Isotope Studies of the CMR Compounds La1−xCaxMnO3+δ

Oxygen isotope effect studies of the ferromagnetic Curie temperature Tc of La1−xCaxMnO3 are presented. The isotope exponent α0=−ΔlnTc/Δlnm0 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 Tc 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 Tc, becoming significantly smaller at and below Tc. IR reflectivity data show a much larger zero frequency IR conductivity than dc conductivity. The IR peaks are independent of temperature between 150 and 295 K.