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

Abstract

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 Mössbauer spectroscopy on 57Fe-doped samples. DSC is used to determine phase transition temperatures (TOO). All the compounds crystallize in space group R-3 at room temperature. An incommensurate structural modulation is observed below TOO = 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 TOO = 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 TOO = 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) Å and c = 6.33939(1) Å in CdMn7O12 at 113 K). CdMn7O12 shows quite strong superstructure reflections at 113 K, while PbMn7O12 has extremely small superstructure reflections. Below TOO, the c lattice parameter of all the compounds increases with decreasing temperature (down to 113 K). Mössbauer 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 Im3 modification of CaMn7O12 and PbMn7O12 are also reported.