High-temperature Crystal Chemistry Research Articles

High temperature structure and vibrational properties of ScXO4 (X= V5+ and P5+): A comparative study

ScVO4 and ScPO4 represent the zircon (xenotime) type structures with smallest trivalent cations, and that enable them to host both transition metal and rare-earth ions for applied optical materials. Thus, their crystal chemistry and thermophysical properties becomes relevance for their application in non-ambient conditions. In this report, high temperature crystal chemistry and vibrational properties of ScVO4 and ScPO4, as observed from in situ high temperature powder XRD and Raman spectroscopic studies, are reported. The comparative analyses of the results indicate that, though both are isostructural, they show drastically different thermal expansion behavior. In case of ScVO4, the c-axis shows significantly larger expansion compared to a-axis, while in ScPO4 the thermal expansion along and a and c-axes are more or less similar. At ambient condition, the thermal expansion anisotropy in ScPO4 and ScVO4 are 1.02 and 3.97, respectively. Additionally, ScPO4 shows relatively lower coefficient of volume thermal expansion compared to ScVO4, (αv = 23.64 × 10−6 K−1 for ScPO4 and 26.09 × 10−6 K−1 for ScVO4), and is contributed by the expansion of ScO8 units in their structures. The thermal expansion coefficients of ScO8 unit in ScPO4 and ScVO4 are 36.3 × 10−6 K−1 and 39.1 × 10−6 K−1, respectively. Temperature evolution of Raman modes indicates weakening of all the modes, except a symmetric stretching mode, with increasing temperature. The anharmonic analyses of the Raman modes indicate that implicit contributions in ScVO4 and ScPO4 are appreciably higher than the explicit contributions, and hence the changes in mode wavenumbers with volume play dominating role in governing their thermal expansion behaviors. Further, it is concluded that ScPO4 is characterized by more or less like rigid unit cell compared to ScVO4.

Re-investigation of ‘minasgeraisite-(Y)’ from the Jaguaraçu pegmatite, Brazil and high-temperature crystal chemistry of gadolinite-supergroup minerals

AbstractThe chemical composition (including B, Be and Li), the Raman spectrum and the crystal-structure evolution (at the temperature range 27–1000°C) of a Mn-bearing, Bi-rich gadolinite-subgroup mineral from the Jaguaraçu Pegmatite, Brazil (type-locality of minasgeraisite-(Y)) was studied. Elemental mapping revealed that the crystal investigated has complex chemical zonation with various Bi (~8–24 wt.% Bi2O3), Ca (~8–10 wt.% CaO) and Y (~11–17 wt.% Y2O3) content. The sample investigated has all the specific features of the chemical composition of minasgeraisite-(Y), except Ca excess and, thus, should be considered as hingganite-(Y). The Raman spectrum of the sample under study has bands at 140, 179, 243, 350, 446, 519, 559, 625, 902, 973, 3224, 3353, 3532 and 3763 cm–1, and is similar to that of hingganite-(Y) / -(Nd). Crystal-structure refinement confirmed that the crystal in question should be considered as hingganite-(Y) and is in line with the previously obtained data on gadolinite-subgroup minerals from the Jaguaraçu Pegmatite. High-temperature single-crystal X-ray diffraction studies revealed that the mineral starts to decompose above 800°C. We can conclude that beryllosilicates are most stable at high-temperature conditions within the gadolinite supergroup and that species with a higher M-site occupancy have higher stability upon heating.

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.

Crystallography and high-temperature crystal chemistry of anhydrous borosilicates of alkali and alkali-earth metals

The crystal structures and thermal behavior of anhydrous borate-silicates of alkali and alkali-earth metals are briefly overviewed. In silicoborates (B/Si > 1), the boron atoms occur in triangle and tetrahedral coordination, as in borates; in two of three known silicoborates, the unique layered and chained polyanions are found. The known borosilicates (B/Si ≤ 1) have a framework structure formed from tetrahedrally coordinated atoms of boron and silicon; some borosilicates belong to the known structural types of aluminosilicates, while the others are only structurally similar to them and silica modifications, whereas their tetrahedral frameworks are topologically identical to that of aluminosilicates and SiO2. The exception are borosilicates Li4B4Si8O24 and lisitsynite KBSi2O6, having the original frameworks. In line with the similarity of chemical composition and commonality of crystal structure, thermal behavior of borosilicates appears to be similar to that of aluminosilicates and silicates, however, borosilicates have considerably lower temperatures of formation, polymorphic transformations, and melting.

High Temperature Crystal Chemistry and Oxygen Permeation Properties of the Mixed Ionic–Electronic Conductors LnBaCo2O5 + δ ( Ln = Lanthanide )

The high temperature crystal chemistry and oxygen permeation properties of the cation-ordered perovskite oxides [lanthanide , Nd, and Sm] have been investigated in comparison with the cation-disordered perovskite. The (, Nd, and Sm) oxides exhibit a metal-insulator transition at , as evidenced by total conductivity measurements and high temperature X-ray diffraction data and an oxygen vacancy order–disorder transition at in air, as evidenced by an orthorhombic to tetragonal transition. At a given temperature, the oxygen permeation flux decreases from to Nd to Sm due to the changes in crystal symmetry and lattice strain. The oxygen permeation mechanism in the is bulk-diffusion-limited rather than surface-exchange-limited for membrane thickness .

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

We report the lattice thermal expansion of zircon-(xenotime-)type LuPO 4 and LuVO 4 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 LuVO 4 shows significant anisotropy compared to LuPO 4 . In the studied temperature range, the average axial thermal expansion coefficients of LuPO 4 are α a = 6.0 × 10 -6 and α c = 7.2 × 10 -6 (°C -1 ) and those of LuVO 4 are α a = 3.6 × 10 -6 and α c = 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 LuO 8 polyhedra. The LuO 8 polyhedron in LuVO 4 shows about 30% higher thermal expansion than that in LuPO 4 . The overall thermal expansion behaviors of these two structures are predominantly related to the distortion in the LuO 8 unit, inter-cation distances and spatial arrangement of the Lu-O bonds in the structure.

High temperature crystal chemistry and thermal expansion of synthetic powellite (CaMoO4): A high temperature X-ray diffraction (HT-XRD) study

In this communication, the high temperature crystal chemistry of scheelite type (space group: I41/a) CaMoO4 (powellite) is reported. The details of the high temperature crystal structures were obtained by high temperature X-ray diffractometer (HT-XRD) data in the temperature range of 25–1000 °C. The lattice thermal expansions of this compound show the an-isotropic behavior, with the coefficient of thermal expansion along c-axis (αc) is almost double of that along the a-axis (αa). The average values of thermal expansion coefficient for CaMoO4 are: αa=13.5×10−6 and αc=22.8×10−6 °C. The typical thermal expansion behavior is basically controlled by the temperature-induced expansion of the Ca–O bonds, i.e. by the thermal expansion of the CaO8 polyhedra. The MoO4 polyhedra acts as typical rigid unit, with virtually no expansion. The preliminary data analysis for CaWO4 indicates almost similar values of thermal expansion coefficients, viz. αa=12.7×10−6 and αc=21.4×10−6/°C (for CaWO4) in the same temperature range. The further details are explained in this manuscript.

Filatovite, K(Al, Zn)2(As, Si)2O8, a new mineral species from the Tolbachik volcano, Kamchatka peninsula, Russia

Filatovite, ideally K((Al,Zn)2(As,Si)2O8), has been found in products of fumarolic activity on the second cinder cone of the North breach of the great fissure Tolbachik eruption (1975-76, Kamchatka peninsula, Russia). The mineral occurs as pris- matic crystals up to 0.3 mm and as intergrowths of crystals, in association with alumoklyuchevskite, lammerite, johillerite, sylvite, As-bearing orthoclase, hematite and tenorite. Filatovite is colourless, with vitreous luster and white streak. The mineral is brittle and transparent. It has a good {100} cleavage. Mohs' hardness is 5-6. The calculated density is 2.92 g/cm3. Biaxial, optically nega- tive, ! = 1.532(1), = 1.535(1), # = 1.537(1), 2Vmeas. = 60(10)°, 2Vcalc. = 78°. Optical orientation: X ~ (100), Y and Z are normal to the {001} and {010} faces, respectively. No pleochroism has been observed. The mineral is monoclinic, space group I2/c, a = 8.772(1), b = 13.370(2), c = 14.690(2) A, = 115.944(6)°, V = 1549.1(4) A3, Z = 8 (from single-crystal structure study). The diagnostic lines of the X-ray powder diffraction pattern are (I-d-hkl): 70-4.329- $ 202; 70-3.897- $ 130; 100-3.364- $ 220,$ 204, 040; 50-3.300-004, 40-3.066-132, 60-2.981-042, 40-2.646- $ 242. Chemical composition (wt. %): P2O5 1.63, As2O5 40.60, SiO2 12.35, Al2O3 27.33, CuO 0.83, ZnO 3.85, FeO 0.28, Na2O 0.63, K2O 12.85, total 100.35. The empirical formula of filatovite, calculated on the basis of O = 8, is (K0.92Na0.07)% =0.99((Al1.81Zn0.16Cu0.04Fe0.01)% =2.02(As1.20Si0.70P0.08)% =1.98O8). The simplified formula is K((Al,Zn)2(As,Si)2O8). The detailed chemical formula can be written as K((Al2-xZnx)(As1+xSi1-x)O8), with x ~ 0.20. Filatovite is a new mineral of the feldspar group and is structurally related to celsian. The name honors Prof. Stanislav K. Filatov (b. 1940), Department of Crystallography, St. Petersburg State University, St. Petersburg, Russia, for his important contributions to high temperature crystal chemistry and crystal chemistry of exhalation minerals.

High-temperature crystal chemistry of normal paraffins with even numbers of carbon atoms

Thermal deformations and phase transformations of evennumbered paraffins CnH2n+2(n=20, 22, and 24) have been investigated in a thermal X-ray diffraction study. In homologs with n=20 and 24, phase transformations occur in two steps: triclinic-to-orthorhombic and orthorhombic-to-hexagonal transitions. An orthorhombic rotator phase of even n-paraffins is described for the first time. A crystal-chemical model of the phase transformations of the homologs is suggested.

High-pressure and high-temperature crystal chemistry of beryllium oxide

The crystal structure of synthetic BeO, bromellite, has been determined at several pressures to 5.0 GPa and several temperatures to 1183 K. The single variable atomic-positional parameter, z of oxygen, does not vary significantly with pressure, but does undergo a small increase with temperature. The hexagonal axial ratio c/a is constant with pressure, but decreases slightly from 1.624±0.001 to 1.623±0.001 between 300 and 1183 K. The observed crystal bulk modulus is 212±3 GPa, if K′ is assumed to be 4. The bulk modulus of the beryllium tetrahedron in BeO is 210 GPa, identical to that of the crystal, and close to the value observed for beryllium tetrahedra in other beryllium minerals. The average volume thermal expansivity of the BeO beryllium tetrahedron between 298 and 1183 K is 2.5±0.2×10−5 K−1, compared to 2.66±0.10×10−5 K−1 for the crystal.

High temperature crystal chemistry.

High temperature crystal chemistry.

High-temperature crystal chemistry of V2O3 and 1% chromium-doped V2O3

High-temperature crystal chemistry of V₂O₃ and 1% chromium-doped V₂O₃

High-temperature Crystal Chemistry of AmBXn Compounds with Particular Reference to Calcium Orthosilicate

High-temperature Crystal Chemistry of A_mBX_n Compounds with Particular Reference to Calcium Orthosilicate