  From Raman spectroscopy, magnetization, and thermal-expansion on the system
La2/3(Ca1-xSrx)1/3MnO3, we have been able to provide a quantitative basis for
the heterogeneous electronic model for manganites exhibiting colossal
magnetoresistance (CMR). We construct a mean-field model that accounts
quantitatively for the measured deviation of TC(x) from the TC predicted by de
Gennes double exchange in the adiabatic approximation, and predicts the
occurrence of a first order transition for a strong coupling regime, in
accordance with the experiments. The existence of a temperature interval
TC<T<T* where CMR may be found is discussed, in connection with the occurrence
of an idealized Griffiths phase.
