METASTABILITY OF THE K2NIF4 TYPE STRUCTURE OF THE SOLID SOLUTION LACA(CRXAL1-X)O-4(O-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-0.10)

摘要

Metastability of the K2NiF4 type aluminate LaCaAlO4 and its chromium diluted solid solution LaCaCrxAl1-xO4 (X less than or equal to 0.10) was evidenced at 1400 degrees C in air, in terms of demixing into the parent structures of the 1:1 intergrowth, i.e. the perovskite and rocksalt type LaAlO3 (LaCrxAl1-xO4) and CaO, respectively. This behaviour is discussed comparatively with YCaAlO4 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+; A(2+) = Ca2+, Sr2+) cations as ''mixing'' the twelve-fold co-ordination of A(3+) in A(3+)AlO(3) perovskite and the six-fold one of A(2+) in A(2t)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. [References: 16]