Size and Vacancies; Unexpected Phase Transitions in Manganese Perovskites

School of Chemistry, The University of Sydney
â—‹Brendan J Kennedy Qingdi Zhou

Manganese containing perovskites have been extensively studied in recent years because of their potential applications as catalysts and magnetoresistive sensors. As is observed in numerous other systems, most manganese perovskites do not exhibit the ideal cubic structure but rather have a distorted variant. The physical and electronic properties of the manganese perovskites are sensitive to the structural distortion and the actual stoichiometry and in some cases the distorted perovskites undergo thermally induced phase transitions.
Using a combination of high resolution synchrotron X-ray and neutron powder diffraction we have studied solid solutions of the type Sr1-xCaxMnO3. For x greater than 0.45 the materials all adopted an orthorhombic structure in Pbnm at room temperature. The diffraction pattern for Sr0.6Ca0.4MnO3 showed the presence of two phases, tetragonal in I4/mcm and orthorhombic in Pbnm. Heating this to 573 K yielded a single phase (I4/mcm) structure. This behaviour is indicative of a first order Pbnm to I4/mcm transition. Variable temperature structural studies of Sr0.5Ca0.5MnO3 show a similar first order Pbnm to I4/mcm transition with extensive two-phase region. Above 573 K the sample is tetragonal (I4/mcm) and this undergoes a continuous transition to cubic near 850 K. We find no evidence for an intermediate Imma phase and suggesting the behaviour of the first row transition metal perovskites AMO3 is different than the analogous heavier second and third row oxides such as SrZrO3 or SrRuO3. Further we find the phase transition behaviour is very sensitive to the environment of the study with both the transition temperature and the nature of the transition being highly influence by oxygen nonstiochiometry.