High‐Temperature Transitions in Rare‐Earth Niobates and TantaIates

The high‐temperature transition monoclinic tetragonal in the rare‐earth tantalates and niobates with the formula AB04 was investigated using a high‐temperature X‐ray diffractometer. The transition was reversible and proceeded by a gradual change in symmetry. The temperature of the first appearance of the tetragonal phase increased as the ionic size of the rare‐earth ions decreased and was considerably higher for the rare‐earth tantalates than for the rare‐earth niobates. The mechanism of the change is discussed.