TEM studies on La2−2xCa1+2xMn2O7 (x=0.5 and 0.25)

The nominal compositions and structures of the compounds La2−2xCa1+2xMn2O7 (x=0.5 and 0.25) prepared by solid-state reaction have been studied by transmission electron microscopy and high-resolution electron microscopy. Besides the observation of the major phase of a two-layer structure in the nominal composition La1.0Ca2.0Mn2O7, the embedded n=3, n=4 Ruddlesden-Popper phases and La rich phase in the two-layer structure were observed as defects. No layered structure was observed in the nominal composition La1.5Ca1.5Mn2O7. Instead, a major phase of orthorhombically distorted perovskite, a minor phase of an ideal cubic perovskite, and a small fraction of calcium-oxide were observed in the sample. The coexistence of the orthorhombic phase and ideal cubic perovskite phase, as well as the same orthorhombic phase with perpendicular directions, was observed in the nominal composition La1.5Ca1.5Mn2O7.