High-Temperature Transformations in Calcium Orthovanadate Studied with Raman Scattering

Abstract

Raman spectra of Ca 3 (VO 4 ) 2 calcium orthovanadate (R3c space group), a high temperature ferroelectric with T c ) 1383 K, are analyzed up to 1483 K with a group theory approach and discussed in relation to Raman spectra of other compounds with the palmierite structure (R3 hm space group). The ferroelectric-paraelectric phase transition at T c ) 1383 K is interpreted as due to the R3c f R3 hm structural transformation. At the 500-800 K temperature range, there occurs a transition to an intermediate between the R3 hm and R3c phases, with all the VO 4 3ions at sites with 3-fold symmetry. There is no soft-mode behavior observed up to 1483 K, suggesting an order-disorder character of the transitions.

Amorphization of Ca3(VO4)2at High Pressure

“…and 1/ A parameters, which are the measures of hardness along the a and c crystallographic axes, respectively, reveals that the axial stiffness along the a axis in Ca (VO ) is over 30% larger than the axial stiffness along the c axis. displacements (450-50 cm\) (25). Up to 8.1 GPa, all the bands shift monotonously toward higher wavenumbers with no mode softening (Fig.…”

Phase transitions in Sr-containing phosphates and vanadates with β-Ca3(PO4)2-related structures

“…Ca 3 (VO 4 ) 2 is isotypic with h-Ca 3 (PO 4 ) 2 and they crystallize in space group (SG) R3c (a c 10 Å and c c 38 Å , Z = 21). Ca 3 (VO 4 ) 2 is a high-temperature (HT) ferroelectric with T c = 1383 K and it has additional anomalies in Raman scattering spectra, dielectric properties, and electronic thermal emission spectra at 550 -800 K [1]. Vanadates Ca 9 M(VO 4 ) 7 (M = Y, La -Lu, and Bi) isotypic with Ca 3 (VO 4 ) 2 also show ferroelectric phase transitions at T c c 1070-1200 K. They demonstrate large second-harmonic generation efficiency, especially Ca 9 Bi(VO 4 ) 7 (about 140 times quartz) [2].…”

Structural Evolution in BiNbO4

“…The Raman spectra of a ceramic sample of α-BiNbO 4 over the temperature range of 25–600 °C are shown in Figure S10 in the Supporting Information. All the data were fitted using a Lorentzian peak function model. , The variation of band positions for selected vibrational modes, as a function of temperature, is given in Figure , with changes in other vibrational modes shown in Figure S11 in the Supporting Information. Most of the vibrational modes experienced a red shift with increasing temperature due to the contribution from thermal expansion and anharmonicity of the interatomic potential energy. , Major changes in structure occur over a broad temperature range from 300 °C to 600 °C, as evidenced by the stiffening or vanishing of some phonon bands.…”

“…All the data were fitted using a Lorentzian peak function model. 31,32 The variation of band positions for selected vibrational modes, as a function of temperature, is given in Figure 10, with changes in other vibrational modes shown in Figure S11 in the Supporting Information. Most of the vibrational modes experienced a red shift with increasing temperature due to the contribution from thermal expansion and anharmonicity of the interatomic potential energy.…”

“…Although the changes seen within the cation polyhedra are very small, they reflect subtle changes in the distortion of the niobate sheets.The Raman spectra of a ceramic sample of α-BiNbO 4 over the temperature range of 25−600 °C are shown in FigureS10in the Supporting Information. All the data were fitted using a Lorentzian peak function model 31,32. The variation of band positions for selected vibrational modes, as a function of temperature, is given in Figure10, with changes in other…”