In-situ investigation of Hf₆Ta₂O₁₇ anisotropic thermal expansion and topotactic, peritectic transformation

The anisotropic coefficients of thermal expansion and the peritectic transformation of orthorhombic-Hf₆Ta₂O₁₇ to tetragonal-HfO₂ plus liquid at 2250 °C have been studied by in-situ X-ray powder diffraction from room temperature to complete melting (∼2450 °C) in air, using a quadrupole lamp furnace (QLF) and a conical nozzle levitator (CNL) equipped with a CO₂ laser. The topotactic, peritectic transformation has been fully described by extracting the orientation relationship, lattice variant deformation and a motif (grouping) of cations that relates the two structures at the transformation temperature. The calculation of these two important parameters as well as identification of the motif is facilitated by a knowledge of the anisotropic coefficients of thermal expansion as a function of temperature. Symmetry decomposition has been performed to show that the orthorhombic-Hf₆Ta₂O₁₇ and tetragonal-HfO₂ structures are simply related by polyhedral rotations and loss of 1 mol of oxygen.