Nature of structure defects in rhombohedral series of La1-xAxMnO3+δ (A = Na, K)

A series of polycristalline samples of the type La(1-x)A(x)MnO(3+delta) (A=Na, K), quenched in air at 800 degreesC, have been analyzed by X-ray, sample density and average valence of Mn ions, in order to determine structure type defectiveness and its evolution vs. atomic fraction of Na and K. It is found that from crystallochemical point of view, both the series are highly similar. They represent solid solutions of rhombohedral symmetry and of the perovskite structure type, both extending slightly over x(A) similar to0.25. Variation of the average valence of Mn ions in the series, confirms initial structure deficiency (2Delta) of the binary matrix to be of cationic type i.e. La(1-Delta)Mn(1-Delta)O3+Delta, partly compensated by the excess oxygen (Delta), the latter as presumably located in vacant positions of the metal sublattices. Within the domain of homogeneity, the solid solutions exhibit some anomalies at x(Na) similar to0.138 and at x(K) similar to0.147, both connected with a substantial change of the structure type defectiveness and clearly seen on x(A) variations of the (La=Mn)-site deficiency (Delta). The experimental sample densities indicate that the excess oxygen, present in the matrix, quickly disappears vs. lanthanum substituent (at x(Na) similar to0.04 and at x(K) similar to0.05), but the cationic deficiency persists in the series up to the concentrations of the anomalies given above where it vanishes. On further increase of sodium or potassium content, the crystal structure, being free of any defects, starts to exhibit an increasing level of oxygen deficiency Delta(oxyg) = (3 + 2x -V-Mn)/2. (C) 2002 Elsevier Science B.V. All rights reserved.