S.J. Patwe

Experimental and theoretical investigations on the polymorphism and metastability of BiPO4

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.

Lattice thermal expansion of zircon-type LuPO4 and LuVO4: A comparative study

Abstract We report the lattice thermal expansion of zircon-(xenotime-)type LuPO4 and LuVO4 in the temperature range of 25-1000 °C from high-temperature powder XRD studies. The details of the high-temperature crystal chemistry of both phases have been determined from Rietveld analysis of the powder XRD data. Both the compounds show appreciably higher thermal expansion than analogous zircon-type silicates. Despite isomorphism, the axial thermal expansion of LuVO4 shows significant anisotropy compared to LuPO4. In the studied temperature range, the average axial thermal expansion coefficients of LuPO4 are αa = 6.0 × 10-6 and αc = 7.2 × 10-6 (°C-1) and those of LuVO4 are αa = 3.6 × 10-6 and αc = 11.8 × 10-6 (°C-1). However, the average volume thermal expansion coefficients are almost identical. The differences in the thermal expansion behavior of the two structures originate from differences in the expansion and distortion of the LuO8 polyhedra. The LuO8 polyhedron in LuVO4 shows about 30% higher thermal expansion than that in LuPO4. The overall thermal expansion behaviors of these two structures are predominantly related to the distortion in the LuO8 unit, inter-cation distances and spatial arrangement of the Lu-O bonds in the structure.

Original ArticlesSynthesis and characterization of mixed fluorides Y1−xCa2+1.5xF7 (−1.33 ≤ x ≤ 1.0)

In this paper, we report on the synthesis and characterization of the Y1−xCa2+1.5xF7 (−1.33 ≤ x ≤ 1.0) compounds and the phase equilibria in the CaF2–YF3 system, in the continuation of our earlier studies on Sr and Ba analogues. It was found that the solubility limit of YF3 in the CaF2 lattice is about 33 mol% as compared to about 25 mol% of YF3 in BaF2 and SrF2. The CaF2–YF3 system shows the formation of an ordered cubic superstructure after the solubility limit. A hexagonal tysonite type phase separates out beyond 41 mol% of YF3. A single phasic tysonite type product was obtained at the composition with about 75 mol% of YF3, i.e., Y1.9Ca0.65F7.

Synthesis and characterization of mixed fluorides Y1-xBa2+1.5xF7 (-1.33 ≤ x ≤ 1.0)

Abstract A detailed study of phase equilibria of mixed fluorides in the BaF 2 –YF 3 system was conducted. For this study, a series of compounds with the general formula Y 1−x Ba 2+1.5x F 7 (−1.33 ≤ x ≤ 1.0) were synthesized and characterized by powder X-ray diffraction (XRD). XRD analysis revealed that there exists an extensive solubility range of YF 3 in a BaF 2 system (≈25 mol%), up to the composition Y 0.8 Ba 2.30 F 7 (x = 0.2). Up to this composition, the BaF 2 structure (cubic fluorite) persists. Compositions closer to YF 3 , however, manifested an ordered fluorite-derived structure with a rhombohedral (hexagonal) unit cell. This ordered phase begins to form at Y 0.9 Ba 2.15 F 7 . At Y 1.0 Ba 2.0 F 7 , the composition is a biphasic mixture of the cubic fluorite and the rhombohedral structures. This investigation supports the formation of the earlier reported Y 2 BaF 8 (monoclinic) compound, which we have demonstrated to be a very stable phase in this system. On either side of Y 2 BaF 8 (i.e., towards BaF 2 or YF 3 ), there coexist the additional phases rhombohedral and O-YF 3 , respectively. The Y 2 BaF 8 exists as a line compound in this system.

High-Pressure Crystal Structure, Lattice Vibrations, and Band Structure of BiSbO4

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.

Synthesis and characterization of Y1-xPb2+1 .5xF7 (-1.33 ≤ x ≤ 1.0)

Abstract We report the synthesis and characterization of a series of compounds with general composition Y1-xPb2+1.5xF7, (-1.33≤ x ≤ 1.0) to establish a detailed phase relation in the PbF2-YF3 system. The solid solubility limit of YF3 in PbF2 lattice, under short annealed and slow cooled conditions, is represented as Y0.7Pb2.45F7, (i.e., about 22 mol % of YF3). The existence of a rhombohedral phase with the composition Y1.2Pb1.70F7 was seen beyond the saturated solubility limit. A new phase with an orthorhombic unit cell was identified within the narrow range of compositions Y1.3Pb1.55F7 and Y1.4Pb1.40F7. Subsequent compositions, towards YF3 side, exist as a mixture of the orthorhombic phase and leftover β-YF3. It was also observed that in the present experimental conditions no PbF2 could be incorporated in the lattice of β-YF3.

Synthesis and characterization of M1−xNdxF2+x (M = Sr2+, Ca2+; 0.00 ≤ x ≤ 1.00)

Series of the mixed fluorides with the general composition M{sub 1-x}Nd{sub x}F{sub 2+x} (0.00{<=}x{<=}1.00; M=Sr{sup 2+} and Ca{sup 2+}) were prepared by a vacuum heat treatment of the appropriate mixtures of MF{sub 2} (M=Ca and Sr) and NdF{sub 3}. The products obtained were analyzed by powder X-ray diffraction, which revealed the phase relations in these systems. At the MF{sub 2}-rich compositions a fluorite-type solid solution was observed in both systems. The typical solid solution limits of NdF{sub 3} in the SrF{sub 2} and CaF{sub 2} lattice are about 40 and 45 mol%, respectively. The incorporation of NdF{sub 3} in the fluorite lattice of CaF{sub 2} causes an expansion of the unit cell volume. The unit cell volume of the fluorite lattice in Sr{sub 1-x}Nd{sub x}F{sub 2+x} system surprisingly remains constant throughout the solid solution range. The NdF{sub 3} (tysonite) type phase separates out beyond the solid solution limit, in both systems. Also, it was observed that about 15 and <10 mol% of CaF{sub 2} and SrF{sub 2}, respectively, could be retained in the NdF{sub 3}, maintaining the tysonite lattice. No fluorite or tysonite-related ordered phases were observed in these systems.

Synthesis and characterization of mixed fluoride Y1−xSr2+1.5xF7 (−1.0 < x < 0.5)

A series of mixed fluorides with the general formula Y1−xSr2+1.5xF7 (x = −1.0 < x < 0.5) was synthesized and characterized by powder X-ray diffraction (XRD). It was found that the compound with the stoichiometry Y0.5Sr2.75F7 shows a cubic symmetry (fluorite-type structure) with a = 5.776 A. On decreasing the Sr2+ content or increasing the Y3+ content in Y1−xSr2+1.5xF7, a phase with tetragonal symmetry with a = 11.416 A and c = 13.291 A was observed in the composition YSr2F7, whereas in the composition Y2Sr0.5F7, a phase with hexagonal symmetry with a = 6.882 A and c = 7.032 A was observed. The existence of phases such as YSr2F7 and Y2Sr0.5F7 was confirmed. We observed a coexistence of rhombohedral and hexagonal phases beyond a particular nominal composition in this series of compounds.

Phase Transformation, Vibrational and Electronic Properties of K2Ce(PO4)2: A Combined Experimental and Theoretical Study

Herein we report the high-temperature crystal chemistry of K2Ce(PO4)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 (P21/n) phase reversibly transforms to a tetragonal (I41/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 K2Ce(PO4)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 (αV = 3.83 × 10-6 K-1) compared to that in ambient monoclinic phase (αV = 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 K2Ce(PO4)2 have been obtained. Several lattice and external modes of rigid PO4 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.

Powder XRD study of Ba[sub 4]Eu[sub 3]F[sub 17]: A new anion rich fluorite related mixed fluoride

The compound Ba 4 Eu 3 F 17 was prepared by heating pre-dried BaF 2 and EuF 3 (4:3) at 800 °C for 8 h in static vacuum. The colorless polycrystalline product obtained was characterized by Rietveld refinement of the observed powder diffraction data with a starting model of Ba 4 Y 3 F 17 . The title compound Ba 4 Eu 3 F 17 crystallizes in rhombohedral lattice with lattice parameters, a =11.1787(4) and c =20.5789(10) A, Z =3 (Space group R 3 , No. 148). The Ba 4 Eu 3 F 17 structure can be described as an ordered anion-rich fluorite type structure with the formation of Eu 6 F 37 clusters. There are two crystallographically distinct Ba (CN=10, 11) and one distinct Eu (CN=8). The typical Ba(1)–F, Ba(2)–F, and Eu–F bond lengths range from 2.56 to 2.83 A, 2.54 to 3.25 A, and 2.24 to 2.49 A, respectively. The salient feature of the structure is that the EuF 8 polyhedra share their corner to form a cubo-octahedron of fluoride ions. The cubic BaF 8 polyhedra of BaF 2 are modified to Ba(1)–F 10 and Ba(2)–F 11 polyhedra in this structure. The cubo-octahedron encloses extra fluorine F(8) inside it.