Variation of charge-ordering transitions inR1/3Sr2/3FeO3 (R=La,Pr, Nd, Sm, and Gd)

Abstract

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 ${R}_{1/3}{\mathrm{Sr}}_{2/3}{\mathrm{FeO}}_{3} (R=\mathrm{La},$ Pr, Nd, Sm, and Gd) by transmission electron microscopy and measurements of transport, magnetic, and optical properties. In ${R}_{1/3}{\mathrm{Sr}}_{2/3}{\mathrm{FeO}}_{3},$ 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 ${\mathrm{La}}_{1/3}{\mathrm{Sr}}_{2/3}{\mathrm{FeO}}_{3}$ with least rhombohedral lattice distortion undergoes a CO phase transition with ${T}_{\mathrm{CO}}=198 \mathrm{K}$ accompanying charge disproportionation into nominally ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Fe}}^{5+}$ sites, as well as antiferromagnetic spin ordering. When the R-site ion is changed from $R=\mathrm{La}$ to smaller-size R ion towards $R=\mathrm{Gd},$ and hence the $p\ensuremath{-}d$ hybridization interaction is decreased, ${T}_{\mathrm{CO}}$ is decreased and finally the CO transition disappears for $R=\mathrm{Sm}$ and Gd. The optical conductivity spectra for the $R=\mathrm{La}$ -- Nd compounds show a gap opening below ${T}_{\mathrm{CO}}$ and several activated phonon modes due to the periodic charge modulation. The spectral intensity of the new phonon modes shows a discontinuous increase at ${T}_{\mathrm{CO}}$ reflecting the first-order nature of the CO transition. In the cases of the $R=\mathrm{Sm}$ 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 $p\ensuremath{-}d$ hybridization as realized for $R=\mathrm{La},$ Pr, and Nd is necessary for stabilizing the specific valence-skipping charge-ordered state.