Variation of charge-ordering transitions in R1/3Sr2/3FeO3 (R = La, Pr, Nd, Sm, and Gd)

The change of the electronic and magnetic properties as well as the charge-ordering (CO) transition related with lattice dynamics has been systematically investigated for crystals of R1/3Sr2/3FeO3 (R = La, Pr, Nd, Sm, and Gd) by transmission electron microscopy and measurements of transport, magnetic, and optical properties. In R1/3Sr2/3FeO3, the hybridization of O 2p and Fe 3d states, or the effective d electron hopping interaction, can be controlled to some extent by R-dependent lattice distortion. The La1/3Sr2/3FeO3 with least rhombohedral lattice distortion undergoes a CO phase transition with T-CO = 198 K accompanying charge disproportionation into nominally Fe3+ and Fe5+ sites, as well as antiferromagnetic spin ordering. When the R-site ion is changed from R = La to smaller-size R ion towards R = Gd, and hence the p-d hybridization interaction is decreased, T-CO is decreased and finally the CO transition disappears for R = Sm and Gd. The optical conductivity spectra for the R = La - Nd compounds show a gap opening below T-CO and several activated phonon modes due to the periodic charge modulation. The spectral intensity of the new phonon modes shows a discontinuous increase at T-CO reflecting the first-order nature of the CO transition. In the cases of the R = Sm and Gd compounds with no CO transition, the gap feature is observed over a whole temperature region, while no phonon anomaly is discerned. These results imply that the strong p-d hybridization as realized for R = La, Pr, and Nd is necessary for stabilizing the specific valence-skipping charge-ordered state. [S0163-1829(99)03039-8].