Cation distribution and interatomic interactions in oxides with heterovalent isomorphism: II. Complex aluminates Ln(2)SrAl(2)O(7) (Ln = La, Nd, Gd)

The distribution of rare-earth and strontium cations over two sites in pero vskite-like layered aluminates Ln(2)SrAl(2)O(7), crystallized in the Sr3Ti2 O7 structural type, was studied by the full-profile X-ray diffraction analy sis. The probability of population of oxygen cubooctahedra (Ln,Sr)O-12 with Sr2+ cations and of nine-vertex polyhedra (Ln,Sr)O-9 with Ln(3+) cations w as found to increase in the La, Nd, Gd series. This was demonstrated both b y direct calculations of the population and by analysis of interatomic inte ractions within and between layers of the perovskite cells. Cation-oxygen b onds between the layers was shown to strengthen in the above series, which results in stabilization of the layered structure as a whole.