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

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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 Mossbauer 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). 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 modification of CaMn7O12 and PbMn7O12 are also reported.
Funding Information
  • Ministry of Education, Culture, Sports, Science and Technology
  • Russian Foundation for Basic Research (14-03-00768)

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