Charge ordering and Mössbauer studies of single crystal R1/3Sr2/3FeO3 (R=Pr, Sm, and Nd)

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Abstract
Single crystals of R1/3Sr2/3FeO3 (R=Pr, Nd, and Sm) were synthesized by the floating zone method and their magnetic properties and charge ordering (CO) transition related to lattice dynamics were systematically investigated. Mössbauer spectra of R1/3Sr2/3FeO3 were taken at various temperatures ranging from 12 K to room temperature. The charge disproportionation in Pr1/3Sr2/3FeO3 was detected below 190 K, in which two kinds of iron with valence states Fe3+ and Fe5+ were found with ratio of 2:1. The iron with valence state Fe4+ in Pr1/3Sr2/3FeO3 coexists at and above 150 K, and its ratio increased from 13% to 66% as the temperature rose. The (Nd1−ySmy)1/3Sr2/3FeO3 (y=0.0, 0.2, 0.4, 0.6, and 0.8) with least lattice distortion underwent a CO phase transition at and below TCO=163 K and accompanying the charge disproportionation into nominally Fe3+ and Fe5+ sites as well as a canted antiferromagnetic spin ordering. In this charge ordering state, a sequence of Fe+3Fe+3Fe+5Fe+3Fe+3Fe+5 exists aligned along the [111] direction of the pseudocubic perovskite structure. In this study, the CO at x=2/3 disappeared in the case of R=Sm. Indeed, disappearance of the CO transition was detected by systematic decrease of a spontaneous magnetization with increase of y in the system of (Nd1−ySmy)1/3Sr2/3FeO3. This result shows that the charge ordering state was realized with strong hybridization between the Fe and O atoms.