Oxygen-sintered (Pr,Ca) MnO3: Structure and magnetism at high Ca concentrations

Polycrystalline Pr1-xCaxMnO3 (PCMO), a perovskite manganite, was synthesized at high Ca doping levels, 0.5⩽x⩽1.0, by the ceramic method complemented by a final oxygen sintering. The experimental focus was on finding out whether such an oxygen treatment is beneficial in terms of structural and magnetic properties. Except for x=1.0, whose formation required the oxygen treatment, the unit cells of the samples were largely unaffected by the additional oxygen pressure. The materials x=0.8 and x=0.9 exhibited a reproducible crystalline phase separation, which was directly observed at all experimentally accessible temperatures (from 83 K to 683 K), and a related magnetic phase separation at lower temperatures regardless of the oxygen treatment. What the oxygen sintering did cause was a systematic relaxation of residual crystal strains. This was seen as a consequence of better crystallite homogeneity due to reduced oxygen vacancies. Unexpectedly, the pattern of magnetic transitions shown by PCMO was also found to be insensitive to elevated oxygen partial pressure during sintering. Other synthesis parameters had therefore to be considered in order to explain the comparatively high charge-ordering temperatures found in this study. To summarize, we found that air sintering is sufficient for producing polycrystalline bulk PCMO of good quality, except at x=1.0, where a final oxygen treatment is recommendable in order to avoid magnetostructural defects.