Oxygen isotope effect on the vibrational modes of La1-xCaxMnO3

Raman-scattering experiments have been carried out on polycrystalline sampl es of La0.65Ca0.35Mn O-16(3) and isotope-exchanged La0.65Ca0.35Mn O-18(3) a s a function of temperature. The most interesting feature in the spectra. i s a vibrational mode that occurs at a nominal frequency of omega(0) approxi mate to 230 cm(-1), which is assigned to an out-of-phase rotational mode th at involves motion of the O(1) atoms. The frequency of this phonon depends strongly on the Ca concentration and has been correlated with the strength of the Jahn-Teller lattice distortion as measured by the tolerance factor. As the temperature is lowered through the critical temperature, the phonon frequency omega(0) increases rather abruptly and continues to harden as the temperature is lowered further. In contrast, the linewidth Gamma(0) decrea ses abruptly when the sample passes through the transition from the paramag netic insulating phase to the more metallic ferromagnetic phase. The observ ed temperature dependences of the frequency and linewidth are well describe d by a model that incorporates a double-exchange mechanism in the presence of an electron-phonon interaction. The agreement between experiment and the ory suggests that the temperature dependence of the phonon is determined pr imarily by the spin alignment associated with the double-exchange mechanism . The results for both omega(0) and Gamma(0) imply that there are no signif icant structural modifications associated with the transition to the ferrom agnetic phase. Finally, the linewidth Gamma(0) in the isotope-exchanged sam ple (O-18) is Smaller than in the O-16 sample, an observation that is consi stent with the smaller electron-phonon-coupling constant in the O-18 compou nd. [S0163-1829(99)06413-9].