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 (A³⁺=Y³⁺, La³⁺; A²⁺=Ca²⁺, Sr²⁺) cations as "mixing" the twelve-fold co-ordination of A³⁺ in A³⁺AlO₃ perovskite and the six-fold one of A²⁺ in A²⁺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.