Charge transfer between the transition metal and rare earth ions in perovskites

The project is directed to the rare earth (RE) based perovskites with metal ions of paramagnetic (low-spin Co4+) or diamagnetic (low-spin Co3+, Ga3+) character. It aims in an experimental and theoretical evidence for a stabilization of the mixed valence of selected RE ions in the calcium doped RE1-xCaxMO3 perovskites. This leads in some cases to a first order metal-insulator (M-I) transition that is associated with a charge transfer between the M and RE ions and to the change of their ionic configurations. We plan to identify and characterize new cobaltites and gallates (M = Co, Ga) with rare earths in the mixed valence, including those with occurrence of M-I transition, and investigate them by using different means. The focus is given to the heat capacity at very low temperatures (0.4 - 2 K) and neutron spectroscopy measurements that will serve as powerful local probes as concerns the identification of non-Kramers ions (as Pr3+) and Kramers ions (as Pr4+). The results will be confronted with the ab-initio and effective Hamiltonian calculations.