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

Single crystals of R1/3Sr2/3FeO3 (R = Pr, Nd, and Sm) were synthesized by t he floating zone method and their magnetic properties and charge ordering ( CO) transition related to lattice dynamics were systematically investigated . Mossbauer spectra of R1/3Sr2/3FeO3 were taken at various temperatures ran ging from 12 K to room temperature. The charge disproportionation in Pr1/3S r2/3FeO3 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 st ate Fe4+ in Pr1/3Sr2/3FeO3 coexists at and above 150 K, and its ratio incre ased from 13% to 66% as the temperature rose. The (Nd1-ySmy)(1/3)Sr2/3FeO3 (y = 0.0, 0.2, 0.4, 0.6, and 0.8) with least lattice distortion underwent a CO phase transition at and below T-CO = 163 K and accompanying the charge disproportionation into nominally Fe3+ and Fe5+ sites as well as a canted a ntiferromagnetic spin ordering. In this charge ordering state, a sequence o f 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 disappea red in the case of R = Sm. Indeed, disappearance of the CO transition was d etected by systematic decrease of a spontaneous magnetization with increase of y in the system of (Nd1-ySmy)(1/3)Sr2/3FeO3. This result shows that the charge ordering state was realized with strong hybridization between the F e and O atoms. (C) 2000 American Institute of Physics. [S0021-8979(00)63908 -7].