The change of the electronic structure and lattice dynamics upon the charge-ordering (CO) transition has been investigated for a ${\mathrm{La}}_{1/3}{\mathrm{Sr}}_{2/3}{\mathrm{FeO}}_3$ crystal by measurements of optical spectra. The CO transition, as characterized by sequential 2:1 ordering of nominal ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Fe}}^{5+}{\mathrm{}\left(111\right)\mathrm{}}_c$ sheets at $T_{\mathrm{CO}}=198\mathrm{}\mathrm{K},$ activates several additional optical phonon modes due to the charge modulation and opens an optical gap (up to 2Δ∼0.13 eV). The spectral intensity of the activated phonon mode shows a discontinuous increase at $T_{\mathrm{CO}}$ reflecting the first-order nature of the CO transition. By contrast, the optical gap value increases rather continuously with decreasing temperature below $T_{\mathrm{CO}},$ implying the effect of the concomitant antiferromagnetic spin ordering on the gap magnitude and structure.

• Received 19 June 1998

DOI:https://doi.org/10.1103/PhysRevB.58.R13326