Dalton Transactions View Article Online
PAPER
Cite this: Dalton Trans., 2014, 43, 13370
View Journal | View Issue
Synthesis, structure and luminescence properties of new chloro-germanate phosphors Ca3GeO4Cl2:Eu2+ Xue Chen,a,b Zhiguo Xia*a and Quanlin Liua
Published on 09 July 2014. Downloaded on 16/10/2014 14:19:18.
A kind of novel blue-emitting chloro-germanate phosphor Ca3GeO4Cl2:Eu2+ has been synthesized via a high temperature solid-state method. The crystal structure of the as-prepared phosphor was discussed from the viewpoint of the doping behaviors of the activators. The luminescence properties and thermal stability of Ca3GeO4Cl2:Eu2+ was investigated in detail. Ca3GeO4Cl2:Eu2+ phosphors exhibit a broad-band excitation band in the near ultra-violet region and a blue emission peak at 428 nm, which are both ascribed to the 4f–5d transitions of the Eu2+ ions. The optimum concentration of Eu2+ in the Ca3GeO4Cl2 phosphor was determined to be 3 mol%, and the concentration quenching mechanism was considered to be the dipole–dipole interaction with a critical distance of Rc = 22.11 Å. Thermal stability studies show Received 2nd May 2014, Accepted 9th July 2014 DOI: 10.1039/c4dt01306a www.rsc.org/dalton
1.
that the photoluminescence intensity of the Ca3GeO4Cl2:Eu2+ phosphor at 150 °C was 77% of the initial value at room temperature. The activation energy, E, was calculated to be 0.163 eV suggesting good thermal stability. The variation in lifetime of Ca3GeO4Cl2:Eu2+ phosphor was also discussed to verify different luminescence centers in the lattice.
Introduction
Rare earth (RE) ions have been widely used as activators in the development of novel optical materials for different applications, such as illumination, display and laser communication. RE-doped phosphor materials possess high luminescence efficiency, an abundance of emission colors and stable light output quality, which has drawn much attention in advanced optical materials.1 Among them, Eu2+ is a kind of important activator ion in many kinds of commercial phosphors since the intrinsic 5d→4f transition have intense and broad emission and excitation bands.2,3 It is also known that the emission of Eu2+ is strongly dependent on the crystal fields of the host lattices because 5d orbitals in the outer space are more sensitive to the ligand field.3 The emission color can vary in the broad visible light region ranging from ultraviolet to red. Consequently, Eu2+ becomes a very useful activator for applications in displays, lamps and luminescent paintings.4 Therefore, it is important for the design of a new phosphor in some suitable compounds with a specific crystal field to accommodate Eu2+ ions.
a School of Materials Sciences and Engineering, University of Science and Technology Beijing, Beijing 100083, China. E-mail: [email protected]; Fax: +86-10-8237-7955; Tel: +86-10-8237-7955 b School of Materials Sciences and Technology, China University of Geosciences, Beijing 100083, China
13370 | Dalton Trans., 2014, 43, 13370–13376
Recently, we have made great efforts to explore novel halocontaining phosphors, including the exploration and crystal chemistry study of new inorganic phases, new phosphor systems with multicolor emissions, as well as spectral and chemical stability modifications.5–8 Halo-containing phosphors, including halophosphate, haloborate, halosilicate, and haloaluminate, have received remarkable attention because they possess plenty of structural types and adjustable cation coordination environments, low synthesis temperatures, and excellent chemical and physical stability.5–11 Particularly, halocontaining phosphors exhibit promising optical properties by doping with Eu2+ or Ce3+.9–11 In previous studies, two groups reported the calcium chlorosilicate Ca3SiO4Cl2 phosphors, and excellent photoluminescence properties can be realized in such a system and green and orange-emitting Ca3SiO4Cl2:Eu2+ and color-tunable Ca3SiO4Cl2:Eu2+,Mn2+ phosphors have been developed.12,13 The crystal structure of the Ca3SiO4Cl2 was first reported by Golovastikov in 1970.10 Ca3SiO4Cl2 has a monoclinic crystal structure with the space group of P21/c, which is composed of alternative layers of calcium chloride and dicalcium silicate. Since Si and Ge elements belong to the same group, Si/Ge substitution can form the possible solid-solution phases. Therefore, we believe that Ca3GeO4Cl2 may be also a kind of potential phosphor host for Eu2+ doping, and some controlled luminescence can be realized in such a phosphor system. However, the crystal structure of Ca3GeO4Cl2 was previously reported by Redhammer et al.14 Note that the Ca3GeO4Cl2 compound crystallizes in the orthorhombic space
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 09 July 2014. Downloaded on 16/10/2014 14:19:18.
Dalton Transactions
group Pnma with Z = 4 (single crystal XRD), which is different with the Ca3SiO4Cl2 phase. The structure contains two crystallographically independent Ca atoms. Ca(1)O4Cl2 octahedra are connected via edges and corners to from a three-dimensional network. Interstitial sites of this framework are filled by Ca(2)O4Cl2 trigonal prisms and GeO4 tetrahedra. Although there is no direct relationship between the two phases of Ca3SiO4Cl2 and Ca3GeO4Cl2 phase, the intrinsic photoluminescence properties and rare earth ions doped luminescence behavior have not yet been reported in the Ca3GeO4Cl2 system as far as we know. In this paper, a series of novel blueemitting Ca3GeO4Cl2:Eu2+ phosphors have been developed by using a solid-state reaction technique. The relationship between the crystal structure and the luminescence properties of Eu2+ in this host, as well as luminescence properties of Ca3GeO4Cl2: Eu2+ phosphor in this host, are investigated in detail.
2. Experimental 2.1.
Materials and synthesis
Ca3GeO4Cl2 host powder and Ca3−xGeO4Cl2:xEu2+ phosphors were synthesized by the conventional high temperature solidstate reaction method. The starting materials, i.e. CaCO3 (A.R.), GeO2 (A.R.), CaCl2 (A.R.) and Eu2O3 (99.99%), were used. Eu2+ is considered to enter the Ca2+ sites because of the charge and ionic radius. First, starting materials with stoichiometric amounts were ground and mixed in an agate mortar. Then, the mixture was placed in a small covered corundum crucible, then transferred into a large corundum crucible, and finally sintered in a muffle furnace at 800 °C for 6 hours. The space between the two crucibles provided a carbon monoxide reducing atmosphere to promote the formation of Eu2+. After this, the samples were cooled to room temperature and crushed into a fine powder. 2.2.
Characterization methods
The phase structure of as-prepared samples was carried out on a Shimadzu model XRD-6000 diffractometer using Cu Kα radiation (λ = 0.15406 nm) at 40 kV and 30 mA. Powder diffraction data were obtained using the general structure analysis system (GSAS) software program. Diffuse reflectance spectra of samples were measured with a UV-Vis-NIR spectrophotometer (Shimadzu UV-3600) equipped with an integrating sphere. The face of the integrating sphere was coated with BaSO4, which was used as a reference standard. The excitation and emission spectra were carried out using a JOBIN YVON FluoroMax-3 fluorescence spectrophotometer equipped with a photomultiplier tube operating at 400 V, and a 150 W xenon lamp was used as the excitation lamp. The temperature-dependent luminescence properties were measured on the same spectrophotometer, which was combined with a home-made heating cell and a computer-controlled electric furnace. The decay curves of Eu2+ lifetime values at different emission wavelengths were measured by a FLSP920 fluorescence spectrometer (Edinburgh Instruments) equipped with an nF900 nanosecond flash lamp.
This journal is © The Royal Society of Chemistry 2014
Paper
3. Results and discussion 3.1.
Crystal structure analysis
Ca3GeO4Cl2:Eu2+ powders were prepared by a high temperature solid-state reaction method. The crystallinity and purity of the series of as-prepared Ca3−xGeO4Cl2:xEu2+ (x = 0.01, 0.05, 0.08, 0.10, and 0.15) compounds were checked by X-ray powder diffraction. Fig. 1 shows the typical powder XRD patterns of the selected Ca3−xGeO4Cl2:xEu2+ (x = 0.01 and 0.08) samples. As shown in Fig. 1, the diffraction patterns of the synthesized samples are all indexed to the standard pattern of inorganic crystal structure database (ICSD 249329), and no other phase was observed. Therefore, it indicates that the single-phase Ca3GeO4Cl2 compound can be received by the established synthesis method, and the doped Eu2+ ions did not generate any impurity or induce significant changes in the host structure. In order to obtain a better understanding of the phase purity and the occupancy of Eu2+ ions on Ca2+ sites in Ca2.99GeO4Cl2:0.01Eu2+, the Rietveld structural refinement was performed by using the GSAS program as shown in Fig. 2. Ca3GeO4Cl2 served as an initial structural model. The final refined unit cell parameters and residual factors are summarized in Table 1. The Rietveld analysis results indicated the weighted profile R-factor (Rwp) and the expected R factor (Rp) are 7.90% and 5.83%, respectively, for Ca3GeO4Cl2, which indicates that the results on phase analysis and the occupancy of Eu2+ ions are stable. Furthermore, Fig. 3(a) represents the unit cell of Ca3GeO4Cl2 phosphor with an orthorhombic structure, which consists of one distinct Ge site on the special position 4c, site symmetry m, three different O sites, one distinct Cl position and two different Ca sites, Ca1 and Ca2 (one on the general position 8d, site symmetry 1, and the other on the special position 4c). As is mentioned above and also given in Fig. 3(b), there are two calcium atoms showing the same six-fold coordination. Both Ca1 and Ca2 have four oxygen and two chlorine neighbors; however, the bond length and the
Fig. 1 XRD patterns of as-prepared Ca3GeO4Cl2:0.01Eu2+, Ca3GeO4Cl2: 0.08Eu2+ phosphors and ICSD card (249329) of Ca3GeO4Cl2 compound is also given for comparison.
Dalton Trans., 2014, 43, 13370–13376 | 13371
View Article Online
Paper
Dalton Transactions
3.2.
Luminescence properties
The diffuse reflectance spectra of Ca3−xGeO4Cl2:xEu2+ (x = 0, 0.03, 0.05, 0.08) phosphors are displayed in Fig. 4. An absorption band at about 221 nm was observed for the virgin sample, confirming that light having this particular wavelength was absorbed by this phosphor. The Kubelka–Munk absorption coefficient (K/S) relation was used to calculate the absorption edge from the measured reflectance (R):15,16
Published on 09 July 2014. Downloaded on 16/10/2014 14:19:18.
K ð1 RÞ2 ¼ S 2R
Fig. 2 Powder XRD patterns for Rietveld structure analysis of the selected Ca2.99GeO4Cl2:0.01Eu2+ phosphor.
Table 1 Main parameters of processing and refinement of the Ca2.99GeO4Cl2:0.01Eu2+ sample
Compound
CaMnSi2O6:0.03Eu
Sp. Gr. a/Å b/Å c/Å α/° β/° γ/° V/Å3 2θ-interval/° Rwp/% Rp/% χ2
Pnma(62) 11.687 10.333 5.701 90 90 90 688.47 10–120° 7.90 5.38 3.925
ð1Þ
where K represents the absorption coefficient, S represents the scattering coefficient, and R represents the reflectivity. As given in the inset of Fig. 4, the band gap energy of the Ca3GeO4Cl2 host is calculated to be approximately 3.08 eV by extrapolation. As is also shown in Fig. 4, Eu2+-doped phosphors possess a strong broad absorption in the 300–450 nm n-UV range, which is consistent with the excitation spectrum. The intensities for the absorption band also increase with increasing Eu2+ concentration. The photoluminescence excitation (λem = 428 nm) and emission (λex = 273 nm) spectra of Ca3GeO4Cl2:0.03Eu2+ are presented in Fig. 5. It is obvious that the excitation spectrum monitored at 428 nm, ranging from 200 to 415 nm, exhibits one strong absorption peak located around 263 nm and two weak absorption peaks at about 327 and 359 nm, respectively. All the peaks correspond with the results of the diffuse reflection spectra, which is attributed to the transition from the ground state 4f 7 to the crystal-field split 4f 65d configuration of the doped Eu2+ ions. On the other hand, as is shown in the emission spectrum of Ca2.97GeO4Cl2:0.03Eu2+ under excitation at 365 nm, a blue emission with a typical asymmetrical band extends from 375 to 550 nm with a peak at 428 nm can be seen. Furthermore, the asymmetric emission spectrum indicate that Eu2+ ions occupy more than one site in the lattice, which can be deconvoluted into two Gaussian components
Fig. 3 (a) Crystal structure of Ca3GeO4Cl2 compound, (b) coordination spheres of the two different Ca2+ sites in Ca3GeO4Cl2.
coordination environment are different.14 Therefore, it is believed that the two different Ca2+ sites will possibly produce two different cation environments for the doped ions.
13372 | Dalton Trans., 2014, 43, 13370–13376
Fig. 4 The diffuse reflectance spectra of Ca3GeO4Cl2:xEu2+ (x = 0, 0.03, 0.05, 0.08) phosphors, and the inset shows the extrapolation of the band gap energy for the host.
This journal is © The Royal Society of Chemistry 2014
View Article Online
Dalton Transactions
Paper
Published on 09 July 2014. Downloaded on 16/10/2014 14:19:18.
is increased. Furthermore, Fig. 6 also demonstrates the dependence of the emission intensity on the concentration of Eu2+. The emission intensity of Eu2+ first increased and reached the maximum at a Eu2+ concentration of 0.03, and then the emission intensity decreased with further increase in the concentration. It can be concluded that the optimum doping concentration of Eu2+ is 0.03. After reaching this critical value, the emission intensity decreased sharply as a result of the concentration quenching effect. It is considered that concentration quenching is mainly caused by energy transfer among Eu2+ ions and the probability of this occurring increases as the concentration of Eu2+ increases.18 In this sense, the crystal field strength as Dq is inversely proportional to the fifth power of the bond-length R.19,20 Dq ¼ Fig. 5 Luminescence excitation and PL spectra of the Ca3GeO4Cl2: 0.03Eu2+ phosphor.
that peaked at 427 nm and 451 nm due to the two different Ca2+ sites, as shown in Fig. 5. In the Ca3GeO4Cl2 structure, two calcium sites are available for activator cations and this structural feature was well matched with the PL emission spectrum of Ca, of which there were two emission centers for Eu2+.17 The PLE (λem = 428 nm) spectrum of the selected Ca2.97GeO4Cl2:0.03Eu2+ and the emission spectra of the Ca3−xGeO4Cl2:xEu2+ phosphors under excitation at 365 nm with different concentrations of Eu2+ (ranging from 0.01 to 0.15) at room temperature are depicted in Fig. 6. The PL spectra have similar spectral profiles, except for the emission intensities for this series of Ca3−xGeO4Cl2:xEu2+ phosphors. It is observed that the emission intensity of Eu2+ ion first increases and reaches a maximum at x = 0.03, and the PL spectra are gradually red-shifted as the concentration of Eu2+
ze2 r 4 6R5
Here Dq is the measurement of the crystal field strength, z is the charge or valence of the anion, R is the bond distance between the central ion and its ligands, e is the charge of an electron, and r is the radius of the d wave function. From this equation can see that a shorter bond distance state stronger crystal field strength clearly when the crystal environments are homologous. It is known that the emission of Eu2+ is very strongly dependent on the host lattice.21 When larger Eu2+ ion substitute the Ca2+ ion, the distance between Eu2+ and O2− becomes shorter, the crystal field strength increases, and finally with increasing Eu2+ concentration the emission wavelength is red-shifted. There are two main aspects responsible for the resonant energy-transfer mechanism: one is exchange interaction and the other is multipolar interaction.22 According to Dexter’s theory, non-radiative transitions between Eu2+ ions takes place via electric multipolar interactions. In order to further confirm the process of energy transfer between sensitizers or between a sensitizer and activator, the interaction type can be calculated by eqn (3):23 I=x ¼ k½1 þ βðxÞθ=3 1
Fig. 6 PLE spectra of Ca3GeO4Cl2:0.03Eu2+ and the Eu2+ concentration-dependent PL spectra of Ca3GeO4Cl2 phosphors.
This journal is © The Royal Society of Chemistry 2014
ð2Þ
ð3Þ
In this equation, x is the activator concentration, I/x is the emission intensity (I) per activator concentration (x), k and β are constants for the same excitation condition for a given host crystal, and θ is a function of the multipole–multipole interaction. θ = 6, 8 and 10 corresponds to dipole–dipole (d–d), dipole–quadrupole (d–q), and quadrupole–quadrupole (q–q) interactions, respectively. In order to get a correct θ value, Fig. 7 illustrate the I/x dependence on x on a logarithmic scale. Obviously, an approximate linear relation between lg(I/cEu) and lg(cEu) can be found and the slopes of the straight line is −θ/3 = −1.594, the value of θ can be calculated as 4.782, which is close to 6, indicating that the quenching is through dipole–dipole interactions in the present Ca3GeO4Cl2:Eu2+ phosphors. The concentration quenching of the luminescence is owing to the energy transfer from one activator to another until all
Dalton Trans., 2014, 43, 13370–13376 | 13373
View Article Online
Published on 09 July 2014. Downloaded on 16/10/2014 14:19:18.
Paper
Dalton Transactions
Fig. 7 The relationships of lg(x) versus lg(I/x) in Ca3GeO4Cl2:xEu2+ phosphors beyond the quenching concentration.
the energy is consumed. Therefore, it is necessary to obtain the critical distance (Rc), i.e., the critical separation between donor (activators) and acceptors (quenching site). The critical distance Rc for concentration quenching Rc can be estimated according to the following equation:24 Rc 2
3V 4πxc N
1=3 ð4Þ
where xc is the atom fraction of activator at which the quenching occurs, N is the number of the Ca2+ ions in the unit cell and V is the volume of the unit cell. Using the crystallographic parameters V = 688.47 Å3, xc is 0.03, and N = 4, the value of the critical distance Rc = 22.11 Å is obtained. Because both donor and acceptor transitions are electric dipole allowed, such a long distance (typical critical distances are 5 Å) indicates that multipolar interactions are dominant, and a dipole–dipole mechanism appears to be most probable. Fig. 8 shows the temperature dependent emission spectra for the Ca2.97GeO4Cl2:0.03Eu2+ phosphor under excitation at 365 nm, and the relative emission intensities as a function of temperature are given in the inset Fig. 8. As can be seen, the relative PL intensity slowly decreases with increasing temperature. When the temperature was raised up to 150 °C, the PL intensity of the sample drops to 77% of the initial value at room temperature. Thermal quenching can be used to explain this phenomenon. The excited luminescent center is thermally activated by the phonon interaction, and then thermally released via the crossing point between the ground and excited states. In order to better understand the thermal quenching behavior, the Arrhenius equation was used to calculate the activation energy as follows:25 IðTÞ
I0
Fig. 8 The PL spectra (λex = 365 nm) of Ca3GeO4Cl2:0.03Eu2+ phosphor under different temperatures in the range of 30–300 °C. The inset shows the relative emission intensities as a function of temperature.
E 1 þ c exp kT
13374 | Dalton Trans., 2014, 43, 13370–13376
ð5Þ
Fig. 9 The activation energy of the thermal quenching of Ca3GeO4Cl2: 0.03Eu2+ phosphor.
where I0 is the initial emission intensity of the phosphor at 30 °C, I(T ) is the intensity at different temperatures, c is a constant, k is the Boltzmann constant (8.62 × 10−5 eV) for the same host, and E is the activation energy for thermal quenching. Fig. 9 presents the best-fit line of the emission thermal quenching model plotted as ln(I0/I) − 1 against 1/kT for the present Ca3GeO4Cl2:0.03Eu2+ phosphor. According to eqn (5), the value of the activation energy, E, of the as-prepared phosphor is calculated to be 0.163 eV, which is similar to another report on the Ca3SiO4Cl2:Eu2+ phosphor.26 The crystal structure and photoluminescence emission spectra analysis verified that there are two different Ca2+ sites, which will induce corresponding emission centers in the Ca3GeO4Cl2:Eu2+ phosphors. In order to further distinguish the two different emission centers, the lifetime for the two emission centers (427 and 451 nm) under excitation at 364 nm
This journal is © The Royal Society of Chemistry 2014
View Article Online
Dalton Transactions
Paper
Acknowledgements The present work was supported by the National Natural Science Foundation of China (grant no. 51002146, 51272242), the Natural Science Foundation of Beijing (2132050), the Program for New Century Excellent Talents in University of Ministry of Education of China (NCET-12-0950), the Beijing Nova Program (Z131103000413047), Beijing Youth Excellent Talent Program (YETP0635), the Funds of the State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University (KF201306) and the Fundamental Research Funds for the Central Universities (FRF-2014).
Published on 09 July 2014. Downloaded on 16/10/2014 14:19:18.
References Fig. 10 Decay curves of Eu2+ emission at different wavelength of 427 and 451 nm in Ca3GeO4Cl2:0.03Eu2+ phosphor under excitation at 364 nm.
was recorded, and is shown in Fig. 10. It was found that the two curves had similar decay behaviors, and both of them can be well fitted by the following single exponential function, eqn (6),27 IðtÞ ¼ A1 expðt=τÞ
ð6Þ
where I and A1 correspond to the luminescence intensity at time t and the initial intensity immediately after the exciting pulse, τ is the lifetime. On the basis of (6), the lifetime values at 427 and 451 nm were determined to be 0.316 and 0.428 μs, respectively. The differences of the lifetimes verified the two different emission centers, however, the minor difference also verified similar crystal field strength effect for the two Ca2+ sites. Therefore, it can be also inferred that there are two Eu2+ emission centres in the present host.
4.
Conclusions
In summary, a series of novel, single-phase, blue-emitting phosphors Ca3GeO4Cl2:Eu2+ were successfully synthesized and studied. The Ca3GeO4Cl2 host has an orthorhombic space group and two different Ca2+ sites. Ca3GeO4Cl2:Eu2+ shows obvious absorption peaks located around 263, 327 and 359 nm. Under excitation at 365 nm, the phosphor exhibited a blue emission band peak at 428 nm due to the 4f–5d transitions of Eu2+. The critical quenching concentration of Eu2+ was about 3 mol%. The value of the critical distance is 22.11 Å, and the corresponding concentration quenching mechanism is considered to be the dipole–dipole interaction. The temperature dependent luminescence behaviors have been also discussed. Different lifetime values of Eu2+ in Ca3GeO4Cl2:Eu2+ also confirmed the two different emission centers in the lattice.
This journal is © The Royal Society of Chemistry 2014
1 Y. Q. Li, A. C. A. Delsing, G. de With and H. T. Hintzen, Chem. Mater., 2005, 17, 3242. 2 B. L. Yuan, X. G. Wang, T. J. Tsuboi, Y. L. Huang and H. J. Seo, J. Alloys Compd., 2012, 512, 144. 3 B. L. Yuan, Y. L. Huang, Y. M. Yu, S. I. Kim and H. J. Seo, Mater. Lett., 2012, 70, 57. 4 R. Stefani, A. D. Maia, E. E. S. Teotonio, M. A. F. Monteiro, M. C. F. C. Felinto and H. F. Brito, J. Solid State Chem., 2006, 179, 1086. 5 Z. G. Xia, J. Q. Zhuang, A. Meijerink and X. P. Jing, Dalton Trans., 2013, 42, 6327. 6 Z. G. Xia, M. S. Molokeev, A. S. Oreshonkov, V. V. Atuchin, R. S. Liu and C. Dong, Phys. Chem. Chem. Phys., 2014, 16, 5952. 7 Z. G. Xia, J. Q. Zhuang and L. B. Liao, Inorg. Chem., 2012, 51, 7202. 8 Z. G. Xia, R. S. Liu, K. W. Huang and V. Drozd, J. Mater. Chem., 2012, 22, 15183. 9 W. Lü, Z. D. Hao, X. Zhang, X. J. Wang and J. H. Zhang, Opt. Mater., 2011, 34, 261. 10 E. N. Treushnikov, V. V. Ilyukhin and N. V. Belov, Dokl. Akad. Nauk SSSR, 1970, 193, 1048. 11 N. Karkada, D. Porob, P. Kumar and A. A. Setlur, ECS Trans., 2011, 3333, 39. 12 W. J. Ding, J. Wang, Z. M. Liu, M. Zhang, Q. Su and J. K. Tang, J. Electrochem. Soc., 2008, 155, J122. 13 J. Liu, H. Z. Lian, J. Y. Sun and C. S. Shi, Chem. Lett., 2005, 34, 1340. 14 G. J. Redhammer, G. Roth and G. Amthauer, Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 2007, 63, i69. 15 N. Yamashita, J. Phys. Soc. Jpn., 1973, 35, 1089. 16 C. H. Huang and T. M. Chen, Inorg. Chem., 2011, 5012, 5725. 17 H. S. Roh, S. Hur, H. J. Song, I. J. Park, D. K. Yim, D. W. Kim and K. S. Hong, Mater. Lett., 2012, 70, 37. 18 Y. C. Chiu, C. H. Huang, T. J. Lee, W. R. Liu, Y. T. Yeh, S. M. Jang and R. S. Liu, Opt. Express, 2011, 19, 331–339. 19 L. Chen, C. C. Lin, C. W. Yeh and R. S. Liu, Materials., 2010, 3, 2172. 20 W. B. Im, Y. I. Kim, N. N. Fellows, H. Masui, G. A. Hirata, S. P. DenBaars and R. Seshadri, Appl. Phys. Lett., 2008, 93, 091905.
Dalton Trans., 2014, 43, 13370–13376 | 13375
View Article Online
Paper
25 E. Pavitra and J. S. Yu, Ceram. Int., 2013, 39, 1029. 26 I. Baginskiy and R. S. Liu, J. Electrochem. Soc., 2009, 156, G29. 27 J. Zhou, Z. G. Xia, M. X. Yang and K. Shen, J. Mater. Chem., 2012, 22, 21935.
Published on 09 July 2014. Downloaded on 16/10/2014 14:19:18.
21 G. Blasse and B. C. Grabmaier, Luminescent Materials, Springer-Verlag, Berlin, 1994. 22 C. H. Huang and T. M. Chen, J. Phys. Chem. C, 2011, 115, 2349. 23 G. Blasse, Phys. Lett., 1968, 28, 444. 24 G. Blasse, Philips Res. Rep., 1969, 24, 131.
Dalton Transactions
13376 | Dalton Trans., 2014, 43, 13370–13376
This journal is © The Royal Society of Chemistry 2014
PAPER
Cite this: Dalton Trans., 2014, 43, 13370
View Journal | View Issue
Synthesis, structure and luminescence properties of new chloro-germanate phosphors Ca3GeO4Cl2:Eu2+ Xue Chen,a,b Zhiguo Xia*a and Quanlin Liua
Published on 09 July 2014. Downloaded on 16/10/2014 14:19:18.
A kind of novel blue-emitting chloro-germanate phosphor Ca3GeO4Cl2:Eu2+ has been synthesized via a high temperature solid-state method. The crystal structure of the as-prepared phosphor was discussed from the viewpoint of the doping behaviors of the activators. The luminescence properties and thermal stability of Ca3GeO4Cl2:Eu2+ was investigated in detail. Ca3GeO4Cl2:Eu2+ phosphors exhibit a broad-band excitation band in the near ultra-violet region and a blue emission peak at 428 nm, which are both ascribed to the 4f–5d transitions of the Eu2+ ions. The optimum concentration of Eu2+ in the Ca3GeO4Cl2 phosphor was determined to be 3 mol%, and the concentration quenching mechanism was considered to be the dipole–dipole interaction with a critical distance of Rc = 22.11 Å. Thermal stability studies show Received 2nd May 2014, Accepted 9th July 2014 DOI: 10.1039/c4dt01306a www.rsc.org/dalton
1.
that the photoluminescence intensity of the Ca3GeO4Cl2:Eu2+ phosphor at 150 °C was 77% of the initial value at room temperature. The activation energy, E, was calculated to be 0.163 eV suggesting good thermal stability. The variation in lifetime of Ca3GeO4Cl2:Eu2+ phosphor was also discussed to verify different luminescence centers in the lattice.
Introduction
Rare earth (RE) ions have been widely used as activators in the development of novel optical materials for different applications, such as illumination, display and laser communication. RE-doped phosphor materials possess high luminescence efficiency, an abundance of emission colors and stable light output quality, which has drawn much attention in advanced optical materials.1 Among them, Eu2+ is a kind of important activator ion in many kinds of commercial phosphors since the intrinsic 5d→4f transition have intense and broad emission and excitation bands.2,3 It is also known that the emission of Eu2+ is strongly dependent on the crystal fields of the host lattices because 5d orbitals in the outer space are more sensitive to the ligand field.3 The emission color can vary in the broad visible light region ranging from ultraviolet to red. Consequently, Eu2+ becomes a very useful activator for applications in displays, lamps and luminescent paintings.4 Therefore, it is important for the design of a new phosphor in some suitable compounds with a specific crystal field to accommodate Eu2+ ions.
a School of Materials Sciences and Engineering, University of Science and Technology Beijing, Beijing 100083, China. E-mail: [email protected]; Fax: +86-10-8237-7955; Tel: +86-10-8237-7955 b School of Materials Sciences and Technology, China University of Geosciences, Beijing 100083, China
13370 | Dalton Trans., 2014, 43, 13370–13376
Recently, we have made great efforts to explore novel halocontaining phosphors, including the exploration and crystal chemistry study of new inorganic phases, new phosphor systems with multicolor emissions, as well as spectral and chemical stability modifications.5–8 Halo-containing phosphors, including halophosphate, haloborate, halosilicate, and haloaluminate, have received remarkable attention because they possess plenty of structural types and adjustable cation coordination environments, low synthesis temperatures, and excellent chemical and physical stability.5–11 Particularly, halocontaining phosphors exhibit promising optical properties by doping with Eu2+ or Ce3+.9–11 In previous studies, two groups reported the calcium chlorosilicate Ca3SiO4Cl2 phosphors, and excellent photoluminescence properties can be realized in such a system and green and orange-emitting Ca3SiO4Cl2:Eu2+ and color-tunable Ca3SiO4Cl2:Eu2+,Mn2+ phosphors have been developed.12,13 The crystal structure of the Ca3SiO4Cl2 was first reported by Golovastikov in 1970.10 Ca3SiO4Cl2 has a monoclinic crystal structure with the space group of P21/c, which is composed of alternative layers of calcium chloride and dicalcium silicate. Since Si and Ge elements belong to the same group, Si/Ge substitution can form the possible solid-solution phases. Therefore, we believe that Ca3GeO4Cl2 may be also a kind of potential phosphor host for Eu2+ doping, and some controlled luminescence can be realized in such a phosphor system. However, the crystal structure of Ca3GeO4Cl2 was previously reported by Redhammer et al.14 Note that the Ca3GeO4Cl2 compound crystallizes in the orthorhombic space
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 09 July 2014. Downloaded on 16/10/2014 14:19:18.
Dalton Transactions
group Pnma with Z = 4 (single crystal XRD), which is different with the Ca3SiO4Cl2 phase. The structure contains two crystallographically independent Ca atoms. Ca(1)O4Cl2 octahedra are connected via edges and corners to from a three-dimensional network. Interstitial sites of this framework are filled by Ca(2)O4Cl2 trigonal prisms and GeO4 tetrahedra. Although there is no direct relationship between the two phases of Ca3SiO4Cl2 and Ca3GeO4Cl2 phase, the intrinsic photoluminescence properties and rare earth ions doped luminescence behavior have not yet been reported in the Ca3GeO4Cl2 system as far as we know. In this paper, a series of novel blueemitting Ca3GeO4Cl2:Eu2+ phosphors have been developed by using a solid-state reaction technique. The relationship between the crystal structure and the luminescence properties of Eu2+ in this host, as well as luminescence properties of Ca3GeO4Cl2: Eu2+ phosphor in this host, are investigated in detail.
2. Experimental 2.1.
Materials and synthesis
Ca3GeO4Cl2 host powder and Ca3−xGeO4Cl2:xEu2+ phosphors were synthesized by the conventional high temperature solidstate reaction method. The starting materials, i.e. CaCO3 (A.R.), GeO2 (A.R.), CaCl2 (A.R.) and Eu2O3 (99.99%), were used. Eu2+ is considered to enter the Ca2+ sites because of the charge and ionic radius. First, starting materials with stoichiometric amounts were ground and mixed in an agate mortar. Then, the mixture was placed in a small covered corundum crucible, then transferred into a large corundum crucible, and finally sintered in a muffle furnace at 800 °C for 6 hours. The space between the two crucibles provided a carbon monoxide reducing atmosphere to promote the formation of Eu2+. After this, the samples were cooled to room temperature and crushed into a fine powder. 2.2.
Characterization methods
The phase structure of as-prepared samples was carried out on a Shimadzu model XRD-6000 diffractometer using Cu Kα radiation (λ = 0.15406 nm) at 40 kV and 30 mA. Powder diffraction data were obtained using the general structure analysis system (GSAS) software program. Diffuse reflectance spectra of samples were measured with a UV-Vis-NIR spectrophotometer (Shimadzu UV-3600) equipped with an integrating sphere. The face of the integrating sphere was coated with BaSO4, which was used as a reference standard. The excitation and emission spectra were carried out using a JOBIN YVON FluoroMax-3 fluorescence spectrophotometer equipped with a photomultiplier tube operating at 400 V, and a 150 W xenon lamp was used as the excitation lamp. The temperature-dependent luminescence properties were measured on the same spectrophotometer, which was combined with a home-made heating cell and a computer-controlled electric furnace. The decay curves of Eu2+ lifetime values at different emission wavelengths were measured by a FLSP920 fluorescence spectrometer (Edinburgh Instruments) equipped with an nF900 nanosecond flash lamp.
This journal is © The Royal Society of Chemistry 2014
Paper
3. Results and discussion 3.1.
Crystal structure analysis
Ca3GeO4Cl2:Eu2+ powders were prepared by a high temperature solid-state reaction method. The crystallinity and purity of the series of as-prepared Ca3−xGeO4Cl2:xEu2+ (x = 0.01, 0.05, 0.08, 0.10, and 0.15) compounds were checked by X-ray powder diffraction. Fig. 1 shows the typical powder XRD patterns of the selected Ca3−xGeO4Cl2:xEu2+ (x = 0.01 and 0.08) samples. As shown in Fig. 1, the diffraction patterns of the synthesized samples are all indexed to the standard pattern of inorganic crystal structure database (ICSD 249329), and no other phase was observed. Therefore, it indicates that the single-phase Ca3GeO4Cl2 compound can be received by the established synthesis method, and the doped Eu2+ ions did not generate any impurity or induce significant changes in the host structure. In order to obtain a better understanding of the phase purity and the occupancy of Eu2+ ions on Ca2+ sites in Ca2.99GeO4Cl2:0.01Eu2+, the Rietveld structural refinement was performed by using the GSAS program as shown in Fig. 2. Ca3GeO4Cl2 served as an initial structural model. The final refined unit cell parameters and residual factors are summarized in Table 1. The Rietveld analysis results indicated the weighted profile R-factor (Rwp) and the expected R factor (Rp) are 7.90% and 5.83%, respectively, for Ca3GeO4Cl2, which indicates that the results on phase analysis and the occupancy of Eu2+ ions are stable. Furthermore, Fig. 3(a) represents the unit cell of Ca3GeO4Cl2 phosphor with an orthorhombic structure, which consists of one distinct Ge site on the special position 4c, site symmetry m, three different O sites, one distinct Cl position and two different Ca sites, Ca1 and Ca2 (one on the general position 8d, site symmetry 1, and the other on the special position 4c). As is mentioned above and also given in Fig. 3(b), there are two calcium atoms showing the same six-fold coordination. Both Ca1 and Ca2 have four oxygen and two chlorine neighbors; however, the bond length and the
Fig. 1 XRD patterns of as-prepared Ca3GeO4Cl2:0.01Eu2+, Ca3GeO4Cl2: 0.08Eu2+ phosphors and ICSD card (249329) of Ca3GeO4Cl2 compound is also given for comparison.
Dalton Trans., 2014, 43, 13370–13376 | 13371
View Article Online
Paper
Dalton Transactions
3.2.
Luminescence properties
The diffuse reflectance spectra of Ca3−xGeO4Cl2:xEu2+ (x = 0, 0.03, 0.05, 0.08) phosphors are displayed in Fig. 4. An absorption band at about 221 nm was observed for the virgin sample, confirming that light having this particular wavelength was absorbed by this phosphor. The Kubelka–Munk absorption coefficient (K/S) relation was used to calculate the absorption edge from the measured reflectance (R):15,16
Published on 09 July 2014. Downloaded on 16/10/2014 14:19:18.
K ð1 RÞ2 ¼ S 2R
Fig. 2 Powder XRD patterns for Rietveld structure analysis of the selected Ca2.99GeO4Cl2:0.01Eu2+ phosphor.
Table 1 Main parameters of processing and refinement of the Ca2.99GeO4Cl2:0.01Eu2+ sample
Compound
CaMnSi2O6:0.03Eu
Sp. Gr. a/Å b/Å c/Å α/° β/° γ/° V/Å3 2θ-interval/° Rwp/% Rp/% χ2
Pnma(62) 11.687 10.333 5.701 90 90 90 688.47 10–120° 7.90 5.38 3.925
ð1Þ
where K represents the absorption coefficient, S represents the scattering coefficient, and R represents the reflectivity. As given in the inset of Fig. 4, the band gap energy of the Ca3GeO4Cl2 host is calculated to be approximately 3.08 eV by extrapolation. As is also shown in Fig. 4, Eu2+-doped phosphors possess a strong broad absorption in the 300–450 nm n-UV range, which is consistent with the excitation spectrum. The intensities for the absorption band also increase with increasing Eu2+ concentration. The photoluminescence excitation (λem = 428 nm) and emission (λex = 273 nm) spectra of Ca3GeO4Cl2:0.03Eu2+ are presented in Fig. 5. It is obvious that the excitation spectrum monitored at 428 nm, ranging from 200 to 415 nm, exhibits one strong absorption peak located around 263 nm and two weak absorption peaks at about 327 and 359 nm, respectively. All the peaks correspond with the results of the diffuse reflection spectra, which is attributed to the transition from the ground state 4f 7 to the crystal-field split 4f 65d configuration of the doped Eu2+ ions. On the other hand, as is shown in the emission spectrum of Ca2.97GeO4Cl2:0.03Eu2+ under excitation at 365 nm, a blue emission with a typical asymmetrical band extends from 375 to 550 nm with a peak at 428 nm can be seen. Furthermore, the asymmetric emission spectrum indicate that Eu2+ ions occupy more than one site in the lattice, which can be deconvoluted into two Gaussian components
Fig. 3 (a) Crystal structure of Ca3GeO4Cl2 compound, (b) coordination spheres of the two different Ca2+ sites in Ca3GeO4Cl2.
coordination environment are different.14 Therefore, it is believed that the two different Ca2+ sites will possibly produce two different cation environments for the doped ions.
13372 | Dalton Trans., 2014, 43, 13370–13376
Fig. 4 The diffuse reflectance spectra of Ca3GeO4Cl2:xEu2+ (x = 0, 0.03, 0.05, 0.08) phosphors, and the inset shows the extrapolation of the band gap energy for the host.
This journal is © The Royal Society of Chemistry 2014
View Article Online
Dalton Transactions
Paper
Published on 09 July 2014. Downloaded on 16/10/2014 14:19:18.
is increased. Furthermore, Fig. 6 also demonstrates the dependence of the emission intensity on the concentration of Eu2+. The emission intensity of Eu2+ first increased and reached the maximum at a Eu2+ concentration of 0.03, and then the emission intensity decreased with further increase in the concentration. It can be concluded that the optimum doping concentration of Eu2+ is 0.03. After reaching this critical value, the emission intensity decreased sharply as a result of the concentration quenching effect. It is considered that concentration quenching is mainly caused by energy transfer among Eu2+ ions and the probability of this occurring increases as the concentration of Eu2+ increases.18 In this sense, the crystal field strength as Dq is inversely proportional to the fifth power of the bond-length R.19,20 Dq ¼ Fig. 5 Luminescence excitation and PL spectra of the Ca3GeO4Cl2: 0.03Eu2+ phosphor.
that peaked at 427 nm and 451 nm due to the two different Ca2+ sites, as shown in Fig. 5. In the Ca3GeO4Cl2 structure, two calcium sites are available for activator cations and this structural feature was well matched with the PL emission spectrum of Ca, of which there were two emission centers for Eu2+.17 The PLE (λem = 428 nm) spectrum of the selected Ca2.97GeO4Cl2:0.03Eu2+ and the emission spectra of the Ca3−xGeO4Cl2:xEu2+ phosphors under excitation at 365 nm with different concentrations of Eu2+ (ranging from 0.01 to 0.15) at room temperature are depicted in Fig. 6. The PL spectra have similar spectral profiles, except for the emission intensities for this series of Ca3−xGeO4Cl2:xEu2+ phosphors. It is observed that the emission intensity of Eu2+ ion first increases and reaches a maximum at x = 0.03, and the PL spectra are gradually red-shifted as the concentration of Eu2+
ze2 r 4 6R5
Here Dq is the measurement of the crystal field strength, z is the charge or valence of the anion, R is the bond distance between the central ion and its ligands, e is the charge of an electron, and r is the radius of the d wave function. From this equation can see that a shorter bond distance state stronger crystal field strength clearly when the crystal environments are homologous. It is known that the emission of Eu2+ is very strongly dependent on the host lattice.21 When larger Eu2+ ion substitute the Ca2+ ion, the distance between Eu2+ and O2− becomes shorter, the crystal field strength increases, and finally with increasing Eu2+ concentration the emission wavelength is red-shifted. There are two main aspects responsible for the resonant energy-transfer mechanism: one is exchange interaction and the other is multipolar interaction.22 According to Dexter’s theory, non-radiative transitions between Eu2+ ions takes place via electric multipolar interactions. In order to further confirm the process of energy transfer between sensitizers or between a sensitizer and activator, the interaction type can be calculated by eqn (3):23 I=x ¼ k½1 þ βðxÞθ=3 1
Fig. 6 PLE spectra of Ca3GeO4Cl2:0.03Eu2+ and the Eu2+ concentration-dependent PL spectra of Ca3GeO4Cl2 phosphors.
This journal is © The Royal Society of Chemistry 2014
ð2Þ
ð3Þ
In this equation, x is the activator concentration, I/x is the emission intensity (I) per activator concentration (x), k and β are constants for the same excitation condition for a given host crystal, and θ is a function of the multipole–multipole interaction. θ = 6, 8 and 10 corresponds to dipole–dipole (d–d), dipole–quadrupole (d–q), and quadrupole–quadrupole (q–q) interactions, respectively. In order to get a correct θ value, Fig. 7 illustrate the I/x dependence on x on a logarithmic scale. Obviously, an approximate linear relation between lg(I/cEu) and lg(cEu) can be found and the slopes of the straight line is −θ/3 = −1.594, the value of θ can be calculated as 4.782, which is close to 6, indicating that the quenching is through dipole–dipole interactions in the present Ca3GeO4Cl2:Eu2+ phosphors. The concentration quenching of the luminescence is owing to the energy transfer from one activator to another until all
Dalton Trans., 2014, 43, 13370–13376 | 13373
View Article Online
Published on 09 July 2014. Downloaded on 16/10/2014 14:19:18.
Paper
Dalton Transactions
Fig. 7 The relationships of lg(x) versus lg(I/x) in Ca3GeO4Cl2:xEu2+ phosphors beyond the quenching concentration.
the energy is consumed. Therefore, it is necessary to obtain the critical distance (Rc), i.e., the critical separation between donor (activators) and acceptors (quenching site). The critical distance Rc for concentration quenching Rc can be estimated according to the following equation:24 Rc 2
3V 4πxc N
1=3 ð4Þ
where xc is the atom fraction of activator at which the quenching occurs, N is the number of the Ca2+ ions in the unit cell and V is the volume of the unit cell. Using the crystallographic parameters V = 688.47 Å3, xc is 0.03, and N = 4, the value of the critical distance Rc = 22.11 Å is obtained. Because both donor and acceptor transitions are electric dipole allowed, such a long distance (typical critical distances are 5 Å) indicates that multipolar interactions are dominant, and a dipole–dipole mechanism appears to be most probable. Fig. 8 shows the temperature dependent emission spectra for the Ca2.97GeO4Cl2:0.03Eu2+ phosphor under excitation at 365 nm, and the relative emission intensities as a function of temperature are given in the inset Fig. 8. As can be seen, the relative PL intensity slowly decreases with increasing temperature. When the temperature was raised up to 150 °C, the PL intensity of the sample drops to 77% of the initial value at room temperature. Thermal quenching can be used to explain this phenomenon. The excited luminescent center is thermally activated by the phonon interaction, and then thermally released via the crossing point between the ground and excited states. In order to better understand the thermal quenching behavior, the Arrhenius equation was used to calculate the activation energy as follows:25 IðTÞ
I0
Fig. 8 The PL spectra (λex = 365 nm) of Ca3GeO4Cl2:0.03Eu2+ phosphor under different temperatures in the range of 30–300 °C. The inset shows the relative emission intensities as a function of temperature.
E 1 þ c exp kT
13374 | Dalton Trans., 2014, 43, 13370–13376
ð5Þ
Fig. 9 The activation energy of the thermal quenching of Ca3GeO4Cl2: 0.03Eu2+ phosphor.
where I0 is the initial emission intensity of the phosphor at 30 °C, I(T ) is the intensity at different temperatures, c is a constant, k is the Boltzmann constant (8.62 × 10−5 eV) for the same host, and E is the activation energy for thermal quenching. Fig. 9 presents the best-fit line of the emission thermal quenching model plotted as ln(I0/I) − 1 against 1/kT for the present Ca3GeO4Cl2:0.03Eu2+ phosphor. According to eqn (5), the value of the activation energy, E, of the as-prepared phosphor is calculated to be 0.163 eV, which is similar to another report on the Ca3SiO4Cl2:Eu2+ phosphor.26 The crystal structure and photoluminescence emission spectra analysis verified that there are two different Ca2+ sites, which will induce corresponding emission centers in the Ca3GeO4Cl2:Eu2+ phosphors. In order to further distinguish the two different emission centers, the lifetime for the two emission centers (427 and 451 nm) under excitation at 364 nm
This journal is © The Royal Society of Chemistry 2014
View Article Online
Dalton Transactions
Paper
Acknowledgements The present work was supported by the National Natural Science Foundation of China (grant no. 51002146, 51272242), the Natural Science Foundation of Beijing (2132050), the Program for New Century Excellent Talents in University of Ministry of Education of China (NCET-12-0950), the Beijing Nova Program (Z131103000413047), Beijing Youth Excellent Talent Program (YETP0635), the Funds of the State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University (KF201306) and the Fundamental Research Funds for the Central Universities (FRF-2014).
Published on 09 July 2014. Downloaded on 16/10/2014 14:19:18.
References Fig. 10 Decay curves of Eu2+ emission at different wavelength of 427 and 451 nm in Ca3GeO4Cl2:0.03Eu2+ phosphor under excitation at 364 nm.
was recorded, and is shown in Fig. 10. It was found that the two curves had similar decay behaviors, and both of them can be well fitted by the following single exponential function, eqn (6),27 IðtÞ ¼ A1 expðt=τÞ
ð6Þ
where I and A1 correspond to the luminescence intensity at time t and the initial intensity immediately after the exciting pulse, τ is the lifetime. On the basis of (6), the lifetime values at 427 and 451 nm were determined to be 0.316 and 0.428 μs, respectively. The differences of the lifetimes verified the two different emission centers, however, the minor difference also verified similar crystal field strength effect for the two Ca2+ sites. Therefore, it can be also inferred that there are two Eu2+ emission centres in the present host.
4.
Conclusions
In summary, a series of novel, single-phase, blue-emitting phosphors Ca3GeO4Cl2:Eu2+ were successfully synthesized and studied. The Ca3GeO4Cl2 host has an orthorhombic space group and two different Ca2+ sites. Ca3GeO4Cl2:Eu2+ shows obvious absorption peaks located around 263, 327 and 359 nm. Under excitation at 365 nm, the phosphor exhibited a blue emission band peak at 428 nm due to the 4f–5d transitions of Eu2+. The critical quenching concentration of Eu2+ was about 3 mol%. The value of the critical distance is 22.11 Å, and the corresponding concentration quenching mechanism is considered to be the dipole–dipole interaction. The temperature dependent luminescence behaviors have been also discussed. Different lifetime values of Eu2+ in Ca3GeO4Cl2:Eu2+ also confirmed the two different emission centers in the lattice.
This journal is © The Royal Society of Chemistry 2014
1 Y. Q. Li, A. C. A. Delsing, G. de With and H. T. Hintzen, Chem. Mater., 2005, 17, 3242. 2 B. L. Yuan, X. G. Wang, T. J. Tsuboi, Y. L. Huang and H. J. Seo, J. Alloys Compd., 2012, 512, 144. 3 B. L. Yuan, Y. L. Huang, Y. M. Yu, S. I. Kim and H. J. Seo, Mater. Lett., 2012, 70, 57. 4 R. Stefani, A. D. Maia, E. E. S. Teotonio, M. A. F. Monteiro, M. C. F. C. Felinto and H. F. Brito, J. Solid State Chem., 2006, 179, 1086. 5 Z. G. Xia, J. Q. Zhuang, A. Meijerink and X. P. Jing, Dalton Trans., 2013, 42, 6327. 6 Z. G. Xia, M. S. Molokeev, A. S. Oreshonkov, V. V. Atuchin, R. S. Liu and C. Dong, Phys. Chem. Chem. Phys., 2014, 16, 5952. 7 Z. G. Xia, J. Q. Zhuang and L. B. Liao, Inorg. Chem., 2012, 51, 7202. 8 Z. G. Xia, R. S. Liu, K. W. Huang and V. Drozd, J. Mater. Chem., 2012, 22, 15183. 9 W. Lü, Z. D. Hao, X. Zhang, X. J. Wang and J. H. Zhang, Opt. Mater., 2011, 34, 261. 10 E. N. Treushnikov, V. V. Ilyukhin and N. V. Belov, Dokl. Akad. Nauk SSSR, 1970, 193, 1048. 11 N. Karkada, D. Porob, P. Kumar and A. A. Setlur, ECS Trans., 2011, 3333, 39. 12 W. J. Ding, J. Wang, Z. M. Liu, M. Zhang, Q. Su and J. K. Tang, J. Electrochem. Soc., 2008, 155, J122. 13 J. Liu, H. Z. Lian, J. Y. Sun and C. S. Shi, Chem. Lett., 2005, 34, 1340. 14 G. J. Redhammer, G. Roth and G. Amthauer, Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 2007, 63, i69. 15 N. Yamashita, J. Phys. Soc. Jpn., 1973, 35, 1089. 16 C. H. Huang and T. M. Chen, Inorg. Chem., 2011, 5012, 5725. 17 H. S. Roh, S. Hur, H. J. Song, I. J. Park, D. K. Yim, D. W. Kim and K. S. Hong, Mater. Lett., 2012, 70, 37. 18 Y. C. Chiu, C. H. Huang, T. J. Lee, W. R. Liu, Y. T. Yeh, S. M. Jang and R. S. Liu, Opt. Express, 2011, 19, 331–339. 19 L. Chen, C. C. Lin, C. W. Yeh and R. S. Liu, Materials., 2010, 3, 2172. 20 W. B. Im, Y. I. Kim, N. N. Fellows, H. Masui, G. A. Hirata, S. P. DenBaars and R. Seshadri, Appl. Phys. Lett., 2008, 93, 091905.
Dalton Trans., 2014, 43, 13370–13376 | 13375
View Article Online
Paper
25 E. Pavitra and J. S. Yu, Ceram. Int., 2013, 39, 1029. 26 I. Baginskiy and R. S. Liu, J. Electrochem. Soc., 2009, 156, G29. 27 J. Zhou, Z. G. Xia, M. X. Yang and K. Shen, J. Mater. Chem., 2012, 22, 21935.
Published on 09 July 2014. Downloaded on 16/10/2014 14:19:18.
21 G. Blasse and B. C. Grabmaier, Luminescent Materials, Springer-Verlag, Berlin, 1994. 22 C. H. Huang and T. M. Chen, J. Phys. Chem. C, 2011, 115, 2349. 23 G. Blasse, Phys. Lett., 1968, 28, 444. 24 G. Blasse, Philips Res. Rep., 1969, 24, 131.
Dalton Transactions
13376 | Dalton Trans., 2014, 43, 13370–13376
This journal is © The Royal Society of Chemistry 2014
