Pressure and Temperature Study on the Structural Stability of GdNbO4

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Pressure and Temperature Study on the Structural Stabilitly of GdNbO:Eu Jiwei Hou, Ru Zhou, Jianwu Zhang, Rucheng Dai, Zhongping Wang, Zengming Zhang, and Zejun Ding J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 22 Jun 2017 Downloaded from http://pubs.acs.org on June 23, 2017

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The Journal of Physical Chemistry

Pressure and Temperature Study on the Structural Stabilitly of GdNbO4:Eu3+ Jiwei Hou1, Ru Zhou1, Jianwu Zhang1*, Zhongping Wang2, Zengming Zhang2* and Zejun Ding1 1

Department of Physics, University of Science and Technology of China, Hefei, 230026, P. R. China

2

The Centre for Physics Experiments, University of Science and Technology of China, Heifei, 230026, P. R. Chian

* Corresponding author: [email protected], [email protected]

ABSTRACT: Lanthanide niobates is an excellent nonlinear optical and high dielectric constant material. However, development and application of these novel perporties depend on an understanding of its structural stability. In this work, Raman and luminescence spectra of GdNbO4 with different Eu3+ ions doping concentrations under high pressure and low temperature were obtained. The evolution of Raman spectra indicated that an irreversible structural phase transition of GdNbO4 from monoclinic phase to M’-fergusonite phase occurred at 6.2 GPa, and the phase transition was completed at about 13.1 GPa. An obvious discontinuity on Eu3+ ions luminescence intensity ratio between 5D0→7F2 and 5

D0→7F1 transition line was observed with increasing pressure. The results indicated that the site

symmetry of Eu3+ ions in GdNbO4:Eu3+ was changed, which was in agreement with high pressure Raman spectra data. Meanwhile, Raman and luminescence spectra showed that the crystal structure remained stable in the temperature range of 20–300 K.

Introduction The family of ABO4 ternary oxides were attractive host materials for phosphors. Because of some peculiar chemical and physical properties, numerous applications for these materials were already reported and under development, such as phosphors, catalysts, and repositories for radioactive wastes.1-3 The ABO4 ctystal have insteresting novel properties due to a particular configuration of BO43tetrahedra. This class of materilas usually crystallize in three structural types, zircon, scheelite and fergusonite, depending on the sizes of the B3+ element.4 Many works have proved that compression was an efficient way to improve our understandingof the physical and chemsical properties of these materials.5-6 High pressure have been performed on many scientific studies, and it played a key role in our understanding of structural stability and phase transition properties. Numerous important researchs on pressure behavior of ABO4 oxides had been carried out by experiments and theoretical calculations.7-8 Pressure has been applied extensively to studying the structural, electronic and optical properties of these materials. Such as phase transition between zircon and scheelite in minerals was used to study the evolution of Earth’s mantle and crust in geophysics and geochemistry.4,5 Of the crystals in this family, the zircon-scheelite-fergusonite sequence of phase transition have been observed using Raman, photoluminescence spectroscopy and X-ray diffraction experiments, in tungstates, germanates and vanadates.9-10 However, the crystal strcture evolution of fergusonite structure has not been well explored under high pressure. Lanthanide niobates exhibit fergusonite structure, as one of the ABO4 oxides, these also have considerable potential for numerous applications due to their novel physical and chemical properties, such as high dielectric constants, low phonon frequencies, photoelastic and nonlinear optical properties as well as excellent chemical, mechanical and thermal stability.11-14 They are well-known as self-activated phosphor with a strong blue emission in the UV region on excitation due to a

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charge-transfer between tetragonal [NbO4]3- molecular ions.15-16 Lanthanide niobates crystallize in two common forms of crystal structure: low temperature monoclinic phase with fergusonite (monoclinic: C2/c) and high temperature tetragonal phase with scheelite structure (tetragonal, I4/a). It is widely known it can undergo a reversible phase transition from monoclinic to tetragonal, and the transition temperature depends on the type of rare earth ions.17 In addition, niobium oxide compounds also exist in two types of coordinated structures: tetrahedrally coordinated NbO4 and octahedrally coordinated NbO6 with different extents of oxygen distortion.18 The tetrahedrally coordinated NbO4 is not a typical structure in niobium oxides because Nb atom is too large to be loaded into an oxygen anion tetrahedron. Only a few rare earth orthoniobates, ANbO4 (A=Y, Yb, Sm and La) have tetrahedron coordination.19-20 As one of lanthanide niobates, gadolinium orthoniobate (GdNbO4) with fergusonite structure has been known as essentially inert materials in visible region with excitation of UV light. The disappearance of luminescence of GdNbO4 is due to the energy transfer from NO6 group to Gd3+ ions, resulting in concentration quenching of Gd3+ ions.21-22 For this reason, the luminescence properties of GdNbO4 phosphor by Eu3+, Tb3+ and Tm3+ doping have rerely been reported. The luminescence of Eu3+ ions was often used as a probe to estimate the strength of Eu-O bonds and information of local field around the Eu3+ ions sites in materials. In oreder to understand the crystal structure stability, high pressure have been applied on many scientific studies. The pressure dependent Eu3+ luminescence were been used as a technique to observe the structure informaition of materials under different pressures.23-24 To our best knowledge, the structure and physical properties of fergusonite-type GdNbO4 under high pressure are still largely an uncharted territory. In this work, we used Raman and luminescence spectra of GdNbO4:Eu3+ under different pressures and temperatures to investigate the structure evolution of GdNbO4. Raman spectra indicated that an irreversible structural transition of GdNbO4 occurred at 6.2 GPa, and the phase transition was completed about 13.1 GPa. Luminescence intensity ratio of Eu3+ ions between 5D0→7F2 and 5D0→7F1 transition line showed that the site symmetry of Eu3+ ions in GdNbO4:Eu3+ was changed. Meanwhile, Raman and luminescence spectra demonstrate that the crystal structure of GdNbO4:Eu3+ remained stable in a temperature range of 20–300 K.

Experimental Monoclinic phase GdNbO4:Eu3+ polycrystalline was prepared using a traditional solid state reaction method according to the reported method.25 In a typical procedure, powder sample of GdNbO4:Eu3+ was fabricated with Eu2O3, Gd2O3 and Nb2O5 as raw materials. A stoichiometric amounts of the raw materials was thoroughly ground and sintered in an alumina crucible at 1200 ºC for 4 h and then cooled down to ambient temperature. The final GdNbO4:Eu3+ polycrystalline was obtained with several cycles of grinding and sintering to ensure homogeneity. All chemicals were analytical parity and used without further purification. The crystal structure of the obtained samples were determined by X-ray powder diffraction apparatus (XRD) analyses using a SmartLab diffractometer with a Cu Kα radiation source, with 30 kV accelerating voltage and 160 mA tube current. High pressure experiments were carried out by a symmetric diamond anvil cell (DAC) with 600 µm diameter culets (Fig. 1a and 1b). A stainless steel gasket was preindented by diamonds and a 170 µm hole was drilled by a spark erode (BETSA, MH20M) to be used as a sample chamber. The 4:1 mixture of methanol and ethanol was used as pressure transmitting medium and the pressure was determined


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using the R1 ruby fluorescence technique (Fig 1c).26 The high pressure dependent Raman and luminescence spectra were measured using an integrated laser Raman system (LABRAMHR, Jobin Yvon) with a confocal microscope and a thermoelectrically cooled multichannel charge-coupled device detection system; the spectral resolution was better than 1.0 cm-1. The line 514.5 nm of an argon laser was used as excitation source with a power below than 10 mW. All spectra were measured in the backscattering geometry at room temperature. Temperature dependent Raman spectra were recorded at low temperature through cycle compression of high purity helium. At every step of data collection, the system was held for 15 min to allow the system to reach thermal equilibrium.

Results and discussion Fig. 2 shows the representative XRD patterns of GdNbO4 with different Eu3+ ions doping concentrations, and the diffraction peaks of samples could be perfectly indexed to the monoclinic crystalline GdNbO4 with fergusonite structure (JCPDS file: 22-1104). There are no extra diffraction peaks caused by impurity phase or raw materials, indicating that the synthesized products were of high purity. Moreover, this also confirmed that the introduced Eu3+ ions substitute Gd3+ ions in the crystal lattice of GdNbO4. This may be due to their similar ionic radii and physical properties. GdNbO4 with fergusonite structure crystallizes in monoclinic system, space group: C2/c, with four structural units and 12 per unit cell at room temperature. Using the site group method of Rousseau eat al.27 we would expect a total of 36 phonon modes, including 18 gerade Raman modes, 15 ungerade IR modes and 3 acoustic modes, in the crystal structure. The group theory analysis predicts that there are 18 optical Raman active phonon modes at the center of the Brillouin zone: Γ=8 Ag+10 Bg

(1)

where, the Ag and Bg vibrational modes represent parallel and cross polarized Raman scattering, respectively. The Raman modes of the monoclinic GdNbO4:Eu3+ (1.0% Eu3+) with fergusonite structure at ambient temperature are shown in Fig. 3. In view of the complexity of Raman modes, a detailed analysis was carried out by fitting the Raman spectrum with Lorentzian curves. The Raman spectrum was divided into three different wavenumber regions for batter visualization (Fig. 3a, 3b and 3c). The fitting results evidenced all the eighteen Raman modes theoretically predicted. Besides the eighteen Raman vibrational modes, the asterisks represent the intrinsic transitions of the Eu3+ ions (5D1→7F1) in Fig.3c.28 The Raman vibrational modes are identified and well matched the results early reported.17 The vibrational modes with the low frequencies are attributed to external vibrations with the NbO43– units and their packing. The symmetric Nb–O vibrational modes give rise to resonances near 332 and 812 cm–1, and the anti-symmetric modes correspond to frequencies between 400 to 470 cm–1 and 630 to 700 cm–1.19 In order to investigate the effect of pressure on structural stability of the GdNbO4:Eu3+ (1.0% Eu3+), an in situ Raman and luminescence measurement were performed with pressure up to 26 GPa. Fig. 4a shows the evolution of Raman spectra for the sample under different pressures at room temperature. At pressure up to 6.2 Gpa, a new additional broad and weak Raman mode appears at 291.7 cm–1. Another new Raman mode with frequency of 141.2 cm-1 appears as the pressure increase to 10.4 Gpa. When the pressure increases to 13.1 Gpa, two new Raman modes appear at 196.2 and 268.7 cm-1. In addition, the intensities of these new Raman modes become sharper and stronger with the



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pressure, which provides new evidence for structural changes. During the process pressure is released to ambient condition, the Raman spectrum maintained a pattern similar to the high pressure phase (Fig. 4b). This fact indicates that the pressure-induced phase transition is irreversible for GdNbO4:Eu3+ and the recovered sample should have the same structure as that at high pressure. The pressure dependence of Raman frequencies for the observed vibrational modes of GdNbO4:Eu3+ are plotted in Fig. 4c and 4d. The discontinuities of Raman modes with increasing pressure are clearly observed in the range of 6.2-13.1 Gpa. It can be concluded that a pressure induced phase transition occurred at 6.2 GPa, and the two phases coexist over a pressure range of 6.2–13.1 GPa. Each Raman modes remain stabilize and no new Raman modes appear even as pressure is increased further, indicating that the high pressure phase only exists above 13.1 GPa. And the coexistence of the two phases within a wide pressure range is similar to the pressure-induced phase transition of GdVO4.29 In addition, some Raman modes gradually weakened and disappeared with the increasing pressure. This is because pressure affects the Raman intensities by changing in bond polarizabilities and the anharmonicity. The Raman modes frequencies, pressure coefficients and the Grüneision parameters γ from Eq. (2) are given in Table 1 for different Raman modes at room temperature. As observed in Fig. 4c and 4d, All the Raman modes shift to higher frequency except the anti-symmetric vibrational mode at 668 cm–1, which displays soft mode behavior with increasing pressure. The Raman modes’ blue shifting indicates that the force constants between atoms involved in these vibrational modes are becoming stronger, because the high pressure reduces the atoms distance and increases the atoms interaction. From the pressure coefficients of the Raman modes frequencies and the modulus B0 , we obtained the Grüneision parameters γ , defined by:

γ =−

∂ lnν B0 dν = ∂ ln V ν dP

For monoclinic phase GdNbO4 (Table 1), where volume and

(2)

v

is the frequency of the vibrational mode,

V

is the

P is the pressure, the value of B0 (132.63 GPa) is taken from Ref. [30]. Pressure

coefficients are obtained by linear fitting the data in different pressure regions for monoclinic GdNbO4. The vibrational frequencies of all most Raman modes of fergusonite structure exhibite a positive pressure, although with somewhat different slopes and curvatures. The low frequencies modes have the high Grüneision parameters, this is because the modes are sensitive to changes of volume. Moreover, the vibrational mode shows a negative pressure coefficient at 668.1 cm-1. A similar Raman mode soft behavior has been observed in other ABO4 oxides, it seems to be related to mechanical instabilities induced by pressure.7,8,31 The pressure-induced phase transition of ABO4 oxides have been studied extensively for many different crystal structures, such as phase transition from scheelite to fergusonite in molybnates and tungstates,7,31 phase transition from zircon to scheelite in vanadates and chromates.29,32 Based on a large number of theoretical works and experiments, a general conclusion was that the radii ratios rA/rO and rB/rO between cation and oxygen increase with pressure, resulting in an increase of cation coordination in the high pressure modifications.7,33 Most of the zircon structure ABO4 oxides are known to undergo a phase transition from zircon-type to scheelite-type under pressure. Furthermore,


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zircon-monazite-scheelite sequence of stable structure was suggested as a likely pressure driven route for zircon-type ABO4 oxides.34 Pressure-induced phase transitions are generally along the sequence of increasing structural compactness. So far, there are very few structural studies under high pressure for niobates with fergusonite structure. Mariathasan et al. observed phase transition of niobates from fergusonte phase to scheelite phase as temperature rises. They pointed out that reducing temperature in niobates was analogous to increasing pressure, which was in agreement with the sequence of pressure-induced phase transition from scheelite to fergusonite structure observed in tungstates.35 According to the investigation of pressure-induced phase transition behavior in molybdates and tungstates, it is reasonable to infer that the pressure-induced phase transition of GdNbO4 follows the sequences: scheelite→fergusonite→M’-fergusonite. The luminescence spectra of GdNbO4:Eu3+ (1.0% Eu3+) under different pressures are illustrated in Fig. 5a. In this figure, the 5D0→7FJ (J=1, 2) transition lines of Eu3+ ions are shown, and the 5D0→7FJ (J=3, 4) transition lines are neglected because they are disturbed by the ruby fluorescence. The emission spectrum of Eu3+ ion strongly depends on the local environment surrounding Eu3+ ions. the split number of emission peak is also response to the site symmetry. C2/c space group induces the splitting of 5D0-7FJ (J=1, 2, 3, 4) transition of Eu3+. The transition lines within 590-600 nm are signed to the magnetic dipole transition 5D0→7F1, the strong emission of 610-620 nm correspond to the electric dipole transition 5D0→7F2 of Eu3+ ions. With the increasing of pressure, these emission intensities decrease. All transition lines broaden and gradually merge with the increasing pressure due to the pressure gradient of the non-hydrostatic condition. Transition lines of 5D0→7F1 exhibit a red shift and the transition lines of 5D0→7F2 show a blue shift. The red shift of the 5D0→7F1 transition lines indicate stronger Eu–O bond, and the interaction between Eu3+ ions and oxygen is strengthened as the pressure increases, which agrees with the pressure behavior of many other Eu3+ compounds reported.36-37 For Eu3+ ions in crystal, the 5D0→7F1 transition is driven by magnetic dipole transition and the emission intensity is supposed to be practically insensitive to the crystal field, while 5D0→7F2 transition is an electric dipole type and its emission intensity changes drastically with deviation from the centrosymmetry of the Eu3+ ions site in crystal field. In a site with inversion symmetry, the 5D0→7F1 magnetic dipole transition dominates; In contrast, the electronic dipole transition of 5D0→7F2 becomes stronger. Therefore, the intensity ratio of the transition 5D0→7F2 to 5D0→7F1, (IR(2/1)), is a good criterion for the symmetry of Eu3+ ions site in crystal field. As an asymmetric factor, IR(2/1) provides the valuable information for local environment surrounding the Eu3+ ions in host matrix.38-39 The decrease in IR(2/1) with high pressure due to a decrease in the covalence between Eu3+ and its surrounding ligads.40 The evolution of IR(2/1) of GdNbO4:Eu3+ under various pressures is shown in Fig. 5b, an obvious discontinuity in the linearity in the range of 6.2–13.1 GPa indicates a possible phase transition. The value of IR(2/1) decreases as the pressure increases, indicating that energy transfer from the host to Eu3+ ions is strengthened. This could be due to either a decrease in the covalence of Eu-O bonds or an increase in the average Eu-O bond distance with increasing pressure. This luminescence characteristics of GdNbO4:Eu3+ under high pressure show that a pressure-induced phase transition occurs at 6.2 GPa, which is in agreement with the result measured by high pressure Raman spectra. In situ Raman measurements spectra study of GdNbO4:Eu3+ (1.0% Eu3+) for different temperatures in the 20–300 K range are carried out. The evolution of Raman active modes of



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GdNbO4:Eu3+ at several temperatures under ambient temperature are shown in Fig 6a. Except for different slopes and curvatures, all Raman vibrational modes slightly shift to high wavenumber with decreasing temperature. This indicates that the force constants between atoms involved in these vibrational modes are becoming stronger due to lattice effects at low temperature. The temperature dependence of Raman frequencies for the observed vibrational modes of GdNbO4:Eu3+ are plotted in Fig. 6c and 6d. New Raman modes and discontinuities of Raman frequencies with decreasing temperature are not observed in the range of 20-300 K. Raman modes remains stable as the temperature decreases, indicating that the crystal structure of GdNbO4:Eu3+ is stable in the range of 20-300 K. The luminescence spectra of GdNbO4:Eu3+ at several temperatures under ambient pressure is given in Fig. 6b. With decreasing temperature, the transition lines of 5D0→7FJ (J=1, 2) exhibit a slight red shift toward longer wavelengths, which agrees with other Eu3+ ions compounds reported at low temperature.28 The evolution of asymmetric factor (IR(2/1)) of GdNbO4:Eu3+ under different temperature at ambient pressure are shown in Fig. 6e. The of linearity remains constant in the range from 20-300 K, indicating the asymmetry of the local environment surrounding the Eu3+ ions in crystal structure is unchanged. These luminescence characteristics of GdNbO4:Eu3+ under low temperature show that the crystal structure remain stable in the temperature range from 20 K to 300 K, which is consistent with the results observed by low temperature Raman spectra.

Conclusions We synthesized fergusonite type GdNbO4 with different Eu3+ ions doping concentrations using a simple solid state reaction method, and made a Raman and luminescence study of the crystal structure stability of the sample under high pressure and low temperature. The evolution of Raman spectra under high pressure indicates that an irreversible structural phase transition from monoclinic phase to M’-fergusonite phase of GdNbO4 occurs at 6.2 GPa, and the phase transition is completed about 13.1 GPa. The changes on Eu3+ ions luminescence from 5D0→7FJ (J=1, 2) transition line in GdNbO4:Eu3+ are also measured under high pressure. An obvious discontinuity on Eu3+ ions luminescence intensity ratio between 5D0→7F2 and 5D0→7F1 transition line indicate that the site symmetry of Eu3+ ions in GdNbO4:Eu3+ is changed, which is in good agreement with the high pressure Raman spectra data. In addition, Raman and luminescence spectra show that crystal structure of GdNbO4:Eu3+ remains stable in the temperature range of 20–300 K.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 11174265) and by “973” projects (No. 2011CB932801).


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Luminescent Properties of GdNbO4:RE3+ (RE=Tm, Dy) Nanocrystalline Phosphors via the Sol-Gel Process. J. Phys. Chem. C 2013, 117, 21972-21980. (22) Liu, X. M.; Chen, C.; Li, S. L.; Dai, Y. H.; Guo, H. Q.; Tang, X. H.; Xie Y.; Yan, L. S. Host-Sensitized and Tunable Luminescence of GdNbO4:Ln3+ (Ln3+=Eu3+/Tb3+/Tm3+) Nanocrystalline Phosphors with Abundant Color. Inorg. Chem. 2016, 55, 10383-10396. (23) Gong, C.; Li, Q. J.; Liu, R.; Hou, Y.; Wang, J. X.; Dong, X. T.; Liu, B.; Tan, X.; Liu, J.; Yang, K., et al. Structural Phase Transition and Photoluminescence Properties of YF3:Eu3+ Nanocrystals under High Pressure. J. Phys. Chem. C 2014, 118, 22739-22745. (24) Song, W. S.; Huang, G. X. Y.; Dai, R. C.; Wang, Z. P.; Zhang, Z. M. Raman Scattering and Photoluminescence Investigation of YBO3:Eu3+ under High Temperature and High Pressure. J. Mater. Chem. C 2015, 3, 2405-2412. (25) Zhou, R.; Kou, Y.; Wei, X.; Duan, C.; Chen, Y.; Yin, M. Broadband Downconversion Based Near-Infrared Quantum Cutting via Cooperative Energy Transfer in YNbO4:Bi3+, Yb3+ Phosphor. Appl. Phys. B 2012, 107, 483-487. (26) Mao, H. K.; Xu, J.; Bell, P. M. Calibration of the Ruby Pressure Gauge to 800 kbar Under Quasi-Hydrostatic Conditions. J. Geophys. Res. 1986, 91, 4673-4676. (27) Rousseau, D. L.; Bauman, R. P.; Porto, S. P. S. Normal Mode Determination in Crystals. J. Raman Spectrosc. 1981, 10, 253-290. (28) Graca, M. P. F.; Peixoto, M. V.; Ferreira, N.; Rodrigues, J.; Nico, C.; Costa, F. M.; Monteriro, T. Optical and Dielectric Behaviour of EuNbO4 Crystals. J. Mater. Chem. C 2013, 1, 2913-2919. (29) Zhang, C. C.; Zhang, Z. M.; Dai, R. C.; Wang, Z. P.; Zhang, J. W.; Ding, Z. J . High-Pressure Raman and Luminescence Study on the Phase Transition of GdNbO4:Eu3+ Microcrystals. J. Phys. Chem. C 2010, 114, 18279-18282. (30) Ding, S. J.; Liu, W. P.; Zhang, Q. L.; Peng, F.; Luo, J. Q.; Dou, R. Q.; Sun, G. H.; Sun, D. L. Crystal Growth, Defects, and Mechanical and Spectral Properties of a Novel Mixed Laser Crystal Nd:GdYNbO4. Appl. Phys. A 2017, 123, 70. (31) Maczka, M.; Souza Filho, A. G.; Paraguassu, W.; Freire, P. T. C.; Mendes Filho, J.; Huanuza, J. Pressure-Induced Structural Phase Transitions and Amorphization in Selected Molybdates and Tungstates. Prog. Mater. Sci. 2012, 57, 1335-1381. (32) Santamaría-Pérez, D.; Kumar, R. S.; Dossantos-García, A. J.; Errandonea, D.; Chuliá-Jordán, R.; Seaz-Puche, R.; Rodríguez-Hernández, P.; Muñoz, A. High-Pressure Transition to the Post-Barite Phase in BaCrO4 Hashemite. Phys. Rev. B 2012, 86, 094116. (33) Fukunage, O.; Yamaoka, S. Phase Transformations in ABO4 Type Compounds Under High Pressure. Phys. Chem. Minerals 1979, 5, 167-177. (34) Tatsi, A.; Stavrou, E.; Boulmetis, Y. C.; Kontos, A. G.; Raptis, Y. S.; Raptis, C. Raman Study of Tetragonal TbPO4 and Observation of a First-Order Phase Transition at High Pressure. J. Phys.: Condens. Mater. 2008, 20, 425216. (35) Mariathasan, J. W. E.; Finger, L. W.; Hazen, R. M. High-Pressure Behavior of LaNbO4. Acta Crystallogr. B 1985, 41, 179-184. (36) Errandonea, D.; Lacomba-Perales, R.; Ruiz-Fuertes, J.; Segura, A.; Achary, S. N.; Tyagi, A. K. High-Pressure Structural Investigation of Several Zircon-Type Orthovanadates. Phys. Rev. B 2009, 79, 184104. (37) Rao, R.; Sakuntala, T.; Achary, S. N.; Tyagi, A. K. High Pressure Behavior of ZrGeO4: A Raman Spectroscopic and Photoluminescence Study. J. Appl. Phys. 2009, 106, 123517. (38) Machon, D.; Dmitriev, V. P.; Sinitsyn, V. V.; Lucazeau, G. Eu2(MoO4)3 Single at High Pressure: Structural


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Phase Transitions and Amorphization Probed by Fluorescence Spectroscopy. Phys. Rev. B 2004, 70, 094117. (39) Zeng, Q. G.; Ding, Z. J.; Zhang, Z. M.; Sheng, Y. Q. Photoluminescence and Raman Spectroscopy Studies of Eu(OH)3 Rods at High Pressures. J. Phys. Chem. C 2010, 114, 4895-4900. (40) Chen, G.; Haire, R. G.; Peterson, J. R. Effect of Pressure on Amorphous Eu(OH)3: A Luminescence Study. J. Phys. Chem. Solids 1995, 56, 1095-1100.



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Table 1 Raman modes of fergusonite-type GdNbO4:Eu3+ (1.0% Eu3+) at ambient pressure compared with studies in the literature, and their corresponding Raman frequencies, pressure coefficients and Grüneision parameters. This work 120.5 123.6 129.3 -181.1 188.6 -219.7 235.1 --318.2 332.0 338.3 367.1 417.5 431.8 458.7 648.9 668.1 688.3 811.9

Siqueira et al. [18] 121.6 123.8 128.2 -181.0 189.0 -220.0 235.1 --316.7 331.4 337.1 362.4 417.0 432.5 459.1 648.5 667.1 688.0 812.2

0-6.2 GPa 1.867 2.819 2.512 2.819 2.995

dν/dP (cm-1/GPa) 6.2-26.5 GPa 0.760

0.828 0.874 1.026

2.600 2.287

1.858 2.331 3.323 3.355 2.373 3.082 3.492 2.883 -0.813 2.627 1.784

1.935 2.815 2.490

113.4

1.657


γ 1.757 2.586 2.203 -1.765 1.801 -1.342 1.103 --0.662 0.796 1.114 1.036 0.644 0.809 0.863 0.504 -0.138 0.433 0.249

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Fig. 1 High pressure generating device, (a) schematic of diamond anvil cell, (b) photograph of high pressure generating device, (c) Ruby fluorescence changes with pressure. Fig. 2 XRD patterns of GdNbO4 with different Eu3+ ions doping concentrations, (a) 0.0%, (b) 0.5%, (c) 1.0% and (d) 2.0%. Fig. 3 Raman spectra for GdNbO4:Eu3+ (1.0% Eu3+) in the region of 80-900 cm-1: (a) 80-260 cm-1, (b) 270-480 cm-1, (c) 630-900 cm-1, the asterisks are the intrinsic transitions of Eu3+ ions. Experiment data are in closed ball, whereas the fitting curves are represented by red lines, green lines represent the phonon modes adjusted by Lorentzian curves. Fig. 4 Evolution of Raman spectra of GdNbO4:Eu3+ (1.0% Eu3+) at high pressure, (a) Raman spectra of GdNbO4:Eu3+ (1.0% Eu3+) at selected pressures, (b) Raman spectra of GdNbO4:Eu3+ (1.0% Eu3+) as the pressure is released to atmospheric pressure, the curves marked by asterisks represent the new Raman modes. The dependence of pressure on shifts of Raman mode frequencies: (c) 100-400 cm-1 and (d) 300-900 cm-1. Fig. 5 Evolution of luminescence spectra of GdNbO4:Eu3+ (1.0% Eu3+) at high pressure, (a) luminescence of GdNbO4:Eu3+ (1.0% Eu3+) at various pressures, (b) the dependence of pressure on luminescence intensity ratio IR(2/1) of I(5D0→7F2) to I(5D0→7F1) transition line, and the solid line represents the linear fitting for different pressure regions. Fig.6 Raman and luminescence of GdNbO4:Eu3+ (1.0% Eu3+) at low temperature, (a) Raman spectra and (b) luminescence of GdNbO4:Eu3+ (1.0% Eu3+) at selected temperatures. The dependence of temperature on the shifts of Raman mode frequencies: (c) 100-345 cm-1 and (d) 350-850 cm-1, (e) the dependence of temperature on luminescence intensity ratio IR(2/1) of I(5D0 → 7F2) to I(5D0 → 7F1) transition line and solid line represent the linear fitting.



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Fig. 1


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Fig.2



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Fig. 3


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Fig. 4



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Fig.5


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Fig.6



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TOC Graphic


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Pressure and Temperature Study on the Structural Stability of GdNbO4

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