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A new bismuth-based host material, i.e., Sr 2 BiF 7 , is explored in this work. Undoped and lanthanide ion-doped Sr 2 BiF 7 nanomaterials are prepared using a simple coprecipitation technique at 120 °C. The undoped nanomaterials exhibit a blue color under 365 nm excitation. The downshifting and upconversion photoluminescent properties of Er and Yb codoped Sr 2 BiF 7 nanomaterials are investigated. The optimum up-conversion luminescence is produced by nanomaterials doped with 5% Yb 3+ and 0.2% Er 3+ . These nanomaterials show blue and magenta colors upon excitation at 365 and 395 nm wavelengths, respectively. Sr 2 BiF 7 material doped with Er 3+ shows green emission, while the codoped Er 3+ , Yb 3+ nanomaterials exhibit an orange-red color under 980 nm light. A specific amount of polyvinyl chloride (PVC) is used for producing luminescent ink with these nanoparticles for multimode anticounterfeiting applications. The letters and patterns written with luminescent ink based on Er 3+ , Yb 3+ doped nanomaterials show blue, magenta, and orange-red colors under 365, 395, and 980 nm light, respectively. These results establish that this material can be effectively used as a multimode photoluminescent covert tag to combat counterfeiting.

The monoclinic polymorph of Co 3 P 2 O 8 (space group P 2 1 / c ), isomorphic to farringtonite (Mg 3 P 2 O 8 ) type orthophosphates, was studied up to 21 GPa using synchrotron powder X-ray diffraction and density-functional theory simulations to investigate the influence of pressure in the crystal structure. This study revealed a pressure induced structural phase transition for monoclinic cobalt phosphate, Co 3 P 2 O 8 , and the details of crystal structure of the new high-pressure polymorph were delineated. The evolution of XRD pattern with pressure indicate that the onset of a phase transition occurs around 2.90(5) GPa, and the low- and high- pressure phases coexist up to 10.3(1) GPa. The high-pressure phase also has a monoclinic lattice (space group P 2 1 / c ), and a discontinuous change of unit-cell volume of about 6.5% occurs at the transition. A reorganization of atomic positions with a change in the cobalt coordination sphere occurs in the phase transition. Notably, the high-pressure polymorph has a defect-olivine-type structure like chopinite-type orthophosphates. Using a third-order Birch-Murnaghan equation of state, the bulk moduli of the low pressure (LP) phase (75(2) GPa) and high-pressure (HP) phase (92(2) GPa) were determined. For the low-pressure polymorph, the principal axes of compression and their compressibility were also determined. Density-functional theory calculations also provided the frequencies of Raman- and infrared-active modes which can be used for mode assignment in future experiments.

Coupling of orbital degree of freedom with a spin exchange, i.e., Kugel-Khomskii-type interaction (KK), governs a host of material properties, including colossal magnetoresistance, enhanced magnetoelectric response, and photoinduced high-temperature magnetism. In general, KK-type interactions lead to deviation in experimental observables of coupled Hamiltonian near or below the magnetic transition. Using diffraction and spectroscopy experiments, here we report anomalous changes in lattice parameters, electronic states, spin dynamics, and phonons at four times the Néel transition temperature (T_{N}) in CrVO_{4}. The temperature is significantly higher than other d-orbital compounds such as manganites and vanadates, where effects are limited to near or below T_{N}. The experimental observations are rationalized using first-principles and Green's function-based phonon and spin simulations that show unprecedentedly strong KK-type interactions via a superexchange process and an orbital-selective spin-phonon coupling coefficient at least double the magnitude previously reported for strongly coupled spin-phonon systems. Our results present an opportunity to explore the effect of KK-type interactions and spin-phonon coupling well above T_{N} and possibly bring various properties closer to application, for example, strong room-temperature magnetoelectric coupling.

We have studied by means of angle-dispersive powder synchrotron X-ray diffraction the structural behavior of KCaPO 4 , SrKPO 4 , and K 2 Ce(PO 4 ) 2 under high pressure up to 26, 25, and 22 GPa, respectively. For KCaPO 4 , we have also accurately determined the crystal structure under ambient conditions, which differs from the structure previously reported. Arguments supporting our structural determination will be discussed. We have found that KCaPO 4 undergoes a reversible phase transition. The onset of the transition is at 5.6 GPa. It involves a symmetry decrease. The low-pressure phase is described by space group P 3̅ m 1 and the high-pressure phase by space group Pnma . For KSrPO 4 and K 2 Ce(PO 4 ) 2 , no evidence of phase transitions has been found up to the highest pressure covered by the experiments. For the three compounds, the linear compressibility for the different crystallographic axes and the pressure-volume equation of states are reported and compared with those of other phosphates. The three studied compounds are among the most compressible phosphates. The results of the study improve the knowledge about the high-pressure behavior of complex phosphates., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

In this study, we have demonstrated the application of sodium manganese oxide for the chemisorption of toxic acidic gases at room temperature. The fabricated alkali ceramic has Na 0.4 MnO 2 , Na 2 Mn 3 O 7 , and Na x MnO 2 phases with a surface area of 2.6 m 2  g -1 . Na-Mn oxide was studied for oxidation of H 2 S, SO 2 , and NO 2 gases in the concentration range of 100-500 ppm. The material exhibited a high uptake capacity of 7.13, 0.75, and 0.53 mmol g -1 for H 2 S, SO 2 , and NO 2 in wet conditions, respectively. The material was reusable when regenerated simply by soaking the spent oxide in a NaOH-H 2 O 2 solution. While the H 2 S chemisorption process was accompanied by sulfide, sulfur, and sulfate formation, the SO 2 chemisorption process yielded only sulfate ions. The NO 2 chemisorption process was accomplished by its conversion to nitrite and nitrate ions. Thus, the present work is one of the first reports on alkali ceramic utilization for room-temperature mineralization of acidic gases., (© 2023. The Author(s).)

The orthorhombic polymorph of Ca 2 Zr 5 Ti 2 O 16 (space group Pbca ) has been studied by powder X-ray diffraction under high pressures up to 30 GPa using synchrotron radiation. We have found evidence of a structural phase transition at 12-13 GPa. The phase transition causes an enhancement of the crystal symmetry. The high-pressure phase is tetragonal, being described by space group I 4 1 / acd . The space groups of the high- and low-pressure phases have a group/subgroup relationship. However, the phase transition is accompanied by a discontinuous change in the unit-cell volume, indicating that the phase transition can be classified as first order. We have also performed density functional theory calculations. These simulations support the occurrence of the orthorhombic-to-tetragonal transition. The pressure-volume equation of state and axial compressibilities have been determined for both polymorphs. The results are compared with previous studies in related oxides., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

Phase pure Na 0.4 MnO 2 microrods crystallized in the orthorhombic symmetry were fabricated for the wet oxidation of H 2 S and SO 2 gases at room temperature. The material was found highly effective for the mineralization of low concentrations of acidic gases. The material was fully regenerable after soaking in a basic H 2 O 2 solution., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

Zircon-type NdVO 4 and scheelite-type PrVO 4 have been studied by means of Raman spectroscopy up to approximately 20 GPa. In the first compound, zircon-scheelite and scheelite-fergusonite phase transitions are reported at 6.4(3) and 19.6(4) GPa, respectively. In the case of scheelite-type PrVO 4 , a reversible phase transition to a PbWO 4 -III structure is observed at 16.8(5) GPa. In both cases, a scheelite-type structure is recovered in a metastable state at low pressures. The pressure evolution of the Raman modes is also reported. Our experimental findings are supported by ab initio calculations, which allowed us to discuss the role of mechanic and dynamical instabilities in the phase transition mechanisms., (© 2021 IOP Publishing Ltd.)

The intriguing coupling phenomena among spin, phonon, and charge degrees of freedom in materials having magnetic, ferroelectric and/or ferroelastic order have been of research interest for the fundamental understanding and technological relevance. We report a detailed study on structure and phonons of Al 0.5 Ga 0.5 FeO 3 (ALGF), a lead-free magnetoelectric material, carried out using variable temperature dependent powder neutron diffraction and Raman spectroscopy. Neutron diffraction studies suggest that Al 3+ ions are distributed in one tetrahedrally (BO 4 ) and three octahedrally (BO 6 ) coordinated sites of the orthorhombic (Pc2 1 n) structure and there is no structural transition in the temperature range of 7-800 K. Temperature dependent field-cooled and zero-field-cooled magnetization studies indicate ferrimagnetic ordering below 225 K (T N ), and that is reflected in the low temperature powder neutron diffraction data. An antiferromagnetic type arrangement of Fe 3+ ions with net magnetic moment of 0.13 μ B /Fe 3+ was observed from powder neutron diffraction analysis and it corroborates the findings from magnetization studies. At the magnetic transition temperature, no drastic change in lattice strain was observed, while significant changes in phonons were observed in the Raman spectra. The deviation of several mode frequencies from the standard anharmonicity model in the ferrimagnetic phase (below 240 K) is attributed to coupling effect between spin and phonon. Spin-phonon coupling effect is discernable from Raman bands located at 270, 425, 582, 695, 738, and 841 cm -1 . Their coupling strengths (λ) have been estimated using our phonon spectra and magnetization results. BO n (n = 4, 6) libration (restricted rotation) mode at 270 cm -1 has the largest coupling constant (λ∼ 2.3), while the stretching vibrations located at 695 and 738 cm -1 have the lowest coupling constant (λ∼ 0.5). In addition to the libration mode, several internal stretching and bending modes of polyhedral units are strongly affected by spin ordering.

The effects of pressure on the crystal structure of aurophilic tetragonal gold iodide have been studied by means of powder X-ray diffraction up to 13.5 GPa. We found evidence of the onset of a phase transition at 1.5 GPa that is more significant from 3.8 GPa. The low- and high-pressure phases coexist up to 10.7 GPa. Beyond 10.7 GPa, an irreversible process of amorphization takes place. We determined the axial and bulk compressibility of the ambient-pressure tetragonal phase of gold iodide up to 3.3 GPa. This is extremely compressible with a bulk modulus of 18.1(8) GPa, being as soft as a rare gas, molecular solids, or organometallic compounds. Moreover, its response to pressure is anisotropic.

We present a study of the pressure dependence of the structure of partially hydrated hexagonal CePO 4 up to 21 GPa using synchrotron powder X-ray diffraction. At a pressure of 10 GPa, a second-order structural phase transition is observed, associated with a novel polymorph. The previously unknown high-pressure phase has a monoclinic structure with a similar atomic arrangement as the low-pressure phase, but with reduced symmetry, belonging to space group C2. Group-subgroup relations hold for the space symmetry groups of both structures. There is no detectable volume discontinuity at the phase transition. Here we provide structural information on the new phase and determine the axial compressibility and bulk modulus for both phases. They are found to have an anisotropic behavior and to be much more compressible than the denser monazite-like polymorph of CePO 4 . In addition, the isothermal compressibility tensor for the high-pressure structure is reported at 10 GPa and the direction of maximum compressibility described. Finally, the possible role of water and the pressure medium in the high-pressure behavior is discussed. The results are compared with those from other rare-earth orthophosphates.

The zircon to scheelite phase boundary of ErVO 4 has been studied by high-pressure and high-temperature powder and single-crystal X-ray diffraction. This study has allowed us to delimit the best synthesis conditions of its scheelite-type phase, determine the ambient-temperature equation of state of the zircon and scheelite-type structures, and obtain the thermal equation of state of the zircon-type polymorph. The results obtained with powder samples indicate that zircon-type ErVO 4 transforms to scheelite at 8.2 GPa and 293 K and at 7.5 GPa and 693 K. The analyses yield bulk moduli K 0 of 158(13) GPa for the zircon phase and 158(17) GPa for the scheelite phase, with a temperature derivative of d K 0 /d T = -[3.8(2)] × 10 -3 GPa K -1 and a volumetric thermal expansion of α 0 = [0.9(2)] × 10 -5 K -1 for the zircon phase according to the Berman model. The results are compared with those of other zircon-type vanadates, raising the need for careful experiments with highly crystalline scheelite to obtain reliable bulk moduli of this phase. Finally, we have performed single-crystal diffraction experiments from 110 to 395 K, and the obtained volumetric thermal expansion (α 0 ) for zircon-type ErVO 4 in the 300-395 K range is [1.4(2)] × 10 -5 K -1 , in good agreement with previous data and with our experimental value given from the thermal equation of state fit within the limits of uncertainty.

The high-pressure behavior of the crystalline structure FeVO 4 has been studied by means of X-ray diffraction using a diamond-anvil cell and first-principles calculations. The experiments were carried out up to a pressure of 12.3 GPa, until now the highest pressure reached to study an FeVO 4 compound. High-pressure X-ray diffraction measurements show that the triclinic P1̅ (FeVO 4 -I) phase remains stable up to ≈3 GPa; then a first-order phase transition to a new monoclinic polymorph of FeVO 4 (FeVO 4 -II') with space group C2/ m is observed, having an α-MnMoO 4 -type structure. A second first-order phase transition is observed around 5 GPa toward the monoclinic ( P2/ c) wolframite-type FeVO 4 -IV structure, which is stable up to 12.3 GPa in coexistence with FeVO 4 -II'. The unit cell volume reductions for the first and second phase transitions are Δ V = -8.5% and -13.1%. It was observed that phase transitions are irreversible and both high-pressure phases remain stable once the pressure is released. Calculations were performed at the level of the generalized gradient approximation plus Hubbard correction (GGA+ U) and with the hybrid Heyd-Scuseria-Ernzerhof (HSE06) exchange-correlation functional in order to have a good representation of the pressure behavior of FeVO 4 . We found that theoretical results follow the pressure evolution of structural parameters of FeVO 4 , in good agreement with the experimental results. Also, we analyze FeVO 4 -II (orthorhombic Cmcm, CrVO 4 -type structure) and -III (orthorhombic Pbcn, α-PbO 2 -type structure) phases and compare our results with the literature. Going beyond the experimental results, we study some possible post-wolframite phases reported for other compounds and we found a phase transition for FeVO 4 -IV to raspite (monoclinic P2 1 / c) type structure (FeVO 4 -V) at 36 GPa (Δ V = -8.1%) and a further phase transition to the AgMnO 4 -type (monoclinic P2 1 / c) structure (FeVO 4 -VI) at 66.5 GPa (Δ V = -3.7%), similar to the phase transition sequence reported for InVO 4 .

Negative thermal expansion material TaVO 5 is recently reported to have pressure induced structural phase transition and irreversible amorphization at 0.2 and above 8 GPa, respectively. Here, we have investigated the high pressure phase of TaVO 5 using in situ neutron diffraction studies. The first high pressure phase is identified to be monoclinic P2 1 / c phase, same as the low temperature phase of TaVO 5 . On heating, amorphous TaVO 5 transformed to a new crystalline phase, which showed signatures of higher coordination of vanadium indicating pressure induced amorphization (PIA). PIA observed in TaVO 5 might be due to the kinetic hindrance of pressure induced decomposition (PID) into a compound with higher coordination of vanadium. Mechanism of PIA observed in TaVO 5 is investigated by carrying out ex situ Raman, XRD, XPS, and XAS measurements. We have also proposed a pressure-temperature phase diagram of TaVO 5 qualitatively delineating the phase boundaries between the ambient orthorhombic, monoclinic, and amorphous phases.

The structural stability and phase transition behavior of tetragonal (I4/m) hollandite type K 2 Fe 2 Ti 6 O 16 have been investigated by in situ high pressure X-ray diffraction using synchrotron radiation and a diamond anvil cell as well as by variable temperature powder neutron and X-ray diffraction. The tetragonal phase is found to be stable in a wider range of temperatures, while it reversibly transforms to a monoclinic (I2/m) structure at a moderate pressure, viz. 3.6 GPa. The pressure induced phase transition occurs with only a marginal change in structural arrangements. The unit cell parameters of ambient (t) and high pressure (m) phases can be related as a m ∼ a t , b m ∼ c t , and c m ∼ b t . The pressure evolution of the unit cell parameters indicates anisotropic compression with β a = β b ≥ β c in the tetragonal phase and becomes more anisotropic with β a ≪ β b < β c in the monoclinic phase. The pressure-volume equations of state of both phases have been obtained by second order Birch-Murnaghan equations of state, and the bulk moduli are 122 and 127 GPa for tetragonal and monoclinic phases, respectively. The temperature dependent unit cell parameters show nearly isotropic expansion, with marginally higher expansion along the c-axis compared to the a- and b-axes. The tetragonal to monoclinic phase transition occurs with a reduction of unit cell volume of about 1.1% while the reduction of unit cell volume up to 6 K is only about 0.6%. The fitting of temperature dependent unit cell volume by using the Einstein model of phonons indicates the Einstein temperature is about 266(18) K.

Monazite-type BiPO 4 , LaPO 4 , CePO 4 , and PrPO 4 have been studied under high pressure by ab initio simulations and Raman spectroscopy measurements in the pressure range of stability of the monazite structure. A good agreement between experimental and theoretical Raman-active mode frequencies and pressure coefficients has been found which has allowed us to discuss the nature of the Raman-active modes. Besides, calculations have provided us with information on how the crystal structure is modified by pressure. This information has allowed us to determine the equation of state and the isothermal compressibility tensor of the four studied compounds. In addition, the information obtained on the polyhedral compressibility has been used to explain the anisotropic axial compressibility and the bulk compressibility of monazite phosphates. Finally, we have carried out a systematic discussion on the high-pressure behavior of the four studied phosphates in comparison to results of previous studies.

The effects of high pressure on the crystal structure of orthorhombic (Pnma) perovskite-type cerium scandate were studied in situ under high pressure by means of synchrotron X-ray powder diffraction, using a diamond-anvil cell. We found that the perovskite-type crystal structure remains stable up to 40 GPa, the highest pressure reached in the experiments. The evolution of unit-cell parameters with pressure indicated an anisotropic compression. The room-temperature pressure-volume equation of state (EOS) obtained from the experiments indicated the EOS parameters V 0 = 262.5(3) Å 3 , B 0 = 165(7) GPa, and B 0 ' = 6.3(5). From the evolution of microscopic structural parameters like bond distances and coordination polyhedra of cerium and scandium, the macroscopic behavior of CeScO 3 under compression was explained and reasoned for its large pressure stability. The reported results are discussed in comparison with high-pressure results from other perovskites.

β-Eucryptite (LiAlSiO 4 ) is known to show super-ionic conductivity above 700 K. We performed inelastic neutron scattering measurements in β-eucryptite over 300-900 K and calculated the phonon spectrum using classical molecular dynamics (MD) simulations. The MD simulations were used to interpret the inelastic neutron spectra at high temperatures. The calculated diffusion coefficient for Li showed superionic conduction above 1200 K in the perfect crystal. The presence of defects was found to enhance diffusion and lower the temperature for Li diffusion. The calculated trajectory of Li atoms at higher temperatures shows that preferential movement of the Li atom is along the hexagonal c-axis, which is further confirmed by the ab initio calculated activation energy profile for cooperative lithium ion displacements. The inter- and intra-channel correlated motion of Li along the hexagonal c-axis gives the minimum energy pathway for Li ion conduction in LiAlSiO 4 .

This work reports the structural and vibrational properties of nanocrystals of corundum-type In 2 O 3 (rh-In 2 O 3 ) at high pressures by using angle-dispersive x-ray diffraction and Raman scattering measurements up to 30 GPa. The equation of state and the pressure dependence of the Raman-active modes of the corundum phase in nanocrystals are in good agreement with previous studies on bulk material and theoretical simulations on bulk rh-In 2 O 3 . Nanocrystalline rh-In 2 O 3 showed stability under compression at least up to 20 GPa, unlike bulk rh-In 2 O 3 which gradually transforms to the orthorhombic Pbca (Rh 2 O 3 -III-type) structure above 12-14 GPa. The different stability range found in nanocrystalline and bulk corundum-type In 2 O 3 is discussed.

Herein we report the high-temperature crystal chemistry of K 2 Ce(PO 4 ) 2 as observed from a joint in situ variable-temperature X-ray diffraction (XRD) and Raman spectroscopy as well as ab initio density functional theory (DFT) calculations. These studies revealed that the ambient-temperature monoclinic (P2 1 /n) phase reversibly transforms to a tetragonal (I4 1 /amd) structure at higher temperature. Also, from the experimental and theoretical calculations, a possible existence of an orthorhombic (Imma) structure with almost zero orthorhombicity is predicted which is closely related to tetragonal K 2 Ce(PO 4 ) 2 . The high-temperature tetragonal phase reverts back to ambient monoclinic phase at much lower temperature in the cooling cycle compared to that observed at the heating cycle. XRD studies revealed the transition is accompanied by volume expansion of about 14.4%. The lower packing density of the high-temperature phase is reflected in its significantly lower thermal expansion coefficient (α V = 3.83 × 10 -6 K -1 ) compared to that in ambient monoclinic phase (α V = 41.30 × 10 -6 K -1 ). The coexistences of low- and high-temperature phases, large volume discontinuity in transition, and large hysteresis of transition temperature in heating and cooling cycles, as well as drastically different structural arrangement are in accordance with the first-order reconstructive nature of the transition. Temperature-dependent Raman spectra indicate significant changes around 783 K attributable to the phase transition. In situ low-temperature XRD, neutron diffraction, and Raman spectroscopic studies revealed no structural transition below ambient temperature. Raman mode frequencies, temperature coefficients, and reduced temperature coefficients for both monoclinic and tetragonal phases of K 2 Ce(PO 4 ) 2 have been obtained. Several lattice and external modes of rigid PO 4 units are found to be strongly anharmonic. The observed phase transition and structures as well as vibrational properties of both ambient- and high-temperature phases were complimented by DFT calculations. The optical absorption studies on monoclinic phase indicated a band gap of about 2.46 eV. The electronic structure calculations on ambient-temperature monoclinic and high-temperature phases were also carried out.

Herein we report the evolution of the crystal structure of K 3 Gd 5 (PO 4 ) 6 in the temperature range from 20 K to 1073 K, as observed from variable temperature X-ray diffraction and Raman spectroscopic studies. K 3 Gd 5 (PO 4 ) 6 has an open tunnel containing a three dimensional structure built by [Gd 5 (PO 4 ) 6 ] 3- ions which in turn are formed of PO 4 tetrahedra and GdO n (n = 8 and 9) polyhedra. The empty tunnels in the structure are occupied by K + ions and maintain charge neutrality in the lattice. Evolution of unit cell parameters with temperature shows a systematic increase with temperature. The average axial thermal expansion coefficients between 20 K and 1073 K are: α a = 10.6 × 10 -6 K -1 , α b = 5.5 × 10 -6 K -1 and α c = 16.4 × 10 -6 K -1 . The evolution of distortion indices of the various coordination polyhedra with temperature indicates a gradual decrease with increasing temperature, while those of Gd2O 9 and K2O 8 polyhedra show opposite trends. The overall anisotropy of the lattice thermal expansion is found to be controlled largely by the effect of temperature on GdO n polyhedra and their linkages. Temperature dependent Raman spectroscopic studies indicated that the intensities and wavenumbers of most of the Raman modes decrease continuously with increasing temperature. Anharmonic analyses of Raman modes indicated that the lattice, rigid translation and librational modes have larger contributions towards thermal expansion of K 3 Gd 5 (PO 4 ) 6 compared to high frequency internal modes. The temperature and field dependent magnetic measurements indicated no long range ordering down to 2 K and the observed effective magnetic moment per Gd 3+ ion and the Weiss constant are 7.91 μ B and 0.38 K, respectively.

Herein we report results of pressure- and temperature-dependent Raman scattering studies on Pr 2 Ti 2 O 7 . Pressure-dependent studies performed up to 23 GPa suggest a reversible phase transition above 15 GPa with subtle changes. Temperature-dependent investigations performed in the range of 77-1073 K showed anomalous temperature dependence of some of the Raman modes. Temperature-dependent X-ray diffraction data indicated no structural transition but nonlinear expansion of unit-cell parameters with increasing temperature. With increasing temperature, the structure dilates anisotropically, and volume of coordination polyhedra around all the atoms expands. Also with increasing temperature the distortions in coordination polyhedra around all the atoms decrease, and appreciable decrease is observed in Pr(1)O 10 and Pr(3)O 9 units. The pressure evolution of Raman-mode frequencies was analyzed for both ambient as well as high-pressure phases, and mode Grüneisen parameters for ambient pressure phase were obtained. The temperature evolution of Raman-mode frequencies was analyzed to obtain the explicit and implicit anharmonic components, and it was found that some of the modes attributable to TiO 6 octahedra and PrO n polyhedra have dominating explicit anharmonic component. Comparison of the structural data with the temperature dependence of Raman modes suggests that the anomalous behavior in Raman modes is due to phonon-phonon interaction.

Variable-temperature Raman spectroscopic and synchrotron X-ray diffraction studies were performed on BaTe2O6 (orthorhombic, space group: Cmcm), a mixed-valence tellurium compound with a layered structure, to understand structural stability and anharmonicity of phonons. The structural and vibrational studies indicate no phase transition in it over a wider range of temperature (20 to 853 K). The structure shows anisotropic expansion with coefficients of thermal expansion in the order αb ≫ αa > αc, which was attributed to the anisotropy in bonding and structure of BaTe2O6. Temperature evolution of Raman modes of BaTe2O6 indicated a smooth decreasing trend in mode frequencies with increasing temperature, while the full width at half-maximum (fwhm) of all modes systematically increases due to a rise in phonon scattering processes. With the use of our earlier reported isothermal mode Grüneisen parameters, thermal properties such as thermal expansion coefficient and molar specific heat are calculated. The pure anharmonic (explicit) and quasiharmonic (implicit) contribution to the total anharmonicity is delineated and compared. The temperature dependence of phonon mode frequencies and their fwhm values are analyzed by anharmonicity models, and the dominating anharmonic phonon scattering mechanism is concluded in BaTe2O6. In addition to the lattice modes, several external modes of TeOn (n = 5, 6) are found to be strongly anharmonic. The ab initio electronic structure calculations indicated BaTe2O6 is a direct band gap semiconductor with gap energy of ∼2.1 eV. Oxygen orbitals, namely, O-2p states in the valence band maximum and the sp-hybridized states in the conduction band minimum, are mainly involved in the electronic transitions. In addition a number of electronic transitions are predicted by the electronic structure calculations. Experimental photoluminescence results are adequately explained by the ab initio calculations. Further details of the structural and vibrational properties are explained in the manuscript.

We report composition dependent structure evolution from SrTiO3 to SrFe0.5Ta0.5O3 by powder X-ray and neutron diffraction studies of SrTi1-2xFexTaxO3 (0.00 ≤ × ≤ 0.50) compositions. Structural studies reveal cubic (Pm3m) perovskite-type structure of the parent SrTiO3 for x up to 0.075 and cation disordered orthorhombic (Pbnm) perovskite-type structure for x ≥ 0.33. A biphasic region consisting of a mixture of cubic and orthorhombic structures is found in the range for 0.10 ≤ × ≤ 0.25. Dielectric studies reveal transformation from a normal dielectric to relaxor like properties with increasing Fe(3+) and Ta(5+) concentration. Dielectric response is maximum at x = 0.33 in the series. The results establish a protocol for designing new lead-free relaxor materials based on the co-substitution of Fe(3+) and Ta(5+) for Ti(4+) in SrTiO3. A complex interplay of strain effects arising from distribution of cations at the octahedral sites of the perovskite structure controls the dielectric properties.

The high-pressure crystal structure, lattice-vibrations, and electronic band structure of BiSbO4 were studied by ab initio simulations. We also performed Raman spectroscopy, infrared spectroscopy, and diffuse-reflectance measurements, as well as synchrotron powder X-ray diffraction. High-pressure X-ray diffraction measurements show that the crystal structure of BiSbO4 remains stable up to at least 70 GPa, unlike other known MTO4-type ternary oxides. These experiments also give information on the pressure dependence of the unit-cell parameters. Calculations properly describe the crystal structure of BiSbO4 and the changes induced by pressure on it. They also predict a possible high-pressure phase. A room-temperature pressure-volume equation of state is determined, and the effect of pressure on the coordination polyhedron of Bi and Sb is discussed. Raman- and infrared-active phonons were measured and calculated. In particular, calculations provide assignments for all the vibrational modes as well as their pressure dependence. In addition, the band structure and electronic density of states under pressure were also calculated. The calculations combined with the optical measurements allow us to conclude that BiSbO4 is an indirect-gap semiconductor, with an electronic band gap of 2.9(1) eV. Finally, the isothermal compressibility tensor for BiSbO4 is given at 1.8 GPa. The experimental (theoretical) data revealed that the direction of maximum compressibility is in the (0 1 0) plane at ∼33° (38°) to the c-axis and 47° (42°) to the a-axis. The reliability of the reported results is supported by the consistency between experiments and calculations.

The compression behavior of zircon-type samarium orthovanadate, SmVO4, has been investigated using synchrotron-based powder x-ray diffraction and ab initio calculations of up to 21 GPa. The results indicate the instability of ambient zircon phase at around 6 GPa, which transforms to a high-density scheelite-type phase. The high-pressure phase remains stable up to 21 GPa, the highest pressure reached in the present investigations. On pressure release, the scheelite phase is recovered. The crystal structure of the high-pressure phase and the equations of state for the zircon- and scheelite-type phases have been determined. Various compressibilities, such as the bulk, axial and bond compressibilities, estimated from the experimental data are found to be in good agreement with the results obtained from theoretical calculations. The calculated elastic constants show that the zircon structure becomes mechanically unstable beyond the transition pressure. Overall there is good agreement between the experimental and theoretical findings.

In this manuscript we report crystal structure of a new complex binary phosphate K2Ce(4+)(PO4)2 in K2O-P2O5-CeO2 system prepared by solid state reaction at moderate temperature conditions. The prepared material was characterized by powder X-ray diffraction using lab source and synchrotron radiation as well as thermal analyses, Raman scattering, FTIR, and X-ray photoelectron spectroscopic studies. The crystal structure of the compound has been determined from powder XRD data by ab initio structure solution in direct space followed by Rietveld refinements. K2Ce(PO4)2 crystallizes in a monoclinic (P21/n) lattice with unit cell parameters: a = 9.1060(4), b = 10.8160(5), c = 7.6263(4) Å, β = 111.155(2)°, V = 700.50(6) Å(3). The unit cell contains two distinguishable PO4 tetrahedra and one CeO8 distorted square anti-prism. Raman spectroscopy confirmed the presence of isolated PO4(3-) groups in the structure. These PO4 tetrahedra are connected to one CeO8 polyhedra by sharing one edge and three other CeO8 polyhedra by sharing corners to form the three dimensional structure and empty channels parallel to a-axis. The channels are occupied by two crystallographically distinguishable K(+) ions which maintain the charge neutrality. Contrast to the earlier reported composition K4Ce2P4O15, this study revealed the composition in actual is K4Ce2P4O16 with Ce in 4+ oxidation state and is also supported by X-ray photoelectron spectroscopic and X-ray absorption near edge structure studies. Differential scanning calorimetric studies revealed a structural transition around 525 °C which reverts on cooling with a large thermal hysteresis. At higher temperature it undergoes a loss of oxygen atom and subsequently loss of phosphorus as P2O5. These thermal effects are also supported by in situ high temperature XRD studies. Finally the crystal chemistry of complex phosphates with tetravalent cations is also discussed.

We report a combined experimental and theoretical study of melilite-type germanate, Sr2ZnGe2O7, under compression. In situ high-pressure X-ray diffraction and Raman scattering measurements up to 22 GPa were complemented with first-principles theoretical calculations of structural and lattice dynamics properties. Our experiments show that the tetragonal structure of Sr2ZnGe2O7 at ambient conditions transforms reversibly to a monoclinic phase above 12.2 GPa with ∼1% volume drop at the phase transition pressure. Density functional calculations indicate the transition pressure at ∼13 GPa, which agrees well with the experimental value. The structure of the high-pressure monoclinic phase is closely related to the ambient pressure phase and results from a displacive-type phase transition. Equations of state of both tetragonal and monoclinic phases are reported. Both of the phases show anisotropic compressibility with a larger compressibility in the direction perpendicular to the [ZnGe2O7](2-) sheets than along the sheets. Raman-active phonons of both the tetragonal and monoclinic phases and their pressure dependences were also determined. Tentative assignments of the Raman modes of the tetragonal phase were discussed in the light of lattice dynamics calculations. A possible irreversible second phase transition to a highly disordered or amorphous state is detected in Raman scattering measurements above 21 GPa.

A titania mineral (obtained from East coast, Orissa, India) was investigated by X-ray diffraction (XRD), photoacoustic spectroscopy (PAS), Raman and Electron Paramagnetic Resonance (EPR) studies. XRD studies indicated the presence of rutile (91%) and anatase (9%) phases in the mineral. Raman investigation supported this information. Both rutile and anatase phases have tetragonal structure (rutile: space group P4(2)/mnm, a=4.5946(1) Å, c=2.9597(1) Å, V=62.48(1) (Å)(3), Z=2; anatase: space group I4(1)/amd, 3.7848(2) Å, 9.5098(11) Å, V=136.22(2) (Å)(3), Z=4). The deconvoluted PAS spectrum showed nine peaks around 335, 370, 415,485, 555, 605, 659, 690,730 and 785 nm and according to the ligand field theory, these peaks were attributed to the presence of V(4+), Cr(3+), Mn(4+) and Fe(3+) species. EPR studies revealed the presence of transition metal ions V(4+)(d(1)), Cr(3+)(d(3)), Mn(4+)(d(3)) and Fe(3+)(d(5)) at Ti(4+) sites. The EPR spectra are characterized by very large crystal filed splitting (D term) and orthorhombic distortion term (E term) for multiple electron system (s>1) suggesting that the transition metal ions substitute the Ti(4+) in the lattice which is situated in distorted octahedral coordination of oxygen. The possible reasons for observation of unusually large D and E term in the EPR spectra of transition metal ions (S=3/2 and 5/2) are discussed., (Copyright © 2014 Elsevier B.V. All rights reserved.)

In this work we report the metastability and the energetics of the phase transitions of three different polymorphs of BiPO4, namely trigonal (Phase-I, space group P3(1)21), monoclinic monazite-type (Phase-II, space group P2(1)/n) and SbPO4-type monoclinic (Phase-III, space group P2(1)/m) from ambient and non-ambient temperature powder XRD and neutron diffraction studies as well as ab initio density functional theory (DFT) calculations. The symmetry ambiguity between P2(1) and P2(1)/m of the high temperature polymorph of BiPO4 has been resolved by a neutron diffraction study. The structure and vibrational properties of these polymorphs of the three polymorphs have also been reported in detail. Total energy calculations have been used to understand the experimentally observed metastable behavior of trigonal and monazite-type BiPO4. Interestingly, all of the three phases were found to coexist after heating a single phasic trigonal BiPO4 to 773 K. The irreversible nature of these phase transitions has been explained by the concepts of the interplay of the structural distortion, molar volume and total energy.

A new wolframite-type polymorph of InVO4 is identified under compression near 7 GPa by in situ high-pressure (HP) X-ray diffraction (XRD) and Raman spectroscopic investigations on the stable orthorhombic InVO4. The structural transition is accompanied by a large volume collapse (ΔV/V = -14%) and a drastic increase in bulk modulus (from 69 to 168 GPa). Both techniques also show the existence of a third phase coexisting with the low- and high-pressure phases in a limited pressure range close to the transition pressure. XRD studies revealed a highly anisotropic compression in orthorhombic InVO4. In addition, the compressibility becomes nonlinear in the HP polymorph. The volume collapse in the lattice is related to an increase of the polyhedral coordination around the vanadium atoms. The transformation is not fully reversible. The drastic change in the polyhedral arrangement observed at the transition is indicative of a reconstructive phase transformation. The HP phase here found is the only modification of InVO4 reported to date with 6-fold coordinated vanadium atoms. Finally, Raman frequencies and pressure coefficients in the low- and high-pressure phases of InVO4 are reported.

Zircon-type PrVO4 and SmVO4 have been studied by high-pressure Raman spectroscopy up to 17 GPa. The occurrence of phase transitions has been detected when compression exceeds 6 GPa. The transformations are not reversible. Raman spectra of the high-pressure phases show similarities with those expected for a monazite-type phase in PrVO4 and a scheelite-type phase in SmVO4.The high-pressure phases have been also synthesized using a large-volume press and recovered at ambient conditions. X-ray diffraction measurements of the metastable products recovered after decompression confirms the monazite (PrVO4) and scheelite (SmVO4) structures of the high-pressure phases. Based upon optical properties of the reported new polymorphs, novel applications for rare-earth vanadates are proposed, including photocatalytic hydrogen production.

Recently colossal volume thermal expansion has been observed in the framework compounds Ag(3)Co(CN)(6) and Ag(3)Fe(CN)(6). We have measured phonon spectra using neutron time-of-flight spectroscopy as a function of temperature and pressure. Ab initio calculations were carried out for the sake of analysis and interpretation. Bonding is found to be very similar in the two compounds. At ambient pressure, modes in the intermediate frequency part of the vibrational spectra in the Co compound are shifted slightly to higher energies as compared to the Fe compound. The temperature dependence of the phonon spectra gives evidence for a large explicit anharmonic contribution to the total anharmonicity for low-energy modes below 5 meV. We have found that modes are mainly affected by the change in size of the unit cell, which in turn changes the bond lengths and vibrational frequencies. Thermal expansion has been calculated via the volume dependence of phonon spectra. Our analysis indicates that Ag phonon modes within the energy range 2-5 meV are strongly anharmonic and major contributors to thermal expansion in both systems. The application of pressure hardens the low-energy part of the phonon spectra involving Ag vibrations and confirms the highly anharmonic nature of these modes.

A series of samples with composition Gd(2-y)Ce(y)Zr(2)O(7) (0.0 ≤ y ≤ 2.0) were prepared by the gel combustion method followed by high-temperature reduction. The details of the structural variations as a function of the composition, temperature, and oxygen stoichiometry have been investigated by X-ray diffraction (XRD), high-temperature XRD (HT-XRD), and thermogravimetry. A complete solubility of Gd(3+) in Ce(2)Zr(2)O(7) and Ce(2)Zr(2)O(8) could be achieved by this adaptive preparative method. Analysis of the XRD data revealed a sequential variation of the structural features with oxygen stoichiometry as well as Gd(3+) contents in these compositions. The variation in the unit cell parameter along the compositions has a strong influence on the oxygen uptake behavior in the Gd(2-y)Ce(y)Zr(2)O(7) system, as observed from the thermogravimetric and HT-XRD studies. The preparation and stability of various metastable phases in Gd-Ce-Zr-O have been addressed in detail. The details of the study will be useful for the design and application of a potential redox catalyst and an oxygen storage capacitor.

We present the structural and magnetic properties of Zn(0.95-x)Co(0.05)Al(x)O (x = 0.0 to 0.1) nanoparticles, synthesized by a novel sol-gel route followed by pyrolysis. Powder X-ray diffraction data confirms the formation of a single phase wurtzite type ZnO structure for all the compositions. The Zn(0.95)Co(0.05)O nanoparticles show diamagnetic behavior at room temperature. However, when Al is co-doped with Co with x = 0.0 to 0.10 in Zn(0.95-x)Co(0.05)Al(x)O, a systematic increase in ferromagnetic moment is observed up to x = 0.07 at 300 K. Above x = 0.07 (e.g. for x = 0.10) a drastic decrease in ferromagnetic nature is observed which is concomitant with the segregation of poorly crystalline Al rich ZnO phase as evidenced from TEM studies. Theoretical studies using density functional calculations on Zn(0.95-x)Co(0.05)Al(x)O suggest that the partial occupancy of S2 states leads to an increased double exchange interaction favoring the ferromagnetic ground states. Such ferromagnetic interactions are favorable beyond a threshold limit. At a high level doping of Al, the exchange splitting is reduced, which suppresses the ferromagnetic ordering.