Abstract
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 ${\mathrm{Fe}}^{5+}{(111)}_{c}$ sheets at ${T}_{\mathrm{CO}}=198\mathrm{K},$ activates several additional optical phonon modes due to the charge modulation and opens an optical gap (up to 2\ensuremath{\Delta}\ensuremath{\sim}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.
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