Electron-phonon coupling through the orthorhombic to rhombohedral phase transition in La 2/3(Ca 1−xSr x) 1/3MnO 3 manganites

Abstract

We present a systematic study of the Raman modes of a ferromagnetic series of manganites, La 2/3(Ca 1− x Sr x ) 1/3MnO 3 with 0⩽ x⩽1, that presents two different regimes for the variation of the ferromagnetic order temperature, T C, with Sr doping. This change occurs for x∼0.5 when the temperature of the orthorhombic, Pbnm, to rhombohedral, R-3 c, structural phase transition, T t, coincides with T C. We have analyzed the evolution of the frequencies and widths of the observed Raman modes as a function of Sr doping and temperature. At room temperature, features of the Pbnm structure are detected for compounds up to x=0.6, over the orthorhombic phase limit x=0.45 at 300 K. Octahedra bending modes behave as expected with doping while the tilt mode, which is related to Mn–O–Mn angles and therefore to electronic conduction mechanisms, presents different behaviors in both structures. The tilt frequency is much less sensitive to Mn–O–Mn angle in R-3 c than in Pbnm structure indicating a reduction of electron-phonon coupling. Its width presents an anomalous behavior both as a function of doping and temperature with unexpectedly large width in the rhombohedral phase.