STRUCTURE AND BONDING ANISOTROPY IN INTERGROWTH OXIDES: A CLUE TO THE MANIFESTATION OF BIDIMENSIONALITY IN T-, T'-, AND T*-TYPE STRUCTURES

摘要

The manifestation of bidimensionality in T-, T'-, and T*-type structures of A2BO4 oxides occurs in terms of cooperative structure-bonding properties involving both the metal-O 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 O 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 angstroms in La2NiO4; 3.66 angstroms 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), T'-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, respp. , do not operate. The useful role of controlled excess structure-bonding anisotropy is emphasized. 30 refs.