Local lattice distortions and Raman spectra in the La1-xCaxMnO3 system

Raman spectra were measured for the full range of hole doping of the orthom anganites of the form La1-xCaxMnO3 (0 less than or equal to x less than or equal to 1) from ambient temperature to 75 K. Two high-frequency phonons th at we assign to stretching modes of the oxygen atoms in the MnO6 octahedra soften below the transition temperature to ferromagnetic insulator and disa ppear at a temperature that depends on the Ca concentration. The correlatio n of their properties with the lattice distortions is examined. At room tem perature, and for x similar to 0.35 and x similar to 0.70, phonons attribut ed to the relative rotations of the octahedra show a strong correlation wit h the octahedral distortion and the reported low-temperature modifications of the magnetic properties of these compounds. [S0163-1829(99)02342-5].