Raman scattering in $CaFeO_{3}$ and $La_{0.33}Sr_{0.67}FeO_{3}$ across the charge-disproportionation phase transition
Raman scattering in $CaFeO_{3}$ and $La_{0.33}Sr_{0.67}FeO_{3}$ across the charge-disproportionation phase transition
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DOI：
10.1103/physrevb.71.245110
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发表时间：
2005-06
影响因子：
3.7
通讯作者：
Shankar Ghosh;N. Kamaraju;M. Seto;A. Fujimori;Y. Takeda;S. Ishiwata;S. Kawasaki;M. Azuma;M. Takano;A. Sood
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文献类型：
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作者：
Shankar Ghosh;N. Kamaraju;M. Seto;A. Fujimori;Y. Takeda;S. Ishiwata;S. Kawasaki;M. Azuma;M. Takano;A. Sood
对斜方晶系 $CaFeO_{3}$ 和菱形晶系 $La_{0.33}Sr_{0.67}FeO_{3}$ 进行了温度依赖性显微拉曼研究，目的是研究晶格在转变温度以下形成电荷歧化态 $(Fe^{4+}\rightarrow Fe^{5+}+Fe^{3+})$ 的作用$(T_{co})$ 分别为 290 和 200 K。对 $CaFeO_{3}$ 进行壳模型晶格动力学计算，以指定拉曼模式并确定其振动模式。两个系统的峰位置和各种模式的峰宽度的温度依赖性在各自的转变温度上显示出明显的变化。在$CaFeO_{3}$中，707 $cm^{-1}$处的对称拉伸模式分为两种模式，707 $cm^{-1}$和684 $cm^{-1}$，对应于$FeO_{6}$八面体的呼吸型畸变。相比之下，$La_{0.33}Sr_{0.67}FeO_{3}$中704 $cm^{-1}$处的光谱特征（已分配给拉曼禁止对称拉伸模式）在$T_{co}$以下消失。这些观察结果表明，在整个温度范围内，$CaFeO_{3}$ 中的 $FeO_{6}$ 八面体存在有限的 Jahn-Teller 畸变，而这些畸变仅存在于 $La_{0.33}Sr_{0.67}FeO_{3}$ 中的 $T_{co}$ 以上。 $La_{0.33}Sr_{0.67}FeO_{3}$ 中 307 $cm^{-1}$ 和 380 $cm^{-1}$ 处的两种模式在 $T_{co}$ 处相互接近，表明低于 $T_{co}$ 时菱面体畸变减少。
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 $T_{co}$.