Independent q and 2q Distortions in the Incommensurately Modulated Low-Temperature Structure of NiTa2Se7

Charge ordering (CO) between Fe3+ and Fe5+ species and magnetotransport transition have been shown for the first time in the perovskite Sm1/3Sr2/3FeO3- by electron diffraction microscopy, by 57Fe M?ssbauer spectroscopy, and by measurements of transport and magnetic properties. This compound is charge ordered at TCO=110 K, which is significantly lower than the temperature TN=150 K of the magnetotransport transition. A charge disproportionation 2Fe4+Fe3++Fe5+ is evidenced by M?ssbauer spectroscopy. The electron diffraction study shows the coexistence of orthorhombic and rhombohedral-type domains. A comparison with the other lanthanides (La, Pr, Nd) is also made. This suggests that the size of the Ln3+ ion is not the only parameter which governs the charge ordering in these perovskites, but the oxygen stoichiometry, which influences the Fe5+ formation, may also play a role in the CO phenomenon.