Abstract

The negative thermal expansion material cubic ZrMo₂O₈ is metastable with respect to other polymorphs at all temperatures. On heating to temperatures >390 °C it readily transforms to trigonal ZrMo₂O₈. The kinetics of this transformation have been studied by in situ XRD and were found to be strongly influenced by the precursor chemistry used to prepare the cubic material. The conversion can be described with an Avrami model. An Arrhenius analysis of the estimated rate constants for two samples gave apparent activation energies of 269 and 291 kJ mol⁻¹. Extrapolation of the kinetic data to lower temperatures than those used for the measurements indicates that cubic ZrMo₂O₈ can be used for many years at temperatures below 280 °C without significant transformation.