Structure and Bonding Anisotropy in Intergrowth Oxides: A Clue to the Manifestation of Bidimensionality in T-, T-, and T*-Type Structures

The manifestation of bidimensionality in T-, Tʹ-, and T*-type structures of A2BO4 oxides occurs in terms of cooperative structure–bonding properties involving both the metal–oxygen A(B)–O bonds and the A–A cationic repulsions. The analysis of the A-metal network gives evidence of ninefold cationic coordination very similar to the ninefold oxygen coordination. The short A–A apical distance results in A–A repulsions which are able to balance the A–O apical overbonding. In La2NiO4 as compared to La2CuO4, the contribution of these repulsions is more important (dA–A=3.25 ? in La2NiO4; 3.66 ? in La2CuO4). The perovskite (P)/rock salt (RS) intergrowth, T-type structure, due to strong geometrical constraint, has excess anisotropy which needs to be relieved. In contrast, the perovskite (P)/calcium fluoride (CF), Tʹ- type structure, is nearly free from any excess anisotropy to be relieved. In the (P)/(RS)/(P)/(CF) intergrowth, T*-type structure, owing to a more regular distribution of the A–O bonds, the excess anisotropy is further restricted. Crystal chemical mechanisms such as A substitutions and the oxidation of Cu, which lower the excess charge in the anionic and cationic (P) and (RS) slabs, respectively, do not operate. The useful role of controlled excess structure–bonding anisotropy is emphasized.