure. Beyond 3 GPa, the high angle diffraction peaks become more compressible than other peaks, indicating anisotropic compression likely due to change of bonding. Schematic crystal structures of αNH3 BH3 and β-NH3 BH3 are shown in Fig. 6. The detailed structural information of both α and β phase after the refinement are listed in Table II. 2. Structural determination of the γ phase at high pressure
Up to 9.67 GPa, the appearance of a new peak is clearly visible in Fig. 4, indicating a phase transition. The new phase is persisted up to the highest pressure of 20 GPa reached in the experiment. In order to identify the crystal structure of the new phase, we have considered two experimental structural models21, 23 (P1 and P21 (Z = 4)) and one theoretical model27 (P21 (Z = 2)) to perform the Rietveld refinement. The data set collected at 9.67 GPa was used for structure determination. The observed XRD pattern was well refined with the proposed space group P21 containing two molecules in a unit cell (γ -NH3 BH3 ) as shown in Fig. 5(c), and the structure of γ -NH3 BH3 is shown in Fig. 7. Our result has the same space group P21 with Lin’s,23 but they are essential different. TABLE II. Experimental structural parameters for the α and β phase obtained from synchrotron powder diffraction data. Structure I4mm(Z = 2) a = 5.255(8) Å c = 5.055(1) Å (0.26 GPa and 300 K) Cmc21 (Z = 4) a = 5.9970 Å b = 6.392(1) Å c = 5.9200 Å (2.14 GPa and 300 K)
Atoms B N
B N H H H H
x 0 0
0 0 0.139(1) 0 0 0.165(4)
y 0 0
0.142(6) 0.211(1) 0.293(6) 0.076(2) 0.299(4) 0.039(6)
z 0.003(2) 0.686(9)
0.690(1) 0.933(9) 0.981(3) 1.028(4) 0.577(9) 0.653(8)
FIG. 7. Schematic crystal structures of γ -NH3 BH3 .
Lin’s structural model contains four molecules in a unit cell, with unit cell dimensions a = 7.713 (6), b = 5.375 (4), c = 3.898 (4) Å, β = 97.22 (8)◦ , and four NH3 BH3 molecules can be separated into two groups, which can be denoted as the N1B1 and N2B2 molecule, with N1 (0, 0,0), B1 (0.1429, 0.1254, 0.1178) and N2 (0.2724, 0.6999, 0.6509), B2 (0.3474, 0.4594, 0.6318). While our experimental result contains two NH3 BH3 molecules in a unit cell with unit cell dimensions a = 3.926 (9), b = 4.992 (2), c = 4.1960 Å, β = 92.216 (5)◦ , and two NH3 BH3 molecules are equivalent. Beyond this phase transition, no remarkable changes were observed in the XRD pattern up to 20 GPa except weakening and broadening of diffraction peaks due to pressurization. Fig. 5 shows the Rietveld refinements of the XRD patterns of the α phase at ambient pressure (a) and β phase at 2.14 GPa (b) and γ phase at 9.67 GPa (c). The Rietveld-refined structure information for the P21 model at 9.67 GPa are all listed in Table III. We then calculated the enthalpy difference curves (relative to the Cmc21 structure), which are presented in Fig. 8, it is obvious that the enthalpy of Lin’s structural model (P21 (Z = 4)) is much higher than our result (P21 (Z = 2)). The enthalpy versus pressure plot showed a transition from orthorhombic to monoclinic around 10 GPa which agreed well with our experiment. TABLE III. The structural parameters for the γ phase from RietveldRefined P21 model in NH3 BH3 at 9.67 GPa and room temperature. Structure P21 (Z = 2) a = 3.926(9) Å b = 4.992(2) Å c = 4.196(0) Å β = 92.216(5)◦ (9.67 GPa and 300 K)
Atoms
x
y
z
H H H H H H N B
0.917(4) 0.987(2) 0.645(7) 0.425(1) 0.834(1) 0.5140 0.807(6) 0.6480
0.474(1) 0.173(5) 0.3140 0.363(6) 0.133(8) − 0.020(7) 0.2920 0.186(1)
0.233(9) 0.118(1) 0.018(1) 0.581(1) 0.706(7) 0.426(1) 0.199(1) 0.503(1)
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nearly isotropic for the α phase, it is highly anisotropic for the β and γ phase, with the c-axis and a-axis parameter shrinking the most, respectively. The pressure volume data for three phases have been fitted to the third-order Birch-Murnaghan (BM) equation of state34 7 5 V −3 V −3 3B0 P = − 2 V0 V0 2 V −3 3 B −4 × 1+ −1 , 4 0 V0
FIG. 8. Enthalpy versus pressure plot for NH3 BH3 .
C. Pressure-induced structural evolution and the second-order isostructural phase transition at 5 GPa
In the disordered α phase, the NH3 BH3 molecules are collinear with the c axis. In the high-pressure β phase, the molecules become inclined to the c axis, the inclination of NH3 BH3 molecules with respect to the c axis becomes larger with increasing pressure, because the c axis is more likely to be compressed seen from Fig. 9(a). In the high pressure P21 phase, two NH3 BH3 molecules occupy one unit cell (Fig. 7). The geometric characteristics in the Cmc21 and P21 (Z = 2) phases are similar, and both of the two phases are the staggered conformation. The pressure dependence of the unit cell parameters and the volume of NH3 BH3 per unit cell are shown in Fig. 9. While the cell contraction under pressure is
where V0 is the volume per unit cell at ambient pressure, with V being the volume per unit cell at pressure P given in GPa, B0 is the isothermal bulk modulus, and B0 is the first pressure derivative of the bulk modulus. However, the results did not converge when fitting the high pressure data of Cmc21 phase with the third-order Birch-Murnaghan equation. Although the diffraction patterns in this pressure range have no characteristic change, together with our Raman spectroscopic study in part A, indicate a change around 5 GPa, so we fitted the data below and above 5 GPa, separately, with B0 being 4, the obtained bulk moduli for the phases are 9.9 ± 0.5 and 17.0 ± 3.0 GPa, respectively (shown in Fig. 9(b)). Although the change observed in our Raman spectroscopic studies at about 5 GPa does not result in any characteristic change in XRD, the different compressibility data associated with the phases below and above 5 GPa, much improved fitting statistics by separately considering the two regions, suggest that a secondorder isostructural phase transition occurs at 5 GPa. D. Dihydrogen bonding network in the Cmc21 phase
Our attempts for refining the XRD date for the highpressure phase were limited due to several factors such as
FIG. 9. (a) Variation in the unit cell dimensions as a function of pressure. (b) Variation in the cell volume as a function of pressure and the BM EOS fitting results for the indicated pressure range of the Cmc21 phase before and after 5 GPa (brown and green line).
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at ambient temperature, together with DFT calculations. Our experimental results confirm the pressure induced structural phase transition from I4mm (α-NH3 BH3 ) to orthorhombic Cmc21 structure (β-NH3 BH3 ) at 2.14 GPa. We have also identified the high pressure monoclinic P21 (Z = 2) phase (γ NH3 BH3 ) at 9.67 GPa in experiment supplementary with the theoretical method. Fitting the measured volumetric compression data to the third order Birch-Murnaghan equation of state reveals a bulk modulus of B0 = 9.9 ± 0.5 and 17.0 ± 3.0 GPa (B0 = 4) for the β phase below and above 5 GPa, respectively. Together with our Raman studies, we clarified the splitting of the NBH rock mode in Raman experiment as a second-order isostructural phase transition arising from the number of dihydrogen bondings per molecule in the Cmc21 phase increases from 12 to 14. ACKNOWLEDGMENTS
FIG. 10. Patterns of dihydrogen bonds in the Cmc21 phase. Red lines represent the dihydrogen bonds that already exist when the Cmc21 phase is formed at lower pressure. Green lines represent the dihydrogen bonds introduced at 5 GPa. The configuration is the same as that in Ref. 27.
peak broadening and weak intensity, so the theory analysis is needed to supplement for explaining the experimental results under pressure. The geometry of the strongly covalent NH3 BH3 molecule is nearly invariant in the investigated pressure range, so we focus on the intermolecular interactions-–dihydrogen bonding. A Cambridge structural database (CSD) structural search study established an empirical rule that the proper Hδ+ · · · δ− H interaction distance is in the range 1.7−2.2 Å.35 We counted the number of N−H · · · H−B close contacts in the Cmc21 phase, and found that the number of dihydrogen bondings per molecule in the simulated Cmc21 phase increases from 12 to 14 at about 5 GPa as can be seen in Fig. 10. At low pressure, one B–H bond is instead not involved in any H · · · H interaction, each of the other two B–H bonds is forming three H · · · H contacts, and the H atom in the NH3 end is still connected with two other H atoms. We suggest that at 5 GPa, the H atom on the mirror plane perpendicular to the a axis at the NH3 end connects to another B–H bond, making the total number of dihydrogen bonds per molecule 14. And we can conclude that the origin of the isostructural phase transition is attributed to the number of dihydrogen bondings per molecule in the Cmc21 phase increases from 12 to 14. IV. CONCLUSIONS
The phase transitions in NH3 BH3 were investigated by XRD up to 20 GPa and Raman measurement up to 33 GPa
The authors are grateful to Zhiqiang Chen, Xiaodong Li, and Yanchun Li for their help during the experimental researches. ADXRD experiments of this work were performed at X17C, BNL in USA and 4W2 HP-Station, BSRF assistance in the synchrotron measurement. This work was supported by the National Basic Research Program of China (No. 2011CB808200), Program for Changjiang Scholars and Innovative Research Team in University (No. IRT1132), National Natural Science Foundation of China (Nos. 51032001, 11074090, 10979001, 51025206, 11274137, and 11004074), and National Found for Fostering Talents of Basic Science (No. J1103202). 1 Z.
T. Xiong, C. K. Yong, G. T. Wu, P. Chen, W. Shaw, A. Karkamkar, T. Autrey, M. O. Jones, S. R. Johnson, P. P. Edwards, and W. I. F. David, Nat. Mater. 7(2), 138 (2007). 2 D. A. Dixon and M. Gutowski, J. Phys. Chem. A 109(23), 5129 (2005). 3 N. Blaquiere, S. Diallo-Garcia, S. I. Gorelsky, D. A. Black, and K. Fagnou, J. Am. Chem. Soc. 130, 14034 (2008). 4 F. H. Stephens, V. Pons, and R. T. Baker, Dalton Transactions (Royal Society of Chemistry, Cambridge, England, 2007), p. 2613. 5 M. G. Hu, R. A. Geanangel, and W. W. Wendlandt, Thermochim. Acta 23, 249 (1978). 6 F. Baitalow, J. Baumann, G. Wolf, K. Jaenicke-Rossler, and G. Leitner, Thermochim. Acta 391, 159 (2002). 7 M. E. Bluhm, M. G. Bradley, R. Butterick, U. Kusari, and L. G. Sneddon, J. Am. Chem. Soc. 128, 7748 (2006). 8 H. C. Kelly and V. B. Marriott, Inorg. Chem. 18, 2875 (1979). 9 F. H. Stephens, R. T. Baker, M. H. Matus, D. J. Grant, and D. A. Dixon, Angew. Chem., Int. Ed. 46, 746 (2007). 10 R. J. Keaton, J. M. Blacquiere, and R. T. Baker, J. Am. Chem. Soc. 129, 1844 (2007). 11 F. Y. Cheng, H. Ma, Y. M. Li, and J. Chen, Inorg. Chem. 46, 788 (2007). 12 C. F. Hoon and E. C. Reynhardt, J. Phys. C 16, 6129 (1983). 13 M. E. Bowden, G. J. Gainsford, and W. T. Robinson, Aust. J. Chem. 60, 149 (2007). 14 N. J. Hess, M. E. Bowden, V. M. Parvanov, C. Mundy, S. M. Kathmann, G. K. Schenter, and T. Autrey, J. Chem. Phys. 128, 034508 (2008). 15 R. Custelcean and Z. A. Dreger, J. Phys. Chem. B 107, 9231 (2003). 16 Y. Lin, W. L. Mao, V. Drozd, J. Chen, and L. L. Daemen, J. Chem. Phys. 129, 234509 (2008). 17 Y. Filinchuk, A. Nevidomskyy, D. Chernyshov, and V. Dmitriev, Phys. Rev. B 79, 214111 (2009). 18 J. Nylén, T. Sato, E. Soignard, J. L. Yarger, E. Stoyanov, and U. Häussermann, J. Chem. Phys. 131, 104506 (2009). 19 R. S. Chellappa, M. Somayazulu, V. V. Struzhkin, T. Autrey, and R. J. Hemley, J. Chem. Phys. 131, 224515 (2009).
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Xie, Y. Song, and Z. Liu, Can. J. Chem. 87, 1235 (2009). S. Kumar, X. Ke, J. Zhang, Z. Lin, S. C. Vogel, M. Hartl, S. Sinogeikin, L. Daemen, A. L. Cornelius, C. Chen, and Y. Zhao, Chem. Phys. Lett. 495, 203 (2010). 22 J. Chen, H. Couvy, H. Liu, V. Drozd, L. L. Daemen, Y. Zhao, and C.-C. Kao, Int. J. Hydrogen Energy 35, 11064 (2010). 23 Y. Lin, H. Ma, C. W. Matthews, B. Kolb, S. Sinogeikin, T. Thonhauser, and W. L. Mao, J. Phys. Chem. C 116, 2172 (2012). 24 S. Trudel and D. F. R. Gilson, Inorg. Chem. 42, 2814 (2003). 25 I. Kupenko, L. Dubrovinsky, V. Dmitriev, and N. Dubrovinskaia, J. Chem. Phys. 137, 074506 (2012). 26 M. Ramzan and R. Ahuja, J. Phys. Chem. Solids 71, 1137 (2010). 27 L. Wang, K. Bao, X. Meng, X. Wang, T. Jiang, T. Cui, B. Liu, and G. Zou, J. Chem. Phys. 134, 024517 (2011). 21 R.
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P. Hammersley, S. O. Svensson, M. Hanfland, A. N. Fitch, and D. Häussermann, High Pressure Res. 14, 235 (1996). 29 S. H. Jiao, G. S. Pang, H. W. Liang, Y. Chen, and S. H. Feng, J. Nanopart. Res. 9, 605 (2007). 30 G. Kresse and J. Hafner, Phys. Rev. B 47, 558 (1993); J. Phys. Condens. Matter 6, 8245 (1994). 31 G. Kresse and J. Furthmuller, Comput. Mater. Sci. 6, 15 (1996); Phys. Rev. B 54, 11169 (1996). 32 P. E. Blöchl, Phys. Rev. B 50, 17953 (1994); G. Kresse and D. Joubert, ibid. 59, 1758 (1999). 33 J. P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. 77, 3865 (1996). 34 F. Birch, J. Appl. Phys. 9, 279 (1938). 35 W. T. Klooster, T. F. Koetzle, P. E. M. Siegbahn, T. B. Richardson, and R. H. Crabtree, J. Am. Chem. Soc. 121, 6337 (1999).
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THE JOURNAL OF CHEMICAL PHYSICS 140, 244507 (2014)
Experimental verification of the high pressure crystal structures in NH3 BH3 Yanping Huang, Xiaoli Huang, Zhonglong Zhao, Wenbo Li, Shuqing Jiang, Defang Duan, Kuo Bao, Qiang Zhou, Bingbing Liu, and Tian Cuia) State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, People’s Republic of China
(Received 26 February 2014; accepted 30 May 2014; published online 26 June 2014) A detailed high-pressure study on NH3 BH3 has been carried out using in situ synchrotron X-ray diffraction (XRD) and Raman scattering with a diamond anvil cell up to 20 and 33 GPa, respectively. The Rietveld refinement based on the XRD pattern and analysis of Raman data indicate two first-order phase transitions from the ambient pressure I4 mm structure (α-NH3 BH3 ) to a high pressure Cmc21 phase (β-NH3 BH3 ) at 2.14 GPa, and further into a monoclinic P21 (Z = 2) phase (γ NH3 BH3 ) at 9.67 GPa. Fitting the measured volumetric compression data to the third order BirchMurnaghan equation of state reveals a bulk modulus of B0 = 9.9 ± 0.5 and 17.0 ± 3.0 GPa (with fixed B0 = 4) for the β-NH3 BH3 below and above 5 GPa, respectively. Still, with the splitting of the NBH rock mode in Raman experiment, it is concluded that a second-order isostructural phase transition occurs at 5 GPa. By analyzing the dihydrogen bonding framework, the origin of the isostructural phase transition is attributed to the number of dihydrogen bondings per molecule in the Cmc21 phase increasing from 12 to 14 at 5 GPa. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4884819] I. INTRODUCTION
In recent years, there has been a vast and growing interest in light hydrides due to their potential use as hydrogen storage materials.1, 2 As a promising candidate material for hydrogen storage, ammonia borane (NH3 BH3 ) has attracted significant interest. It has a high gravimetric (19.6 wt. %) and volumetric (145 gH/L) hydrogen density.3, 4 Hydrogen is irreversibly released from ammonia borane through thermolysis at moderate temperatures5–7 and acid- or transition metal-catalyzed8–11 dehydrogenation. While, thermal decomposition of ammonia borane proceeds in three steps and the release of all the hydrogen atoms is only accomplished at 500 ◦ C. At ambient conditions, ammonia borane crystallizes in a tetragonal space group I4mm (α-NH3 BH3 ) with a unit cell containing two molecules.12, 13 This α phase is highly disordered, as the BH3 and NH3 groups undergo a 3-fold rotation about the B–N bond, so that each H atom occupies three available equilibrium positions with equal probability. This α phase transforms to an orthorhombic structure when cooled to 225 K, resulting in a significant change in the lattice dynamics. The new orthorhombic structure was shown to have the space group Pmn21 (Z = 2) with the lattice parameters a = 5.517 Å, b = 4.742 Å, and c = 5.020 Å.14 Although temperature plays an important role in controlling the stability of materials, as an independent thermodynamic parameter, pressure can significantly alter the crystal structures and properties of materials. The effect of pressure on the behavior of NH3 BH3 expands the fundamental understanding of this system and sheds further light onto its potential application as a hydrogen-storage material. The a) Author to whom correspondence should be addressed. Electronic mail:
[email protected]. Tel./Fax: +86-431-85168825.
0021-9606/2014/140(24)/244507/9/$30.00
high-pressure behavior and properties of NH3 BH3 have been studied by both experimental and theoretical study.15–25 Although numerous investigations were made to examine high pressure behavior of NH3 BH3 , the results were still controversial. The earliest in situ Raman spectra study has found that ammonia borane may undergo several phase transitions under high pressure. And they have observed a disorder-order phase transition around 8 kbar.15 At very similar time, Trudel and Gilson reported two phase transitions at 5 and 14 kbar, respectively.24 In some of later studies, Lin et al. identified three phase transitions at approximately 2, 5, and 12 GPa on the basis of vibration bands splitting and changes of the pressure coefficient.16 Xie et al. then discovered several solid-tosolid transformations with phase boundaries roughly at 2.4, 5.5, 8.5, and 10.4 GPa, characterized by distinctive profiles and the pressure dependences of characteristic modes.20 Very recently, Kupenko and Dubrovinsky reported another transition at 27 GPa.25 They identified the phase transition at 2 GPa as a disorder-order phase transition from the α phase to the β phase by means of XRD and thermal conductivity measurements. However, there is no consensus about the behavior of NH3 BH3 under higher pressure. In Ref. 17 no other transitions were observed up to 12 GPa apart from the one at 2 GPa, while one more transition to the triclinic P1 structure was suggested around 8 GPa in Ref. 21 with the highest pressure of 27 GPa. In Ref. 22, a phase transition at 5 GPa was reported and no further transformations were detected upon compression up to 23 GPa. In Ref. 23, a second-order isostructural phase transition and a first-order transition were discovered around 5 GPa and 13 GPa, respectively. Besides, several previous theoretical calculations were done concerning high pressure phases of NH3 BH3 but again with no consensus.26, 27 In order to shed more light on the structural behavior of NH3 BH3 under high pressure and to verify the high
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© 2014 AIP Publishing LLC
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pressure structures reported in the literatures, we have performed synchrotron angle-dispersive XRD and Raman spectra experiments up to 30 GPa. Our experimental results confirm the pressure induced structural phase transition from I4mm (α-NH3 BH3 ) to orthorhombic Cmc21 structure (βNH3 BH3 ) at 2.14 GPa and we have identified the high pressure monoclinic P21 (Z = 2) phase (γ -NH3 BH3 ) at 9.67 GPa in experiment supplementary with the theoretical method. The structure of Cmc21 phase (β-NH3 BH3 ) was further optimized using density functional theoretical (DFT) calculations. The dihydrogen bond, which plays a significant role in determining the behavior of NH3 BH3 , is discussed extensively. For the first time, the high pressure crystal structures of NH3 BH3 have been identified by the joint of the experimental and theoretical results. II. EXPERIMENTAL AND THEORETICAL METHODS
Commercially available crystalline NH3 BH3 powder (>97% purity) was purchased from Sigma Aldrich. The purity was checked at ambient conditions by the Raman spectroscopy and XRD pattern. All observed Raman modes of the starting material were found to be associated with the NH3 BH3 , so it was used without further purification. A symmetric diamond anvil cell (DAC) with 400 μm diamond culets was used for these experiments. Finely ground samples, together with a ruby chip for pressure calibration, were loaded into the sample chamber created by drilling a 100 μm hole in a 50 μm thick stainless steel gasket. No pressure transmitting medium was used because pure NH3 BH3 is quite soft and provides good quasi-hydrostatic conditions. The Raman spectra were measured by an Acton SpctraPro 500i spectrograph with a liquid-nitrogen-cooled CCD detector (Princeton Instruments, 1340 × 100). A solidstate, diode-pumped, frequency-doubled Nd: vanadate laser (λ = 532 nm) was used as the excitation laser. Raman spectra were collected in a back scattering geometry with an 1800 gr/mm holographic grating, and the slit width was selected as 80 μm, corresponding to a resolution of ∼2 cm−1 . The sample image can be collected through an achromatic
lens and then focused onto a CCD detector for visual monitoring during experiments. The synchrotron ADXRD experiments were performed at the wiggler beamline X17C of the National Synchrotron Light Source in Brookhaven National Laboratory (BNL). And a MAR3450 image plate detector was used to record the diffracted X-rays. The wavelength was 0.4075 Å. Twodimensional patterns were radially integrated using the software FIT2D.28 All experiments were carried out at room temperature. The XRD patterns were fitted by Rietveld profile matching using the MATERIALS STUDIO program.29 During each refinement cycle, the fractional coordinates, scale factor, background parameter, isotropic thermal parameters, profile function, and cell parameters were optimized. Using the experimentally determined structures as input, we have performed DFT calculations to further optimize for experimentally obtained structures. Each structure was fully relaxed to an energy minimum at different pressure with the Vienna ab initio simulation package (VASP).30, 31 The allelectron projector augmented waves (PAW) method32 within the generalized gradient approximation (GGA) of Perdew, Burke, and Ernzerhof (PBE) was used.33 During the calculations, the uniform cutoff energy of 520 eV for the wave function expansion into plane waves and Monkhorst-Pack k points meshes were chosen to ensure that all the structures are converged to better than 1 meV/f.u. III. RESULTS AND DISCUSSION A. High pressure Raman spectroscopic measurements on NH3 BH3
The structural phase transformations of NH3 BH3 have been studied by using in situ Raman scattering at high pressures. It provides valuable information on local and cooperative changes during pressure-related transformation between phases. Raman vibrational spectra of NH3 BH3 were measured as a function of pressure up to 33 GPa. The Raman spectrum of the powder sample outside the DAC at ambient temperature is shown in Fig. 1 and listed in Table I. All active Raman
FIG. 1. Raman spectrum collected at ambient conditions.
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TABLE I. Assignments and vibrational frequencies (cm−1 ) of observed Raman modes of NH3 BH3 at ambient condition. Frequency(cm−1 ) This work 728 783 798 1069 1161 1187 1377 1597 2280 2373 3251 3316
Ref. 10
Ref. 11
Ref. 13
Ref. 26
Ref. 27
Assignment
Mode
726 782 798 1066 1158 1185 1375 1596 2280 2375 3255 3316
726 795 815 1066 1171 1192 1373 1600 2293 2397 3247 3318
728 783 798 1067 1161 1188 1377 1595 2280 2376 3253 3319
728 784 799 1067 1158 1189 n. r. 1596 2278 2375 3250 3314
738 803 817 1093 1164 1192 1376 1596 2291 2382 3251 3312
ν1 ν3 ν4 ν2 ν5 ν6 ν7 ν8 ν9 ν 10 ν 11 ν 12
NBH rock stretch 10 B–N stretch NBH rock BH3 deformation BH3 deformation NH3 deformation NH3 deformation Sym. B–H stretch Asym. B–H stretch Sym. N–H stretch Asym. N–H stretch
modes are measured in five distinct spectral regions. The two weak modes that are assigned as NBH rock ν 1 and ν 2 are observed at 728 and 1069 cm−1 , respectively. The B–N stretching manifests itself in two modes, 11 B–N, ν 3 , at 783 cm−1 and less intensive 10 B–N, ν 4 , at 798 cm−1 . Two BH3 deformation modes ν 5 and ν 6 center at 1161 cm−1 and 1187 cm−1 , respectively. Two NH3 deformation modes ν 7 is at 1377 cm−1 and ν 8 is at 1597 cm−1 . The two peaks at 2280 cm−1 and 2373 cm−1 correspond to the B–H stretching symmetric mode (ν 9 ) and asymmetric mode (ν 10 ), respectively. The N–H stretching vibrations are seen at 3251 cm−1 for symmetric ν 11 and at 3316 cm−1 for asymmetric ν 12 modes. In addition, a weak peak at 3173 cm−1 was observed. Earlier work left this peak unassigned,9, 10 while Hess et al.14 indicated that this mode may be the result of Fermi resonance of the overtone of the 1597 cm−1 and the N–H symmetric stretch fundamental, and disappeared around 6 GPa.
11 B–N
In our experiment, the Raman spectra were recorded from the sample during pressurizing up to 33 GPa at room temperature. The Raman spectra of NH3 BH3 and the Raman frequency shift as a function of pressure for the lattice vibration region (100–700 cm−1 ) are shown in Fig. 2. Dramatic spectral changes arising from phase transition are observed in the lattice region. As can be seen in Fig. 2(a), the lattice mode is not obvious at ambient pressure. Upon compression to 1.76 GPa, there are two clearly visible lattice modes: a high intensity lattice mode 1 at 176 cm−1 and a weak lattice mode 2 at 219 cm−1 . The sharp lattice modes strongly suggest a phase transition from the disordered tetragonal structure at ambient pressure to an ordered structure at about 2 GPa. Upon further compression to 8.43 GPa, a small peak begins to appear at 234 cm−1 (assigned as lattice mode 3) in the high frequency side of the lattice mode 1, indicating a possible beginning structural change in NH3 BH3 . This shoulder peak grows
FIG. 2. (a) Lattice vibration modes of solid NH3 BH3 measured to 33 GPa at 298 K and (b) pressure dependence of the corresponding Raman shift of NH3 BH3 on compression for the lattice mode.
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FIG. 3. Representative Raman spectra of NH3 BH3 of internal vibrational modes and the Raman shift versus pressure in the frequency range 600–3400 cm−1 .
rapidly with a slight increase in pressure to 33 GPa. The relative intensity of all the lattice modes increases with pressure until 18.17 GPa, and all of them move slowly to high frequency in the whole pressure range. Figure 3 shows representative Raman spectra of NH3 BH3 of internal vibrational modes and the Raman shift versus pressure in the frequency range 600–3400 cm−1 as the pressure increased up to 33 GPa. Upon compression from ambient pressure to 1.17 GPa, the Raman spectra change dramatically, indicating substantial changes in the crystal structures. For instance, the low frequency NBH rock mode (728 cm−1 ) splits
into a doublet very close each other, and exhibits blue shift up to 13.1 GPa, where this doublet cannot be resolved directly, and the high frequency mode (1069 cm−1 ) that looks like a broad weak peak at ambient pressure shows slight red-shift and appears as a part of a doublet at 1.17 GPa. Meanwhile, the two adjacent BH3 deformation modes at ambient pressure become shaper and more intense. In addition, the splitting of the asymmetric N–H stretching mode was observed at 1.17 GPa. The splitting as well as the significant narrowing of these modes indicates the onset of the structural phase transition. Up to 2.97 GPa, the high frequency BH3 deformation
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mode centered at 1187 cm−1 at ambient pressure split into two modes (1191 cm−1 and 1207 cm−1 ). The low frequency NH3 deformation (1377 cm−1 ) mode coalesces into the diamond first order peak, only the 1597 cm−1 mode is shown. The doubly degenerate mode ν 8 (NH3 deformation) split into two modes with similar intense. At ambient pressure, two dominant peaks at 2280 and 2373 cm−1 are observed in the B–H stretching region. The intermediate peak at 2328 cm−1 was assigned as the overtone of the symmetric B–H bending mode. It can be seen that the asymmetric B–H stretching mode (2373 cm−1 ) split at 2.97 GPa, and remained separated up to the highest pressure. The phase transition from the ambient phase to high pressure phase proceeds sluggishly, starting at 1.17 GPa and completing at 2.97 GPa. The transition pressure is hence determined to be about 2 GPa by taking the midpoint pressure. This result is consistent with our XRD results. Nevertheless, the splitting at B–N stretching mode is due to an isotopic effect rather than the removal of degeneracy. The 11 B and 10 B contributions to the B–N symmetric stretch are readily apparent up to 13.1 GPa. Both 11 B–N and 10 B–N stretching modes are sharp in the pressure region under 8.43 GPa, whereas 10 B–N stretching mode is much weaker than 11 B–N stretching mode because of the lower abundance of 10 B. Also, we did not observe the splitting of the 11 B–N stretching mode reported in Ref. 11. With further compression to 4.88 GPa, the high frequency NBH rock mode splits into a triplet, nevertheless, no remarkable changes are seen in other regions at this pressure, this simple change is likely due to a change in atomic bonding of the structure. With the pressure increasing, the high frequency BH3 deformation mode split into three peaks at 11.55 GPa, and remained split to the highest pressure. Together with changes in the lattice region, we can conclude that a new transition occurred at about 10 GPa. With continuous compression, there is no obvious change in the number of peaks in all five regions, in addition to weakening and broadening of Raman modes. This implies that the compound has become compact at this pressure range. The pressure dependences of Raman modes can also be seen in Fig. 3. In general, most Raman modes exhibit pressure-induced blue shift, consistent with the finding that the bonds stiffen upon compression. However, in contrast to the other internal Raman modes, both the symmetric and asymmetric N–H stretching mode shifted to low frequency with increasing pressure, indicating the weakening of N–H bond. This is to be believed a result of the lengthening of the N–H bond, owning to the formation and strengthening of dihydrogen bonding (N–H· · ·H–B) as pressure increasing.
J. Chem. Phys. 140, 244507 (2014)
FIG. 4. XRD patterns collected at various pressures for NH3 BH3 with incident wavelength λ = 0.4075 Å. The arrow in the diagram indicates changes in the peak shapes indicating a phase transition.
1. Structural determination of α and β phase at low pressure
Below 2.14 GPa, all diffraction patterns are attributed to the α phase with the lattice parameters a = 5.255 (8) Å and c = 5.055 (1) Å. This is in good agreement with the values determined from the previous study with a = 5.2630 (4) Å and c = 5.0504 (8) Å,13 as shown in Fig. 5(a). At 2.14 GPa the spectrum showed very different features from the lowpressure spectra. We observed that two strongest peaks around 2θ = 6.8◦ merged together. Several new diffraction lines emerged at the higher angles around 2θ = 7.29◦ , 7.78◦ , and 8.28◦ were indicative of a transformation to a high-pressure phase with lower symmetry. After the first transition, the
B. High pressure XRD on NH3 BH3
To further explore the crystal structure of NH3 BH3 , we have carried out in situ high pressure XRD experiment from ambient pressure to 20 GPa. XRD patterns collected at different pressures during compression are shown in Fig. 4. Over the whole pressure range, the sample remains transparent and pressure-induced metallization is so ruled out.
FIG. 5. Background-subtracted Rietveld refinement profile for the α phase at 0.26 GPa (a) and β phase at 2.14 GPa (b) and γ phase at 9.67 GPa (c).
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FIG. 6. Schematic crystal structures of polymorphs: (a) α-NH3 BH3 ; (b) β-NH3 BH3 .
diffraction patterns are identified as the β phase yields an orthorhombic unit cell dimensions with the lattice parameters a = 5.9970 Å, b = 6.392 (1) Å, and c = 5.9200 Å (Fig. 5(b)), in consistent with Filinchuk et al.’s observation.17 We have further performed a high-pressure Raman experiment that is also consistent with a transition at this pressure. Beyond 3 GPa, the high angle diffraction peaks become more compressible than other peaks, indicating anisotropic compression likely due to change of bonding. Schematic crystal structures of αNH3 BH3 and β-NH3 BH3 are shown in Fig. 6. The detailed structural information of both α and β phase after the refinement are listed in Table II. 2. Structural determination of the γ phase at high pressure
Up to 9.67 GPa, the appearance of a new peak is clearly visible in Fig. 4, indicating a phase transition. The new phase is persisted up to the highest pressure of 20 GPa reached in the experiment. In order to identify the crystal structure of the new phase, we have considered two experimental structural models21, 23 (P1 and P21 (Z = 4)) and one theoretical model27 (P21 (Z = 2)) to perform the Rietveld refinement. The data set collected at 9.67 GPa was used for structure determination. The observed XRD pattern was well refined with the proposed space group P21 containing two molecules in a unit cell (γ -NH3 BH3 ) as shown in Fig. 5(c), and the structure of γ -NH3 BH3 is shown in Fig. 7. Our result has the same space group P21 with Lin’s,23 but they are essential different. TABLE II. Experimental structural parameters for the α and β phase obtained from synchrotron powder diffraction data. Structure I4mm(Z = 2) a = 5.255(8) Å c = 5.055(1) Å (0.26 GPa and 300 K) Cmc21 (Z = 4) a = 5.9970 Å b = 6.392(1) Å c = 5.9200 Å (2.14 GPa and 300 K)
Atoms B N
B N H H H H
x 0 0
0 0 0.139(1) 0 0 0.165(4)
y 0 0
0.142(6) 0.211(1) 0.293(6) 0.076(2) 0.299(4) 0.039(6)
z 0.003(2) 0.686(9)
0.690(1) 0.933(9) 0.981(3) 1.028(4) 0.577(9) 0.653(8)
FIG. 7. Schematic crystal structures of γ -NH3 BH3 .
Lin’s structural model contains four molecules in a unit cell, with unit cell dimensions a = 7.713 (6), b = 5.375 (4), c = 3.898 (4) Å, β = 97.22 (8)◦ , and four NH3 BH3 molecules can be separated into two groups, which can be denoted as the N1B1 and N2B2 molecule, with N1 (0, 0,0), B1 (0.1429, 0.1254, 0.1178) and N2 (0.2724, 0.6999, 0.6509), B2 (0.3474, 0.4594, 0.6318). While our experimental result contains two NH3 BH3 molecules in a unit cell with unit cell dimensions a = 3.926 (9), b = 4.992 (2), c = 4.1960 Å, β = 92.216 (5)◦ , and two NH3 BH3 molecules are equivalent. Beyond this phase transition, no remarkable changes were observed in the XRD pattern up to 20 GPa except weakening and broadening of diffraction peaks due to pressurization. Fig. 5 shows the Rietveld refinements of the XRD patterns of the α phase at ambient pressure (a) and β phase at 2.14 GPa (b) and γ phase at 9.67 GPa (c). The Rietveld-refined structure information for the P21 model at 9.67 GPa are all listed in Table III. We then calculated the enthalpy difference curves (relative to the Cmc21 structure), which are presented in Fig. 8, it is obvious that the enthalpy of Lin’s structural model (P21 (Z = 4)) is much higher than our result (P21 (Z = 2)). The enthalpy versus pressure plot showed a transition from orthorhombic to monoclinic around 10 GPa which agreed well with our experiment. TABLE III. The structural parameters for the γ phase from RietveldRefined P21 model in NH3 BH3 at 9.67 GPa and room temperature. Structure P21 (Z = 2) a = 3.926(9) Å b = 4.992(2) Å c = 4.196(0) Å β = 92.216(5)◦ (9.67 GPa and 300 K)
Atoms
x
y
z
H H H H H H N B
0.917(4) 0.987(2) 0.645(7) 0.425(1) 0.834(1) 0.5140 0.807(6) 0.6480
0.474(1) 0.173(5) 0.3140 0.363(6) 0.133(8) − 0.020(7) 0.2920 0.186(1)
0.233(9) 0.118(1) 0.018(1) 0.581(1) 0.706(7) 0.426(1) 0.199(1) 0.503(1)
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nearly isotropic for the α phase, it is highly anisotropic for the β and γ phase, with the c-axis and a-axis parameter shrinking the most, respectively. The pressure volume data for three phases have been fitted to the third-order Birch-Murnaghan (BM) equation of state34 7 5 V −3 V −3 3B0 P = − 2 V0 V0 2 V −3 3 B −4 × 1+ −1 , 4 0 V0
FIG. 8. Enthalpy versus pressure plot for NH3 BH3 .
C. Pressure-induced structural evolution and the second-order isostructural phase transition at 5 GPa
In the disordered α phase, the NH3 BH3 molecules are collinear with the c axis. In the high-pressure β phase, the molecules become inclined to the c axis, the inclination of NH3 BH3 molecules with respect to the c axis becomes larger with increasing pressure, because the c axis is more likely to be compressed seen from Fig. 9(a). In the high pressure P21 phase, two NH3 BH3 molecules occupy one unit cell (Fig. 7). The geometric characteristics in the Cmc21 and P21 (Z = 2) phases are similar, and both of the two phases are the staggered conformation. The pressure dependence of the unit cell parameters and the volume of NH3 BH3 per unit cell are shown in Fig. 9. While the cell contraction under pressure is
where V0 is the volume per unit cell at ambient pressure, with V being the volume per unit cell at pressure P given in GPa, B0 is the isothermal bulk modulus, and B0 is the first pressure derivative of the bulk modulus. However, the results did not converge when fitting the high pressure data of Cmc21 phase with the third-order Birch-Murnaghan equation. Although the diffraction patterns in this pressure range have no characteristic change, together with our Raman spectroscopic study in part A, indicate a change around 5 GPa, so we fitted the data below and above 5 GPa, separately, with B0 being 4, the obtained bulk moduli for the phases are 9.9 ± 0.5 and 17.0 ± 3.0 GPa, respectively (shown in Fig. 9(b)). Although the change observed in our Raman spectroscopic studies at about 5 GPa does not result in any characteristic change in XRD, the different compressibility data associated with the phases below and above 5 GPa, much improved fitting statistics by separately considering the two regions, suggest that a secondorder isostructural phase transition occurs at 5 GPa. D. Dihydrogen bonding network in the Cmc21 phase
Our attempts for refining the XRD date for the highpressure phase were limited due to several factors such as
FIG. 9. (a) Variation in the unit cell dimensions as a function of pressure. (b) Variation in the cell volume as a function of pressure and the BM EOS fitting results for the indicated pressure range of the Cmc21 phase before and after 5 GPa (brown and green line).
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at ambient temperature, together with DFT calculations. Our experimental results confirm the pressure induced structural phase transition from I4mm (α-NH3 BH3 ) to orthorhombic Cmc21 structure (β-NH3 BH3 ) at 2.14 GPa. We have also identified the high pressure monoclinic P21 (Z = 2) phase (γ NH3 BH3 ) at 9.67 GPa in experiment supplementary with the theoretical method. Fitting the measured volumetric compression data to the third order Birch-Murnaghan equation of state reveals a bulk modulus of B0 = 9.9 ± 0.5 and 17.0 ± 3.0 GPa (B0 = 4) for the β phase below and above 5 GPa, respectively. Together with our Raman studies, we clarified the splitting of the NBH rock mode in Raman experiment as a second-order isostructural phase transition arising from the number of dihydrogen bondings per molecule in the Cmc21 phase increases from 12 to 14. ACKNOWLEDGMENTS
FIG. 10. Patterns of dihydrogen bonds in the Cmc21 phase. Red lines represent the dihydrogen bonds that already exist when the Cmc21 phase is formed at lower pressure. Green lines represent the dihydrogen bonds introduced at 5 GPa. The configuration is the same as that in Ref. 27.
peak broadening and weak intensity, so the theory analysis is needed to supplement for explaining the experimental results under pressure. The geometry of the strongly covalent NH3 BH3 molecule is nearly invariant in the investigated pressure range, so we focus on the intermolecular interactions-–dihydrogen bonding. A Cambridge structural database (CSD) structural search study established an empirical rule that the proper Hδ+ · · · δ− H interaction distance is in the range 1.7−2.2 Å.35 We counted the number of N−H · · · H−B close contacts in the Cmc21 phase, and found that the number of dihydrogen bondings per molecule in the simulated Cmc21 phase increases from 12 to 14 at about 5 GPa as can be seen in Fig. 10. At low pressure, one B–H bond is instead not involved in any H · · · H interaction, each of the other two B–H bonds is forming three H · · · H contacts, and the H atom in the NH3 end is still connected with two other H atoms. We suggest that at 5 GPa, the H atom on the mirror plane perpendicular to the a axis at the NH3 end connects to another B–H bond, making the total number of dihydrogen bonds per molecule 14. And we can conclude that the origin of the isostructural phase transition is attributed to the number of dihydrogen bondings per molecule in the Cmc21 phase increases from 12 to 14. IV. CONCLUSIONS
The phase transitions in NH3 BH3 were investigated by XRD up to 20 GPa and Raman measurement up to 33 GPa
The authors are grateful to Zhiqiang Chen, Xiaodong Li, and Yanchun Li for their help during the experimental researches. ADXRD experiments of this work were performed at X17C, BNL in USA and 4W2 HP-Station, BSRF assistance in the synchrotron measurement. This work was supported by the National Basic Research Program of China (No. 2011CB808200), Program for Changjiang Scholars and Innovative Research Team in University (No. IRT1132), National Natural Science Foundation of China (Nos. 51032001, 11074090, 10979001, 51025206, 11274137, and 11004074), and National Found for Fostering Talents of Basic Science (No. J1103202). 1 Z.
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