Metastability of the K₂NiF₄ type structure of the solid solution LaCa(Cr_xAl_1−x)O₄(0⩽x⩽0.10)

Abstract

Metastability of the K₂NiF₄ type aluminate LaCaAlO₄ and its chromium diluted solid solution LaCaCr_xAl_1−xO₄ (x⩽0.10) was evidenced at 1400‡C in air, in terms of demixing into the parent structures of the 1∶1 intergrowth, i.e. the perovskite and rocksalt type LaAlO₃ (LaCr_xAl_1−xO₃) and CaO, respectively. This behaviour is discussed comparatively with YCaAlCO₄ and LaSrAlO₄ whose structures are stable under the same thermodynamic conditions. The results of a structure analysis are used to emphasize the role of the nine-fold co-ordination of the (A ³⁺=Y³⁺, La³⁺; A²⁺=Ca²⁺, Sr²⁺) cations as “mixing” the twelve-fold co-ordination of A ³⁺ in A ³⁺AlO₃ perovskite and the six-fold one of A ²⁺ in A ²⁺O rocksalt. Calcium-oxygen underbonding in the (La, Ca)-O-Al bridge is assumed to trigger the metastability of the intergrowth structure at high temperature.