Prominent part of calcium ordering in the formation and stability of the intergrowth type solid solution La2Sr1−xCaxAl2O7

Abstract

The partial substitution of Ca2+ for Sr2+ in La2SrAl2O7 was performed. The structure of the solid solution La2Sr1−xCaxAl2O7 (x≤0.5) consists in the regular intergrowth of double perovskite P2 slabs with unit rocksalt RS slabs. As the main result of Rietveld structure calculations, Ca2+ is found to replace Sr2+ in the A2 sites of the RS slabs (nine-fold CN); the random occupancy of the A1 sites (12-fold CN) in the middle of the P2 slabs by La3+ and Sr2+ remains nearly unchanged. The formation of the P2–RS intergrowth of the solid solution is easier than that of La2SrAl2O7 but its stability is progressively weakened when the amount of Ca2+ increases. This is the very likely consequence of the simultaneous presence of La3+ and Ca2+ in the RS slabs. The comparison of these ordering phenomena in the aluminates and the corresponding superconducting cuprates is taken into account.