Phase transformations in rare earth niobates

Rare earth niobates present an interesting group of materials due to their reversible ferroelastic to paraelastic phase transformations. These materials are potential large force actuators having spontaneous strains of 6-7 %. So far these phase transformations have mainly been studied either at low temperatures (for instance in LaNbO ₄) or in specimens that have been quenched after heat treatment. In this study, high temperature phase transformations of YNbO ₄ and DyNbO ₄ have been studied in-situ, using synchrotron X-rays. Results on the evolution of lattice parameters of the ferroelastic (monoclinic) phase with temperature, until it transforms to the prototype (tetragonal) phase are presented. The monoclinic to tetragonal phase transformation in YNbO ₄ and DyNbO ₄) is second-order in nature and the transformation temperatures were determined as ∼861°±2°C and ∼842°±2°C, respectively.