Stability of the layered Sr3Ti2O7 structure in La-1.2(Sr1-xCax)(1.8)Mn2O7

Samples with nominal compositions La-1.2 (Sr1-xCax)(1.8)Mn2O7 (x = 0.00, 0. 25, 0.50, 0.65, 0.80 and 1.00) have been prepared and characterized by x-ra y diffraction (XRD) and ac susceptibility techniques. Drastic changes in th e XRD pattern and in the ac susceptibility versus temperature behaviour at x = 0.65 indicate that single phase compounds with layered Sr3Ti2O7 structu re are formed only up to x = 0.50. Rietveld refinement of the XRD patterns of x = 1.00 composition suggests that the compositions with x greater than or equal to 0.65 form multiphase mixtures comprising hole doped perovskite manganates as majority phases and CaO as the minority phase. These results are at variance with the literature reports that the compound La1.2Ca1.8Mn2 O7 forms with layered ST3Ti2TO7 structure.