Low-Temperature Structural Modulations in CdMn7O12, CaMn7O12, SrMn7O12, and PbMn7O12 Perovskites Studied by Synchrotron X-ray Powder Diffraction and Mossbauer Spectroscopy

Structural phase transitions in CdMn7O12, CaMn7O12, SrMn7O12, and PbMn7O12 perovskites are investigated by synchrotron X-ray powder diffraction between 113 and 583 K, differential scanning calorimetry (DSC), and Mossbauer spectroscopy on Fe-57-doped samples. DSC is used to determine phase transition temperatures (T-OO). All the compounds crystallize in space group R-3 at room temperature. An incommensurate structural modulation is observed below T-OO = 265 K in SrMn7O12 with a propagation vector q = (0, 0, 0.9235) at 113 K, similar to the already reported case of CaMn7O12 (with T-OO = 258 K and q = (0, 0, 0.9215) at 113 K). However, superstructure reflections of SrMn7O12 are significantly weaker than those of CaMn7O12. On the other hand, a commensurate structural transition is found in CdMn7O12 below Too = 254 K and in PbMn7O12 below T-OO = 294 K; the transition can be described as from space group R-3 to space group P-3 with the same unit cell dimensions (a = 10.43306(2) angstrom and c = 6.33939(1) angstrom in CdMn7O12 at 113 K). CdMn7O12 shows quite strong superstructure reflections at 113 K, while PbMn7O12 has extremely small superstructure reflections. Below T-OO, the c lattice parameter of all the compounds increases with decreasing temperature (down to 113 K). Mossbauer spectroscopy shows that quadrupole splitting noticeably increases below Too, and we quantitatively explain this increase in CaMn7O12 by structural modulations. Structure parameters of the R-3 modification and high-temperature Im (3) over bar modification of CaMn7O12 and PbMn7O12 are also reported.