Phase Transformations and Thermal Expansion of α- and β‑BiTaO4 and the High-Temperature Modification γ‑BiTaO4

Thermal analysis and X-ray phase analysis under high temperature have defined the order of the BiTaO4 phase transformations in the range of temperature from 25 to 1200 °C: α-BiTaO4 → γ-BiTaO4 → β-BiTaO4 and β-BiTaO4→ γ-BiTaO4 → β-BiTaO4; the new high-temperature phase of bismuth orthotantalate is γ-BiTaO4. It was shown that the transition α-BiTaO4 → β-BiTaO4 is irreversible, in contrast to BiNbO4. The crystal structure of the new bismuth orthotantalate high-temperature modification (γ-BiTaO4) was studied by X-ray powder diffraction. The possible space group of the high-temperature orthorhombic phase was determined as Cmcm, and the parameters of the unit cell were calculated at 900 °C: a = 3.92455(4), b = 15.06243(16), c = 5.63483(6) Å, and V = 333.093(6) Å3. As an alternative, the space group of γ-BiTaO4 was determined as P21/c, and the following unit cell parameters were calculated: a = 7.78151(10), b = 5.63466(6), c = 7.84843(7) Å, β = 104.5858(17)°, and V = 333.033(7) Å3. The method of HTPXRD was used to obtain thermal expansion coefficients (TECs) and to study the thermal deformations of the α, β, and γ modifications of BiTaO4. The average volumetric TECs of all modifications studied had the range of 18.4–33.6 × 10–6 °C–1. The highest anisotropy of thermal expansion was the characteristic of the triclinic phase, β-BiTaO4, whereas α-BiTaO4 had minimal anisotropy and TEC indicating the highest stability of this modification.