# Raman scattering in $CaFeO_{3}$ and $La_{0.33}Sr_{0.67}FeO_{3}$ across the charge-disproportionation phase transition

Ghosh, Shankar and Kamaraju, N and Seto, M and Fujimori, A and Takeda, Y and Ishiwata, S and Kawasaki, S and Azuma, M and Takano, M and Sood, AK (2005) Raman scattering in $CaFeO_{3}$ and $La_{0.33}Sr_{0.67}FeO_{3}$ across the charge-disproportionation phase transition. In: Physical Review B, 71 (24).

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Temperature-dependent micro-Raman studies of orthorhombic $CaFeO_{3}$ and rhombohedral $La_{0.33}Sr_{0.67}FeO_{3}$ were carried out with an aim to study the role of a lattice in the formation of the charge-disproportioned state $(Fe^{4+}\rightarrow Fe^{5+}+Fe^{3+})$ below the transition temperature $(T_{co})$ of 290 and 200 K, respectively. Shell-model lattice-dynamical calculations were performed for $CaFeO_{3}$ to assign the Raman modes and determine their vibrational pattern. The temperature dependence of the peak positions and the peak widths of various modes for both systems show distinct changes across their respective transition temperatures. In $CaFeO_{3}$, the symmetric-stretching mode at 707 $cm^{-1}$ splits into two modes, 707 $cm^{-1}$ and 684 $cm^{-1}$, corresponding to the breathing-type distortion of the $FeO_{6}$ octahedra. In comparison, the spectral feature at 704 $cm^{-1}$ in $La_{0.33}Sr_{0.67}FeO_{3}$, which has been assigned to the Raman-forbidden symmetric-stretching mode, disappears below $T_{co}$. These observations indicate the presence of finite Jahn-Teller distortions of the $FeO_{6}$ octahedra in $CaFeO_{3}$ in the entire temperature range, where as these distortions are present only above $T_{co}$ in $La_{0.33}Sr_{0.67}FeO_{3}$. Two modes at 307 $cm^{-1}$ and 380 $cm^{-1}$ in $La_{0.33}Sr_{0.67}FeO_{3}$ approach each other at $T_{co}$, indicating a reduction of rhombohedral distortions below $T_{co}$.