Observation of a valence transition in (Pr,Ca)CoO3 cobaltites: charge   migration at the metal-insulator transition
  X-ray absorption spectroscopy measurements in Pr0.5Ca0.5CoO3 and (Pr,Y)0.55Ca0.45CoO3 compositions reveal that the valence of praseodymium ions is stable and essentially +3 (Pr [4f 2]) in the metallic state, but abruptly changes when carriers localize approaching the oxidation state +4 (Pr [4f 1]). This mechanism appears to be the driving force of the metal-insulator transition. The ground insulating state of Pr0.5Ca0.5CoO3 is an homogeneous Co3.5-d state stabilized by a charge transfer from Pr to Co sites: 1/2Pr3+ + Co3.5 \to 1/2Pr3+2d + Co3.5-d, with 2d \approx 0.26 e-. 
Jos\'e Luis Garc\'ia-Mu\~noz, Carlos Frontera, Aura J.   Bar\'on-Gonz\'alez, Sergio Valencia, Javier Blasco, Ralf Feyerherm,...
cond-mat.mtrl-sci