Infrared reflectivity spectra were measured for a polycrystalline ${\mathrm{La}}_{0.7}$${\mathrm{Ca}}_{0.3}$Mn${\mathrm{O}}_3$ sample. The internal phonon modes, i.e., the bending and stretching modes, of the Mn${\mathrm{O}}_6$ 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 Grüneisen law. A strong electron-phonon coupling and a crossover from small to large polaronic states are attributed to the observed frequency shifts.

• Received 14 December 1995

DOI:https://doi.org/10.1103/PhysRevLett.77.1877