Oxygen isotope effect on the vibrational modes ofLa1−xCaxMnO3

Raman-scattering experiments have been carried out on polycrystalline samples of ${\mathrm{La}}_{0.65}{\mathrm{Ca}}_{0.35}{\mathrm{Mn}\mathrm{}}^{16}{\mathrm{O}}_3$ and isotope-exchanged ${\mathrm{La}}_{0.65}{\mathrm{Ca}}_{0.35}{\mathrm{Mn}\mathrm{}}^{18}{\mathrm{O}}_3$ as a function of temperature. The most interesting feature in the spectra is a vibrational mode that occurs at a nominal frequency of ${\omega }_0\approx 230{\mathrm{cm}}^{\mathrm{-}1},$ which is assigned to an out-of-phase rotational mode that 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 ${\Gamma }_0$ decreases abruptly when the sample passes through the transition from the paramagnetic insulating phase to the more metallic ferromagnetic phase. The observed temperature dependences of the frequency and linewidth are well described by a model that incorporates a double-exchange mechanism in the presence of an electron-phonon interaction. The agreement between experiment and theory suggests that the temperature dependence of the phonon is determined primarily by the spin alignment associated with the double-exchange mechanism. The results for both ${\omega }_0$ and ${\Gamma }_0$ imply that there are no significant structural modifications associated with the transition to the ferromagnetic phase. Finally, the linewidth ${\Gamma }_0$ in the isotope-exchanged sample ${(}^{18}\mathrm{O})$ is smaller than in the ${}^{16}\mathrm{O}$ sample, an observation that is consistent with the smaller electron-phonon-coupling constant in the ${}^{18}\mathrm{O}$ compound.