Isotope effect on Raman spectra of polycrystalline La0.67Ca0.33MnO3

Micro-Raman spectra were recorded for two samples of polycrystalline La0.67 Ca0.33MnO3, which were synthesized from oxides with oxygen isotopes O-16 an d O-18. Isotope Raman shifts were measured and compared with the theoretica l prediction. In this way the oxygen-related Raman modes were clearly ident ified. Temperature-related blue Raman shifts for O-16 (O-18) vibration mode s at 230 cm(-1) (216 cm(-1)) were unexpectedly observed. These blue Raman s hifts were compared with the blue shift due to lattice parameter variation as the temperature is lowered. The large blue Raman shift that occurs with decrease in temperature demonstrates that the Jahn-Teller distortion may pl ay an important role. The appearance of new peaks at 440-480 and 410-450 cm (-1) for O-16 and O-18 samples, respectively, as the temperature decreases to 140-100 K indicates a possible charge-orbit ordering transition. A conti nuous low-wavenumber signal was observed which is superimposed on the broad phonon Raman lines. It was found that the scattering intensity of the low- wavenumber signal is dramatically reduced as the temperature is lowered. Th e nature of this excitation is not clear. Copyright (C) 2001 John Wiley & S ons, Ltd.