Charge ordering and Mo¨ssbauer studies of single crystal R[sub 1/3]Sr[sub 2/3]FeO[sub 3] (R=Pr, Sm, and Nd).

Single crystals of R[sub 1/3]Sr[sub 2/3]FeO[sub 3] (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. Mo¨ssbauer spectra of R[sub 1/3]Sr[sub 2/3]FeO[sub 3] were taken at various temperatures ranging from 12 K to room temperature. The charge disproportionation in Pr[sub 1/3]Sr[sub 2/3]FeO[sub 3] was detected below 190 K, in which two kinds of iron with valence states Fe[sup 3+] and Fe[sup 5+] were found with ratio of 2:1. The iron with valence state Fe[sup 4+] in Pr[sub 1/3]Sr[sub 2/3]FeO[sub 3] coexists at and above 150 K, and its ratio increased from 13% to 66% as the temperature rose. The (Nd[sub 1-y]Sm[sub y])[sub 1/3]Sr[sub 2/3]FeO[sub 3] (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[sub CO]=163 K and accompanying the charge disproportionation into nominally Fe[sup 3+] and Fe[sup 5+] sites as well as a canted antiferromagnetic spin ordering. In this charge ordering state, a sequence of Fe[sup +3]Fe[sup +3]Fe[sup +5]Fe[sup +3]Fe[sup +3]Fe[sup +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 (Nd[sub 1-y]Sm[sub y])[sub 1/3]Sr[sub 2/3]FeO[sub 3]. This result shows that the charge ordering state was realized with strong hybridization between the Fe and O atoms. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]