Raman scattering in CaFeO₃ and La_0.33Sr_0.67FeO₃ across the charge-disproportionation phase transition

Abstract

Temperature-dependent micro-Raman studies of orthorhombic CaFeO₃ and rhombohedral La_0.33Sr_0.67FeO₃ were carried out with an aim to study the role of a lattice in the formation of the charge-disproportioned state (Fe⁴⁺→Fe⁵⁺+Fe³⁺) below the transition temperature (T_co) of 290 and 200 K, respectively. Shell-model lattice-dynamical calculations were performed for CaFeO₃ 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 CaFeO₃, 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 FeO₆ octahedra. In comparison, the spectral feature at 704 cm⁻¹ in La_0.33Sr_0.67FeO₃, 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 FeO₆ octahedra in CaFeO₃ in the entire temperature range, whereas these distortions are present only above T_co in La_0.33Sr_0.67FeO₃. Two modes at 307 cm⁻¹ and 380 cm⁻¹ in La_0.33Sr_0.67FeO₃ approach each other at T_co, indicating a reduction of rhombohedral distortions below T_co.