Perovskite‐Type Rare‐Earth Niobates and Tantalates

A family of perovskite‐type compounds of composition A O.33 BO 3 was prepared by high‐temperature heat treatment of appropriate mixtures of trivalent rare‐earth oxide (A 2 3+ O 3 ) and niobia or tantala (B 2 5+ O 5 ). The A‐type cations investigated included La, Ce, Pr, Nd, Sm, Gd, Dy, Y, and Yb. The compounds which can be formed with niobia appear to be limited to those containing A‐type cations of radius greater than that of samarium, but with tantala the range extends down to ytterbium. The perovskitelike crystal structures are tetragonal or pseudotetragonal with an axial ratio ∼2. For the majority, the two‐cell pseudotetragonal unit shows either orthorhombic or monoclinic distortions, the lower monoclinic symmetry characterizing those containing the smallest rare‐earth cations, dysprosium, yttrium, and ytterbium. Niobia and tantala compounds of similar structure and composition were formed from ceria and praseodymia and the rare‐earth cations appeared to be substantially reduced to the trivalent state during the high‐temperature reactions.