Frequency shifts of the internal phonon modes in La0.7Ca0.3MnO3

Infrared reflectivity spectra were measured for a polycrystalline La0.7Ca0.3MnO3 sample. The internal phonon modes, i.e., the bending and stretching modes, of the MnO6 octahedra show significant frequency shifts near the Curie temperature T-C. The frequency shifts of the internal phonon modes were found to have a linear relationship with the excess lattice expansion above T-C, which deviates from a prediction of the Gruneisen law. A strong electron-phonon coupling and a crossover from small to large polaronic states are attributed to the observed frequency shifts.