METASTABILITY OF THE K2NIF4 TYPE-STRUCTURE OF THE SOLID-SOLUTION ESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-0.10)

Metastability of the K2NiF4 type aluminate LaCaAlO4 and its chromium d iluted solid solution LaCaCrxAl1-xO4 (X less than or equal to 0.10) wa s evidenced at 1400 degrees C in air, in terms of demixing into the pa rent structures of the 1:1 intergrowth, i.e. the perovskite and rocksa lt type LaAlO3 (LaCrxAl1-xO4) and CaO, respectively. This behaviour is discussed comparatively with YCaAlO4 and LaSrAlO4 whose structures ar e stable under the same thermodynamic conditions. The results of a str ucture analysis are used to emphasize the role of the nine-fold co-ord ination of the (A(3+) = Y3+, La3+; A(2+) = Ca2+, Sr2+) cations as ''mi xing'' the twelve-fold co-ordination of A(3+) in A(3+)AlO(3) perovskit e and the six-fold one of A(2+) in A(2t)O rocksalt. Calcium-oxygen und erbonding in the (La, Ca)-O-Al bridge is assumed to trigger the metast ability of the intergrowth structure at high temperature.