Structural investigation of temperature-induced phase transitions in HfV₂O₇

¹ Laboratoire Matériaux Multiphasés Interfaces, Université de Toulon et du Var, B.P. 132, 83957 La Garde Cedex, France
² Laboratoire de Cristallographie, CNRS, B.P. 166, 38042 Grenoble Cedex 09, France

Corresponding authors: leroux@univ-tln.fr nihoul@univ-tln.fr

Received: 4 October 1999
Accepted: 22 February 2000
Published online: 15 April 2000

Abstract

HfV₂O₇ powders were prepared by means of thermal decomposition of a mixture of vanadyl and hafnyl oxalates in air at 973 K. The room temperature structure and its evolution with temperature was investigated using both electron microscopy (TEM and HREM) and powder diffraction (X-ray and neutron) techniques combined with DSC measurements. HfV₂O₇ exhibits two temperature-induced phase transitions between room temperature and 480 K. Over the highest transition (370 K), HfV₂O₇ crystallizes in a cubic system with a cell parameter of 8.75 Å and space group. A negative thermal expansion is observed in this high temperature phase and can be correlated to anisotropic oxygen atom displacements. On the other hand, the room temperature structure of HfV₂O₇ is also described in a cubic symmetry, but the existence of superstructure reflections leads to a cell parameter three times larger than for the high temperature structure (~ 26.25 Å). Moreover, HREM calculations have shown that the atomic positions, usually used to describe the superstructures of the ZrV₂O₇ compounds, cannot be used in the case of HfV₂O₇ at room temperature.