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Coupled vibrations of Li and SiO4 enable Li-ion diffusion in amorphous Li2Si2O5.

Low dimensional sodium diffusion preferentially through interstitial sites.

Phase pure Na

The high pressure stability of α-Cu3V2O8 has been investigated via complementary high pressure synchrotron X-ray diffraction experiments and theoretical density functional theory calculations. The results of both experiment and theory are in close agreement. The main result of this work is that α-Cu3V2O8 undergoes a pressure-induced chemical decomposition into CuO and V2O5 at a modest pressure of ∼1.35 GPa according to the experimental observations, and at ∼2.45 GPa according to the calculations. The decomposition is investigated with enthalpy calculations and one of the main driving factors is the stability of the octhedral oxygen-coordination of the metal atoms in the decompositon products. Additionally, we determine the bulk modulus of α-Cu3V2O8 to be the lowest of all known vanadates, at 52(2) GPa, due to its crystal structure, and determine the corresponding isothermal compressibility tensor.

We have performed quasielastic neutron scattering (QENS) experiments up to 1243 K and ab initio molecular dynamics (AIMD) simulations to investigate the Na diffusion in various phases of NaAlSiO4 (NASO), namely, low-carnegieite (L-NASO; trigonal), high-carnegieite (H-NASO; cubic) and nepheline (N-NASO; hexagonal) phases. The QENS measurements reveal Na ions localized diffusion behavior in L-NASO and N-NASO, but long-range diffusion behavior in H-NASO. The AIMD simulation supplemented the QENS measurements and showed that excess Na ions in H-NASO enhance the host network flexibility and activate the AlO4/SiO4 tetrahedra rotational modes. These framework modes enable the long-range diffusion of Na across a pathway of interstitial sites. The simulations also show Na diffusion in Na-deficient N-NASO through vacant Na sites along the hexagonal c-axis.

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 Al0.5Ga0.5FeO3 (ALGF), a lead-free magnetoelectric material, carried out using variable temperature dependent powder neutron diffraction and Raman spectroscopy. Neutron diffraction studies suggest that Al3+ ions are distributed in one tetrahedrally (BO4) and three octahedrally (BO6) coordinated sites of the orthorhombic (Pc21n) 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 (TN), and that is reflected in the low temperature powder neutron diffraction data. An antiferromagnetic type arrangement of Fe3+ ions with net magnetic moment of 0.13 μB/Fe3+ 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. BOn (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.

Ultraviolet emitting Gd3+ doped phosphors have recently attracted significant attention from the optical science community owing to their important applications in the areas of photothermal therapy, transilluminators and sensitizer based photoluminescent phosphors (PLPs). It was found in several previous studies that the annealing temperature and water content has a profound influence on the emission intensity and luminescence lifetime of PLPs. In this context, we have synthesized Th(C2O4)2·6H2O:Gd3+ (TOHG) and characterized it using photoluminescence (PL) and electron paramagnetic resonance (EPR) spectroscopy. Emission spectroscopy showed the presence of a singular peak at 311 nm due to an electric dipole transition, suggesting the extremely low symmetry around Gd3+ in TOHG. Luminescence lifetime measurements suggested two different local environments of Gd3+ in TOHG; the shorter lived one (S) is closer to oxygen vacancies and the longer lived one (L) is further away from the oxygen vacancies. The time resolved emission spectrum shows a slightly different peak profile for S and L. The EPR spectra of TOHG suggested that Gd3+ is situated in a nearly eight-fold coordination of oxygen (cubic coordination) at the Th4+ ion. Furthermore, we carried out thermal treatment of TOHG at 250 °C which led to Th(C2O4)2·2H2O:Gd3+(TODG) which on further annealing at 300 °C was converted into anhydrous thorium oxalate (TOAG). X-ray diffraction analysis suggested a triclinic lattice structure for TOHG and a monoclinic and orientationally disordered structure for TOAG. The PL emission intensity and excited state lifetime were found to be greater for TOAG due to the absence of the non-radiative transition provided by water molecules. The EPR spectra of TOAG suggested lowering of the site symmetry to rhombic symmetry in TOAG. This is in agreement with our structural analysis. Such a complete spectrum of work wherein the structure, excited state dynamics and spin properties of Gd3+ were investigated will open up a new avenue into gadolinium photophysics, particularly for researchers to explore actinide oxalate with a view to thermally tuning its properties.

This work highlights the role of defects, excitation energy, and energy transfer in realizing color-tunable Green → Red → White from a single phosphor. In this work, thorium-based K2Th(PO4)2:Eu3+ (KTPE) phosphor was explored for the first time. Rietveld refinement of the powder XRD data showed the formation of a phase pure compound and a reduction in the lattice parameters upon europium doping. Undoped K2Th(PO4)2 (KTP) depicted bright green emission under UV irradiation due to the presence of oxygen vacancies (OVs) in the lattice. Further upon host excitation with 250 nm, complete host-to-Eu3+ energy transfer occurred to produce red emission due to 5D0 → 7F2 transition of europium ions with high asymmetry and a branching ratio with an internal quantum yield (IQY) of 53%. On the other hand, under europium excitation with 395 nm, emission spectra displayed host as well as europium emission, producing near white emission with an IQY of 49%. There were slightly more non-radiative channels under dopant excitation compared to the host. Evaluation of the color coordinates and correlated color temperature suggested that the white light was neutral with perfect amalgamation of red, green, and blue. Lifetime spectroscopy suggested a monoexponential decay behavior owing to the stabilization of Eu3+ at Th4+ ion, leading to the formation of oxygen vacancies and antisite defects. These defects together with the size and charge mismatch produce substantial lattice strain in the KTPE. Such lattice strain combined with the host crystalline field leads to substantial Stark splitting, in turn to a reduction in the symmetry of KTPE. This work gives a complete flavor of structure–property correlation in designing a new color tunable phosphor by controlling defects, excitation energy, and host-to-dopant energy transfer in the complex phosphate host consisting of alkali and actinide ions.

High pressure behavior of K2Zr(PO4)2 has been investigated using combined Raman spectroscopy, x-ray diffraction and first principles calculations. Our studies show that the ambient trigonal phase (P 3 ¯ ) is stable only up to ∼0.5 GPa. Upon compression, the compound shows clear evidences of two first order structural phase transformations at 0.5 GPa and 3 GPa, as observed using x-ray diffraction and spectroscopic measurements. Across these pressures, the spectroscopic and theoretical data show softening of phosphate internal vibrations and collapse of inter-polyhedral bond angle Zr–O–P respectively, which indicate that polyhedral distortion drives the structural phase transitions. The second high pressure phase, observed above 3 GPa, is stable up to the highest pressure (i.e. 21 GPa) in the experiments. However, upon decompression, it reverts back to the ambient phase, with traces of first high pressure phase, on complete release of pressure.

This work discusses about synthesis of relatively unexplored carnegieite type NaAlSiO4 and designing it as efficient europium ion doped red phosphor. The synthesized powder has been systematically characterized using powder X-ray diffraction (XRD), Raman spectroscopy, Thermogravimetry - Differential Thermal Analysis (TG-DTA) and photoluminescence (PL) techniques. The effects of Eu3+ concentration on the photoluminescence properties were investigated. Concentration dependent emission spectroscopic studies suggested quenching at 0.5% doping level due to multipolar interaction. Stark splitting analyses revealed that the point group around Na+ ions decreases with Eu3+ doping and it is found to be C4v in Eu3+ doped NaAlSiO4. The asymmetry ratio was found to increase monotonically with europium concentration due to increasing density of sodium ion vacancies (VNa) which further distort the structure of NaAlSiO4. Lifetime spectroscopic studies suggested stabilization of europium ion at sodium site in two different environment; one nearer to charge compensating defect and one at far off distance. Judd-Ofelt analysis suggested asymmetric and highly polarizable environment around europium ion lead to enhanced red emission. The structure-property correlations indicate the potential of such structures for enhanced luminescence, scintillation and other optoelectronic properties.

Bismuth based host materials for doping rare earth ions gained considerable interest due to the possibility of forming crystalline homogeneous solid solution phases over a wide range and their excellent luminescent properties. In the present manuscript, a facile synthesis method for preparation of nanocrystalline NaBi(MoO4)2 and their optical properties are presented. The influence of various synthetic parameters such as precursor concentration, reaction time and reaction temperature on phase formation have been investigated and revealed that these nanoparticles can be synthesized even at 5 °C within 5 min with a Bi/Mo ratio of 1:4. All the doped and undoped samples show scheelite type tetragonal structure. Formation of solid solution between NaBi(MoO4)2 and NaEu(MoO4)2 over the complete range of compositions could be achieved. A systematic decrease in the unit cell parameters is observed with increasing concentration of Eu3+ ion in NaBi1–xEux(MoO4)2. Investigations on the luminescence properties of europ...

Zircon-type $\mathrm{PrV}{\mathrm{O}}_{4}$ has been studied at high pressures and room temperature by means of synchrotron powder x-ray diffraction. At room temperature, we observed the previously known zircon-to-monazite phase transition at 5.5(4) GPa and a second phase transition from monazite to a monoclinic structure at 12.7(8) GPa, which we identified as a $\mathrm{PbW}{\mathrm{O}}_{4}$-III-type phase. This conclusion is supported by our ab initio calculations, which also predict a scheelite-type phase to be stable at high pressure. Motivated by this finding, we subjected zircon-type $\mathrm{PrV}{\mathrm{O}}_{4}$ samples to high pressure (7 GPa) and temperature (600, 800, and 1000 \ifmmode^\circ\else\textdegree\fi{}C) in a Paris-Edinburgh large volume press cell. As a result, we could synthesize a scheelite-type phase of $\mathrm{PrV}{\mathrm{O}}_{4}$, which was recovered in a metastable state at room conditions, and the unit-cell parameters and the atomic positions have been determined. We also report a substantial increase of the scheelite proportion with time by comparing synthesis of different durations, at the same pressure-temperature conditions. The transition mechanisms of the zircon-, monazite-, and scheelite-type structures as well as the role of kinetic barriers will be discussed. Information on the compressibility of different polymorphs will be reported too.

In this chapter, a brief introduction to fluorine and fluorides with an emphasis on their preparation is presented. The general procedures for preparation of fluorides, in particular inorganic fluorides, are explained. Preparation of F2 and HF, and their properties are discussed in light of their importance to fluoride chemistry. Since the fluorine and fluorides are hazardous, corrosive and reactive materials, the need for special experimental conditions are explained. In addition to F2 and HF, other fluorinating reagents are also briefly mentioned. The common experimental procedures used for fluorides and oxyfluorides, like solid-state reactions, solid–liquid and solid–vapor reactions as well as non-conventional fluorides preparation methods like sol–gel, hydrothermal, displacement and precipitation reactions are presented. The specific examples of preparation and properties of fluorides and oxyfluorides are presented at the end. The chapter is concluded with a mention of the scope of innovative needs in preparative fluorine chemistry.

The structural stability of metastable Zn4V2O9 has been investigated using in-situ Raman spectroscopy at high pressures up to 25 GPa at ambient temperature and high temperature, upto 1073 K at ambient pressure. Pressure dependent Raman studies show pressure induced structural phase transitions around 5 GPa and 14 GPa, followed by an onset of structural disorder above17 GPa. The transition observed at 5 GPa is reversible while the transition to disordered phase is irreversible. The changes in Raman spectra above 14 GPa point out towards an increase in coordination of vanadium atoms. The disordered high pressure phase could be retrieved upon release of pressure. Temperature dependent Raman studies indicated many anomalous phonon modes but none of them show softening behaviour with pressure. The anharmonicities of Raman modes have been estimated for all the observed Raman modes in the ambient monoclinic phase and interestingly most of the modes show positive explicit anharmonic contribution.

We report a study of the high-pressure structural behavior of ZnV2O6, Zn2V2O2, and Zn3V2O8, which has been explored by means of synchrotron powder x-ray diffraction. We found that ZnV2O6 and Zn3V2O8 remain in the ambient-pressure structure up to 15 GPa. In contrast, in the same pressure range, Zn2V2O2 undergoes three phase transitions at 0.7, 3.0, and 10.8 GPa, respectively. Possible crystal structures for the first and second high-pressure phases are proposed. Reasons for the distinctive behavior of Zn2V2O2 are discussed. The compressibility of the different polymorphs has been determined. The response to pressure is found to be anisotropic in all the considered compounds and the room-temperature equations of state have been determined. The bulk moduli of Zn2V2O6 (129(2) GPa) and Zn2V2O8 (120(2) GPa) are consistent with a structural framework composed of compressible ZnO6 octahedra and uncompressible VO4 tetrahedra. In contrast, Zn2V2O2 is highly compressible with a bulk modulus of 58(9) GPa, which is almost half of the bulk modulus of the other two vanadates. The large compressibility of Zn2V2O2 and its sequence of structural transitions are related to the fact that this material is less dense than the other zinc vanadates and to the penta-coordination of Zn atoms by oxygen atoms in Zn2V2O2. A comparison to the high-pressure behavior of related compounds is presented. (C) 2020 Elsevier B.V. All rights reserved., This work was supported by the Spanish Ministry of Science, Innovation and Universities under Grants MAT2016-75586-C4-1-P and PGC2018-097520-A-I00, and by Generalitat Valenciana under Grant Prometeo/2018/123 (EFIMAT). D.S.-P. also acknowledges the financial support of Spanish MINECO for the RyC2014-15643 Ramón y Cajal Grant. CP thanks the financial support from Spanish MINECO under FIS2017-83295-P project. The authors thank ALBA synchrotron for providing beam-time for the XRD experiments (Projects 2016081779 and 2019023362).

The dynamics of Na ions in amorphous $\mathrm{N}{\mathrm{a}}_{2}\mathrm{S}{\mathrm{i}}_{2}{\mathrm{O}}_{5}$, a potential solid electrolyte material for Na battery, have been investigated by employing. the quasielastic neutron scattering (QENS) technique in the temperature range 300 to 748 K. To understand the diffusion pathways and relaxation timescales of Na ionic diffusion, the experimental studies are complemented by ab initio and force-field molecular dynamics simulations. The QENS data are fairly well described by a jump-diffusion model with a mean jump length of about 3 \AA{} and residence time about 9 ps. Our molecular dynamics simulations have predicted that the diffusion of $\mathrm{N}{\mathrm{a}}^{+}$ ions occurs in the amorphous phase of $\mathrm{N}{\mathrm{a}}_{2}\mathrm{S}{\mathrm{i}}_{2}{\mathrm{O}}_{5}$ while absent in the crystalline orthorhombic phase even up to 1100 K. The molecular dynamics simulations have revealed that in the amorphous phase, due to different orientations of silicon polyhedral units, several accessible pathways are opened up for $\mathrm{N}{\mathrm{a}}^{+}$ diffusions. These pathways are not available in the crystalline phase of $\mathrm{N}{\mathrm{a}}_{2}\mathrm{S}{\mathrm{i}}_{2}{\mathrm{O}}_{5}$ due to rigid spatial arrangement of silicon polyhedral units.

We report on investigations of phonons and lithium diffusion in LiAlO$_2$ based on inelastic neutron scattering (INS) measurements of the phonon density of states (DOS) in {\gamma}-LiAlO$_2$ from 473 K to 1073 K, complemented with ab-initio molecular dynamics (AIMD) simulations. We find that phonon modes related to Li vibrations broaden on warming as reflected in the measured phonon DOS and reproduced in simulations. Further, the AIMD simulations probe the nature of lithium diffusion in the perfect crystalline phase ({\gamma}-LiAlO$_2$), as well as in a structure with lithium vacancies and a related amorphous phase. Almost liquid-like super-ionic diffusion is observed in AIMD simulations of the three structures at high temperatures; with predicted onset temperatures of 1800 K, 1200 K, and 600 K in the perfect structure, vacancy structure and the amorphous phase, respectively. In the ideal structure, the Li atoms show correlated jumps; while simple and correlated jumps are both seen in the vacancy structure, and a mix of jumps and continuous diffusion occur in the amorphous structure. Further, we find that the Li-diffusion is favored in all cases by a large librational amplitude of the neighbouring AlO4 tetrahedra, and that the amorphous structure opens additional diffusion pathways due to a broad distribution of AlO4 tetrahedra orientations., Comment: 31 Pages, 14 figures

Negative thermal expansion material TaVO5 is recently reported to have pressure induced structural phase transition and irreversible amorphization at 0.2 and above 8 GPa, respectively. Here, we hav...

Herein we reported structural stability, vibrational and thermal properties of K2Ce[PO4]2, a relatively underexplored complex phosphate of tetravalent Ce4+ from in situ high-pressure Raman spectroscopic investigations up to ~ 28 GPa using a diamond anvil cell. The studies identified the soft phonons that lead to a reversible phase transformation above 8 GPa, and a phase coexistence of ambient (PI) and high pressure (PII) phases in a wider pressure region 6–11 GPa. From a visual representation of the computed eigen vector displacements, the Ag soft mode at 82 cm−1 is assigned as a lattice mode of K+ cation. Pressure-induced positional disorder is apparent from the substantial broadening of internal modes and the disappearance of low frequency lattice and external modes in phase PII above 18 GPa. Isothermal mode Gruneisen parameters γi of the various phonon modes are calculated and compared for several modes. Using these values, thermal properties such as average Gruneisen parameter, and thermal expansion coefficient are estimated as 0.47, and 2.5 × 10−6 K−1, respectively. The specific heat value was estimated from all optical modes obtained from DFT calculations as 314 J-mol−1 K−1. Our earlier reported temperature dependence of phonon frequencies is used to decouple the “true anharmonic” (explicit contribution at constant volume) and “quasi harmonic” (implicit contribution brought out by volume change) contributions from the total anharmonicity. In addition to the 81 cm−1 Ag lattice mode, several other lattice and external modes of PO43- ions are found to be strongly anharmonic.

In the present paper, thermodynamic properties of compounds of ZnO-V2O5 system have been reported. Phase pure compounds of ZnO-V2O5 system viz., ZnV2O6, Zn2V2O7, Zn3V2O8 and Zn4V2O9 have been prepared and characterized. The standard molar enthalpies of formation of these compounds at 298 K ( ∆ f H 298 o ) and their isobaric heat capacity were determined employing calorimetric techniques. From their enthalpy of formation and the temperature dependent molar heat capacity data, thermodynamic functions of ZnV2O6(s), Zn2V2O7(s), Zn3V2O8(s) and Zn4V2O9(s) compounds have been derived. From these derived thermodynamic parameters, the relative stabilities of the compounds were determined.

In this paper, we report the crystal structure and thermodynamic parameters of zirconolite (CaZrTi2O7) which is a potential ceramic matrix for incorporation of nuclear high-level waste. The room temperature structural studies revealed a two layer monoclinic (C2/c) structure for the compound. Thermal expansion behavior of zirconolite was investigated by the high-temperature XRD. The average thermal expansion coefficient of zirconolite, measured in the temperature range 298–1273 K, is found to be: a-axis (A) = 12.410(2) + 1.06(8) × 10−4 [T] + 3.1(5) × 10−8 [T]2/K, b-axis (A) = 7.252(1) + 6.2(4) × 10−5 [T] + 4.9(2.5) × 10−9 [T]2/K and c-axis (A) = 11.351(2) + 7.6(7) × 10−5 [T] + 3.2(4) × 10−8 [T]2/K. The standard molar enthalpy of formation of zirconolite (CaZrTi2O7, s) at 298.15 K determined employing high-temperature solution calorimetry is found to be − 3716.5 ± 3.7 kJ mol−1. The isobaric heat capacity of the CaZrTi2O7(s) has been derived from the enthalpy increment (H T–H 298.15) data. Based on the experimental data for the compound and the available thermodynamic data for the component oxides, thermodynamic functions like: $${\text{Cp}}_{\text{m}}^{\circ } ,S_{\text{m}}^{\circ } , H_{{{\text{m}},{\text{T}}}}^{0} , { }G_{\text{m,T}}^{\circ } , \left( {H_{\text{T}}^{\circ } - H_{298.15}^{\circ } } \right), - \left( {\frac{{G_{\text{T}}^{\circ } - H_{298.15}^{\circ } }}{T}} \right), \Delta_{\text{f}} H_{{{\text{m}},298.15}}^{\circ } \,\, {\text{and}}\,\,\Delta_{\text{f}} G_{{{\text{m}},{\text{T}}}}^{\circ }$$ for zirconolite (CaZrTi2O7, s) have been generated.

Zircon-type NdVO4 and scheelite-type PrVO4 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 PrVO4, a reversible phase transition to a PbWO4-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.

The exponential increase in global energy and clean energy demands lead to the necessity for development of efficient photocatalyst for hydrogen evolution. Herein, photocatalytic hydrogen evolution activity with nano-structured ZnFe2-xGaxO4 (0 ≤ x ≤ 2) solid solution samples synthesized by citrate-gel method at 550 °C is reported. The prepared materials have been characterized for their structure and optical properties. The formation of solid solution with spinel type structure in the complete range of composition and a systematically decreasing trend of unit cell parameters with increasing Ga3+ concentration is observed from powder XRD studies. The band gap of these solid solutions could be tuned from 1.9 to 3.1 eV by increasing the Ga3+ concentration. Without any co-catalyst, ZnFe2O4 shows poor catalytic activity (378 μmol h−1 g−1) for photocatalytic hydrogen evolution from water. Whereas its activity enhances with loading of co-catalyst and touches a maximum activity of 3089 μmol h−1 g−1 with loading 1 wt% of Pt. The catalytic activity increases systematically with increasing Ga concentration in the solid solutions and reaches to 3989 μmol h−1 g−1 for ZnGa2O4 nanomaterials. Electrochemical impedance spectroscopic and transient photocurrent measurements also support the observed increasing trend. The lower catalytic activity of ZnFe2O4 has been attributed to the faster e-h recombination due to lower band gap and inherent oxygen vacancies. A smaller amount of oxygen vacancies and segregated ZnO phase observed in Ga substituted ZnFe2O4 samples favor the e-h pair separation and that facilitate the hydrogen generation from water.

In this report we communicate the behavior of NdF3 and SrF2 doped in MSBR fluoride salt (78 mol% LiF–20 mol% ThF4 –2 mol% UF4) up to 750 °C as observed from DTA and XRD. The eutectic composition of the fuel salt exists as Li3ThF7 and LiF. Formation of phases on addition of SrF2 and NdF3 in the eutectic mixture resulted in marginal shift in the melting point and the system shifted to ternary phase field of Li3ThF7–LiF–NdF3/SrF2. Additionally, incorporation of SrF2 in fuel salt resulted in precipitation of compounds like SrThF6 and a phase in the quaternary Li–Sr–Th–F system.

In this article a brief review of recent progress on preparation and crystal chemistry of rare-earth phosphates have been presented. The article begins with a brief note on the rare-earths and then a description on preparation methods of nano and crystalline samples of binary and complex phosphates. The merits and demerits of the typical procedures are explained in brief. Subsequently the crystal chemistry of various rare-earth phosphates is explained. The diversified crystal chemistry of rare-earth phosphates with compositions as well as with external parameters like pressure and temperature are explained. Typical crystal chemistry of hydrated and anhydrous orthophosphates is also covered. A brief note on phosphates of tetravalent cerium is presented. The high temperature behaviors of complex and metastable phosphates are also discussed. Finally the prospects of rare-earth phosphates for optical and nuclear applications are mentioned in brief.

Herein we report preparation and crystal of Cd 4 Si 2 O 7 F 2 and details of its crystal structure, vibrational and electronic properties from powder XRD, Raman scattering, FTIR studies and ab initio density functional theory (DFT) calculations. Cd 4 Si 2 O 7 F 2 crystallizes in a monoclinic lattice (space group: P 2 1 / c ) with unit cell parameters: a = 7.3211(1), b = 10.2708(2), c = 10.9123(2) Ǻ and β = 107.172(2)°, V = 775.02(3) Ǻ 3 . Highly distorted (CdO,F) 6 and Si 2 O 7 units form the building units of the crystal structure. The structural and vibrational properties were investigated by using Raman and IR spectroscopic studies and supported by DFT calculations. Electronic structure calculations revealed an indirect band gap (∼1.8 eV) in Cd 4 Si 2 O 7 F 2 which reasonably agrees with that observed in DRUV measurements. For comparison DFT calculations on structure and electronic properties of Ca 4 Si 2 O 7 F 2 , were also carried out. Ca 4 Si 2 O 7 F 2 is also found to be an indirect band gap semiconductor, but the band gap is appreciably higher (∼4.6 eV). More diffused 5s orbital of Cd atoms in Cd 4 Si 2 O 7 F 2 lower the conduction band minima and hence show lower band gap.

The third section of closed loop Iodine Sulphur (IS) thermochemical cycle, dealing with HI x processing, suffers from low equilibrium decomposition of HI to hydrogen with a conversion value of only ∼22% at 700 K. Here, we report a significant enhancement in conversion of HI into hydrogen (up to ∼95%) using a zeolite membrane reactor for the first time. The all silica DDR (deca dodecasil rhombohedral) zeolite membrane with dense, interlocked structure was synthesized on the seeded clay alumina substrate by sonication mediated hydrothermal process. The synthesized membranes along with seed crystals were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) and energy dispersive X-ray spectroscopy (EDX). Corrosion studies were carried out by exposing the membrane samples to simulated HI decomposition reaction environment (at 450 °C) for different durations of time upto 200 h. The FESEM, EDX and XRD analyses indicated that no significant changes occurred in the morphology, composition and structure of the membranes. Iodine adsorption on to the membrane surface was observed which got increased with the exposure duration as confirmed by secondary ion mass spectrometry studies. A packed bed membrane reactor (PBMR) assembly was fabricated with integration of in-house synthesized zeolite membrane and Pt-alumina catalyst for carrying out HI decomposition studies. The tube side was chosen as reaction zone and the shell side as the permeation zone. The HI decomposition experiments were carried out for different values of temperature and feed flow rates. DDR zeolite based PBMR was found to enhance the single-pass conversion of HI up to ∼95%. The results indicate that for achieving optimal performance of PBMR, it should be operated with space velocities of 0.2–0.3 s −1 and temperature in the range of 650 K–700 K with permeate side vacuum of 0.12 kg/cm 2 . It is believed that the in-house developed zeolite PBMR shall be a potential technology augmentation in making the IS thermochemical cycle energy efficient.

β-Eucryptite (LiAlSiO4) 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 LiAlSiO4.

Herein we report results of pressure- and temperature-dependent Raman scattering studies on Pr2Ti2O7. 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)O10 and Pr(3)O9 units. The pressure evolution of Raman-mode frequencies was analyzed for both ambient as well as high-pressure phases, and mode Gruneisen parameters for ambient pressure phase were obtained. The t...

Novel self activated green phosphors; K 2 ZnV 2 O 7 (KZV) and K 2 MgV 2 O 7 (KMV) have been prepared and characterized in details. Both KZV and KMV were prepared by solid state reaction and systematically characterized by a number of techniques like X-ray diffraction (XRD), Fourier transformed Infrared (FTIR), Raman spectroscopy, Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), diffuse reflectance spectroscopy (DRS) and time resolved photoluminescence spectroscopy (TRPLS). XRD studies confirm tetragonal melilite type layered phase for both of them. The presence of pyrovanadate group in both is supported by FTIR and Raman spectroscopic investigations. Raman bands of KMV are blue shifted w.r.t to KZV which may be due to shorter bond lengths of MgO compared to ZnO and lower atomic weight of magnesium compared to zinc. DRS measurements indicated the optical band gap of KMV and KZV are 3.14 and 3.12 eV, respectively. Optical measurements on these samples show the emission of green color on ultra violet light irradiation. The origin of such emission is attributed to electronic transition (charge transfer) from filled 2p orbital of oxygen ion (O 2− ) to vacant 3d orbital of vanadium ion (V 5+ ). In KZV there are dual emission band (PL1 and PL2) which are respectively attributed to 3 T 2 → 1 A 1 and 3 T 1 → 1 A 1 transition, and the emission zone varies slightly compared to KMV. KZV emits bluish green (cool green) where KMV emits in yellowish green (warm green) region. This is indeed an important step towards realization of cost effective rare earth free luminescence material. It is also oberved that PL intensity of KZV is very high compared to KMV which is supported by the lifetime measurements.

We present a study of the different high-pressure polymorphs of $\mathrm{GdV}{\mathrm{O}}_{4}$ and its stability. Powder x-ray diffraction and Raman experiments show a phase transition from a zircon- to a scheelite-type structure taking place at 6.8(4) GPa. Ab initio density functional theory calculations support this conclusion. The equations of state of these two phases are reported. In addition, we studied the pressure evolution of the Raman modes for the zircon and scheelite phases, showing good agreement between calculations and experiments. For the sake of completeness, we performed optical-absorption measurements up to 16 GPa, showing a band-gap collapse at the transition point. Beyond 20 GPa a second phase transition to a monoclinic fergusonite structure takes place as a consequence of a mechanical instability. A third transition is observed at around 29.3 GPa in Raman experiments. According to our calculations, this fourth polymorph corresponds to an orthorhombic structure described by space group Cmca. This phase involves an increase of the atomic coordination number of vanadium and gadolinium. The results are compared to those reported on isomorphic compounds.

[EN] 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., The authors are thankful for the financial support of the Spanish Ministerio de Ciencia, Innovacion y Universidades, the Spanish Research Agency (AEI), and the European Fund for Regional Development (FEDER) under Grant MAT2016-75586-C4-1/2-P and of Generalitat Valenciana under Grant Prometeo/2018/123 (EFIMAT). J.A.S. acknowledges the "Ramon y Cajal" fellowship program (RYC-2015-17482) and Spanish Mineco Project FIS2017-83295-P. V.M. acknowledges the "Juan de la Cierva" program (FJCI-2016-27921) for financial support. Experiments were performed at the Materials Science and Powder Diffraction beamline of ALBA Synchrotron. The authors are thankful for the collaboration of ALBA staff and also thank D. Santamaria-Perez for fruitful discussions.

We have studied the electronic properties at ambient pressure and under high pressure of InVO4, InNbO4, and InTaO4 powders, three candidate materials for hydrogen production by means of photocatalytic water splitting using solar energy. A combination of optical absorption and resistivity measurements and band structure calculations have allowed us to determine that these materials are wide band-gap semiconductors with a band-gap energy of 3.62(5), 3.63(5), and 3.79(5) eV for InVO4, InNbO4, and InTaO4, respectively. The last two compounds are indirect band-gap materials, and InVO4 is a direct band-gap material. The pressure dependence of the band-gap energy and the electrical resistivity have been determined too. In the three compounds, the band gap opens under compression until reaching a critical pressure, where a phase transition occurs. The structural transition triggers a band-gap collapse larger than 1.2 eV in the three materials, being the abrupt decrease in the band-gap energy related to an increase in the pentavalent cation coordination number. The phase transitions also cause changes in the electrical resistivity, which can be correlated with changes induced by pressure in the band structure. An explanation to the reported results is provided based upon ab initio calculations. The conclusions attained are of significance for technological applications of the studied oxides.

We report here high-pressure--high-temperature Raman experiments performed on ${\text{BiVO}}_{4}$. We characterized the fergusonite and scheelite phases (powder and single crystal samples) and the zircon polymorph (nanopowder). The experimental results are supported by ab initio calculations, which, in addition, provide the vibrational patterns. The temperature and pressure behavior of the fergusonite lattice modes reflects the distortions associated with the ferroelastic instability. The linear coefficients of the zircon phase are in sharp contrast to the behavior observed in the fergusonite phase. The boundary of the fergusonite-to-scheelite second-order phase transition is given by ${T}_{F\ensuremath{-}\mathrm{Sch}}(\text{K})=\ensuremath{-}166(8)P(\text{GPa})+528(5)$. The zircon-to-scheelite, irreversible, first-order phase transition takes place at ${T}_{Z\ensuremath{-}\mathrm{Sch}}(\text{K})=\ensuremath{-}107(8)P(\text{GPa})+690(10)$. We found evidence of additional structural changes around 15.7 GPa, which in the downstroke were found to be not reversible. We analyzed the anharmonic contribution to the wave-number shift in fergusonite using an order parameter. The introduction of a critical temperature depending both on temperature and pressure allows for a description of the results of all the experiments in a unified way.

Multi-cation substituted pyrochlores are potential matrices for immobilization of long-lived high level nuclear waste. In this paper, we report, the structural and thermodynamic stabilities of aliovalent multi cation substituted Gd2Ti2O7 pyrochlore compounds determined by experimental and theoretical methods. A series of cubic pyrochlore-type Gd2-2xCaxZrxTi2O7 (0.0 ≤ x ≤ 0.4) compounds were synthesized and characterized by XRD and Raman spectroscopy techniques. It is observed that incorporation of Ca2+ and Zr4+ at Gd3+ site does not significantly alter crystal lattice of Gd2Ti2O7. Calorimetric studies were carried out to determine the thermodynamic stability of Gd2-2xCaxZrxTi2O7 compounds. The observed data indicate that standard molar enthalpy of formation of Gd2-2xCaxZrxTi2O7 decreases steadily with increasing dopant concentration which has been attributed to increasing distortion of the polyhedra around the cations. These inferences were further supported by DFT calculations.

Evolution of crystal structure of PbMoO4 (Wulfenite, tetragonal, I41/a) from 5 to 300 K by in situ low temperature powder neutron diffraction studies is reported. The study revealed no structural transition in the complete range of temperature. The unit cell parameters show anisotropic expansion with about 2.2 times higher expansion along c-axis compared to that along a-axis. From the refined structural parameters, it is emphasized that the expansion of Pb–O bonds in PbO8 (bisdisphenoid) units and their orientations along a- and c-axes are the controlling factors. The MoO4 tetrahedra behaves like a rigid unit in the temperature range of this study. The expansion of unit cell volume is fully arising from the expansion of PbO8 bisdisphenoids. The analyses of anisotropic displacement parameters also show almost smooth variation with temperature down to 5 K. The evolutions of structural parameters of PbMoO4 with temperature suggest it for a promising material for cryogenic applications.

Phase evolution from CaZrTi 2 O 7 (zirconolite) to Y 2 Ti 2 O 7 (pyrochlore) have been delineated by systematic characterization of a series of sample with composition as Ca 1−x Zr 1−x Y 2x Ti 2 O 7 (0.00 ≤ x ≤ 1.00) by powder XRD, Raman spectroscopy, SEM and EDS analyses. Comparative analyses of XRD and Raman spectra revealed sequential evolution of phases with increasing concentration of Y 3+ in the compositions. From the XRD studies, three distinct phase fields, namely two layer (2M) and four layer (4M) zirconolite-types and cubic pyrochlore-type are observed in between zirconolite (CaZrTi 2 O 7 ) and Y 2 Ti 2 O 7. 4M-zirconolite phase is observed in a narrow range of composition, viz. 0.35 ≤ x ≤ 0.40 while cubic pyrochlore type phase is observed in the compositions with x ≥ 1.20. The unit cell volume of different phases shows a non-linear increasing trend with Y 3+ ion concentration which has been attributed to the distribution of cations in different structure and change in their coordination number.

In this communication we report the high temperature structural details of pyrochlore-type Pr2Zr2O7, a potential mixed electronic and ionic conducting material, as observed from the in situ variable temperature powder neutron diffraction in vacuum. The evolution of structural parameters with temperature revealed a smooth expansion of the lattice while a feeble decreasing trend in the oxygen parameters of pyrochlore structure. The analyses of the structural parameters further indicate a larger expansion of the Pr–O bonds (α = 11.32 × 10−6 K−1) compared to that of Zr–O bonds (α = 8.83 × 10−6 K−1). The temperature dependent unit cell parameters measured under vacuum deviates from those measured in air and the coefficient of axial thermal expansion (αa) of Pr2Zr2O7 lattice is 10.18 × 10−6 K−1 (in vacuum). The analyses of structural parameters indicated a larger distortion in PrO8 polyhedra compared to ZrO6 octahedra of the Pr2Zr2O7. The differences in high temperature behaviour in air and vacuum have been explained by the contributions of oxidation of Pr3+ to Pr4+.

[EN] Monazite-type BiPO4, LaPO4, CePO4, and PrPO4 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 authors are thankful for the financial support to this research from the Spanish Ministerio de Economia y Competitividad, the Spanish Research Agency, and the European Fund for Regional Development under Grant Nos: MAT2016-75586-C4-1-P/2-P/3-P and MAT2015-71070-REDC. AM and PR-H acknowledge computing time provided by Red Espanola de Supercomputacion (RES) and MALTA-Cluster.

Herein we report the structure, magnetic and dielectric properties of Y2CoMnO6 (YCMO) and Y2NiMnO6 (YNMO) prepared under various conditions. On annealing in inert atmosphere, YCMO sample shows a partial decomposition while YNMO remains unaffected. Both YCMO and YNMO samples show paramagnetic to ferromagnetic transition around 80 K, but only the magnetic properties of YCMO samples depend on the preparation conditions. YCMO samples show antiferromagnetic ground state in zero field and a metamagnetic transition at relatively low field (~2 T). Differences in the annealing environment do not destroy the initial antiparallel spin order in YCMO, but affect the critical fields of metamagnetic transition. The field-dependent magnetic properties indicated soft ferromagnetic nature of YNMO samples. The dielectric measurements show distinct frequency and temperature-dependent features in YCMO samples compared to YNMO samples. A large relative permittivity with e׳ of about 104 is observed in both O2 and air-annealed YCMO samples. Multiple dielectric relaxations are observed in all the annealed YCMO and YNMO samples.

This study was aimed at discerning the enhancement in the visible-light–driven water splitting activity of nanocrystalline CdS photocatalysts because of their doping with a small amount of an aliovalent cation (Ag+ or Cr3+). The CdS/Cr-x (x = 0–2.3 wt%) and CdS/Ag-x (x = 0–1.5 wt%) samples, which were synthesized using a one-step hydrothermal method, were characterized systematically for their crystallographic, morphological, interfacial, and photo-physical properties. The Rietveld refinement of the powder X-ray diffraction data enabled us to quantify the doping-effect on the phase composition and lattice parameters. As compared to pure CdS, the samples containing ∼0.2 wt% of Cr or Ag showed two or tenfold enhancement in the rate of H2 evolution from water, respectively, when sulfide-sulfite ions were used as sacrificial electron donors. This pyramidal trend, i.e. maximum activity for a specific impurity content, which decreased on decreasing as well as increasing the amount of doping, did not occur owing to the presence of a secondary-phase metal sulfide or a dispersed metal cocatalyst. Our study revealed that instead of the widely advocated mechanism involving inter-semiconductor or semiconductor-to-metal electron transfer steps, the doping-modified photoactivity of CdS was governed by certain bulk and surface properties such as the cation-dependent particle nucleation, dominance of hex-CdS facets, and structural defects. Additionally, impurity-induced sub-bandgap charge-trapping states also contributed to the overall quantum efficiency.

Herein we report the structure, thermal and electrical properties of pyrochlore type Pr 2−x Ca x Zr 2 O 7 (x = 0.00, 0.10 and 0.20) compositions. These compounds have been prepared by gel combustion method followed by heating in reducing (Argon/Hydrogen) atmosphere and characterized by powder X-ray diffraction, Raman spectroscopy and ac-impedance studies. The powder XRD and Raman spectroscopic studies revealed the retention of the parent cubic pyrochlore type structure up to the compositions with x = 0.10. The thermal stabilities of the compositions have been investigated by in situ high temperature powder XRD and thermogravimetric studies. A maximum of about 1.25% weight gain due to oxidation of Pr 3+ is observed in all the samples. The typical axial thermal expansion coefficients (α a ) for the compositions with x = 0.00, 0.10 and 0.20 are 8.40 × 10 −6 , 11.29 × 10 −6 and 11.67 × 10 −6 K −1 , respectively. The analyses of impedance and admittance spectra revealed maximum conductivity at x = 0.10, while the activation energy increases with Ca 2+ content. At around 770 K, the conductivities of the compositions with x = 0.00, 0.10 and 0.20 are 2.26 × 10 −5 S/cm, 1.88 × 10 −4 S/cm and 1.23 × 10 −4 S/cm, respectively. The activation energy for electronic conductivity for the composition with x = 0.00 and 0.10 are 0.42 and 0.63 eV, respectively. The temperature dependent conductivity of the composition with x = 0.20 composition revealed a deviation around 700 K where the aviation energy decreased from 0.97 eV to 0.38 eV. The electrical properties Pr 2−x Ca x Zr 2 O 7 have been correlated with structural parameters and explained by the coexisting electronic and ionic conductivities.

From the characterization of a series compositions with general stoichiometry as Ca 1− x Zr 1− x Sm 2 x Ti 2 O 7 (0.00 ≤ x ≤ 1.00), the phase evolution between zirconolite (CaZrTi 2 O 7 ) and pyrochlore type Sm 2 Ti 2 O 7 has been elucidated. All the compositions were prepared by high temperature solid state reaction and characterized by powder X-ray diffraction (XRD) and electron probe for microanalyses (EPMA). Three major phase fields, namely two layer (2-M) or four layer (4-M) monoclinic zirconolite and cubic pyrochlore structure types were observed in this system. In addition, a feeble amount of perovskite type phase is found to coexist with zirconolite phase. 4-M zirconolite phase is observed as single phase field at the composition with x = 0.30 and 0.35, while cubic pyrochlore phase is observed as single phase at the compositions with x ≥ 0.60. Further, the composition and microstructure of coexisting phases are verified by back scattered electron image and EPMA studies.

A series of CdS/Nix nanocomposite photocatalysts, containing ca. 0.6–15 wt% Ni, were synthesized using a one-step hydrothermal method and characterized for their crystallographic, morphological, interfacial, and optical properties. Rietveld refinement of powder XRD data revealed the coexistence of wurtzite (hexagonal) and zinc blende (cubic) phases of CdS in ratios dependent on Ni content. Only a fraction of Ni existed as a secondary phase of NiS while the majority occupied the lattice positions of hexagonal CdS. Whereas up to 10-fold enhancement in H2 evolution compared with pure CdS was observed for samples containing ∼1.5–4.5 wt% Ni, samples with smaller or larger Ni content displayed poor activity for visible-light-induced splitting of water in presence of sulfide–sulphite ions as sacrificial electron donors. In contrary to recent findings, our study reveals that the enhanced CdS photoactivity is not a result of charge transfer between p-type NiS and n-type CdS, Ni-induced visible-region absorbance, or the coating of CdS particles by non-photoactive NiS. Instead, the preparation-dependent hexagonal/cubic CdS phase boundaries and particle morphology may play a crucial role. Additionally, certain Ni-doping-induced sub-bandgap shallow energy levels contribute to charge carrier separation.

High-pressure Raman-scattering measurements and ab initio calculations on NdVO4 have been carried out up to 30 GPa. Our combined experimental and theoretical study confirms that beyond 5.9 GPa NdVO4 undergoes an irreversible zircon to monazite transition. The coexistence of zircon and monazite phases is experimentally observed up to ~8 GPa (which agrees with the theoretical transition pressure), stabilizing the monazite phase as a single phase around 10 GPa. Calculations additionally predict the existence of a second high-pressure phase transition at 12.4 GPa. This reversible phase transition has been experimentally observed beyond 18.1 GPa and remains stable up to 30 GPa. The post-monazite phase is predicted to have a monoclinic structure isomorphic to the BaWO4-II type structure. The calculated structure for the three polymorphs of NdVO4 is reported and the pressure dependence of their Raman modes is discussed., The authors thank the financial support provided by the Spanish government MINECO under Grants no: MAT2013-46649-C4-1/2/3 and MAT2015-71070-REDC.