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

The anisotropic coefficients of thermal expansion and the peritectic transformation of orthorhombic-Hf6Ta2O12 to tetragonal-HfO2 plus liquid at 2250 degrees C have been studied by in-situ X-ray powder diffraction from room temperature to complete melting (similar to 2450 degrees 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-Hf6Ta2O12 and tetragonal-HfO2 structures are simply related by polyhedral rotations and loss of 1 mol of oxygen. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.