Isotope Studies of the CMR Compounds La1AξCaξMnO3Cδ

Oxygen isotope effect studies of the ferromagnetic Curie temperature Tc of La1AxCaxMnO3 are presented. The isotope exponent α0G−∆ ln Tcy∆ ln m0 changes from 0.4 to 0.14 in the range 0.2FxF0.43. 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. 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.