Cation Distribution and Interatomic Interactions in Oxides with Heterovalent Isomorphism: III. Complex Aluminates LnCaAlO4 (Ln = Y,La, Nd,Gd, Ho,Er, Yb)

The heterovalent isomorphism of Ln³+⁺ and Ca^{2 +} cations in stoichiometric LnCaAlO₄ oxides was studied. The X-ray diffractometry of the intensity of Laue background modulations revealed a positive short-range order in Ln^{3 +} and Ca^{2 +} distribution over points of one structural site common for both ions, which results in formation of stable Ln³⁺-Ca²⁺ couples in adjacent nine-coordinate oxygen polyhedra. The degree of order and the interchange energy estimated in the quasichemical approximation depend both on the effective cationic charges and on the number of unpaired f electrons of the lanthanide atom.