Isotope studies of the CMR compounds La1-xCaxMnO3+δ

Oxygen isotope effect studies of the ferromagnetic Curie temperature T-c of La1-xCaxMnO3 are presented. The isotope exponent alpha(0) = -Delta ln T-c/Delta ln m(0) changes from 0.4 to 0.14 in the range 0.2 < x < 0.43. The isotope exponent decreases strongly with increasing tolerance factor, or decreasing lattice distortion. Above T-c the conductivity is characteristic of small polarons. Raman scattering shows a prominent peak at 230 cm(-1). The peak width could be related to site-dependent Jahn-Teller distortions above T-c, becoming significantly smaller at and below T-c. IR reflectivity data show a much larger zero frequency IR conductivity than de conductivity. The IR peaks are independent of temperature between 150 and 295 K.