Stability of Sr3Ti2O7 structure in La-1.2(Sr1-xCax)(1.8)Mn2O7 and Ca3-yLayMn2O7

Structural studies on the samples of nominal compositions of the series, La -1.2(Sr1-xCax)(1.8)Mn2O7 and Ca3-yLayMn2O7, have been carried out using neu tron and X-ray diffraction techniques. These studies show that, unlike the case of the (La,Sr)(3)Mn2O7 system, the samples belonging to the (La,Ca)(3) Mn2O7 system form layered Sr3Ti2O7-type structures in a very restricted com position range. Through a systematic study on the effect of substitution of La for Ca in the layered compound Ca3Mn2O7, the solubility of La in Ca3Mn2 O7 has been determined. The results go against the literature reports, whic h claim that the samples of nominal compositions Ca3-yLayMn2O7 (1.0 less th an or equal to y less than or equal to 2.3) form with layered Sr3Ti2O7 stru ctures. The usefulness of the neutron diffraction technique in unambiguous structural characterization of the layered compounds has also been discusse d.