Pressure-Induced Phase Transitions in Bismutotantalite (BiTaO₄): Insights from Single-Crystal Diffraction and Raman...

In situ high-pressure single-crystal X-ray diffraction and Raman spectroscopy analyses were performed on a natural bismutotantalite with an α-BiTaO₄ structure. The results indicate that α-BiTaO₄ transforms into an orthorhombic phase (HP γ-BiTaO₄), likely through an intermediate orthorhombic phase (HP β-BiTaO₄). The transition pressures are 11.0-11.8 GPa for α-BiTaO₄ → HP β-BiTaO₄ and 13.9-14.4 GPa for HP β-BiTaO₄ → HP γ-BiTaO₄ transition. The phase transitions are reversible. Although the structure of HP β-BiTaO₄ was not successfully solved, the possible space group was determined to be Fmm2, with unit-cell parameters calculated at 12.2 GPa: a = 4.8158(18) Å, b = 33.8880(80) Å, c = 5.2910(5) Å. In contrast, the structure of HP γ-BiTaO₄ was successfully solved and refined at 28.0 GPa, revealing a Pnma space group and unit-cell parameters of a = 9.8999(12) Å, b = 5.0435(16) Å, c = 10.8331(8) Å. The significant volume collapse of 6.2% through the phase transition and the increase in coordination numbers of Bi and Ta from 6 in α-BiTaO₄ to 8/9 in HP γ-BiTaO₄ indicate that the HP γ-BiTaO₄ structure is considerably more compacted. Additionally, the equation of state for both α-BiTaO₄ and HP γ-BiTaO₄ was also studied.