Phase transition in Pr 0.5 Ca 0.5 CoO 3 and related cobaltites

We present an extensive investigation (magnetic, electric and thermal measurements and X-ray absorption spectroscopy) of the Pr 0.5 Ca 0.5 CoO 3 and (Pr 1−y Y y ) 0.7 Ca 0.3 CoO 3 (y = 0.0625 − 0.15) perovskites, in which a peculiar metal-insulator (M-I) transition, accompanied with pronounced structural and magnetic anomalies, occurs at 76 K and 40 − 132 K, respectively. The inspection of the M-I transition using the XANES data of Pr-L 3 -edge and Co-K-edge proofs the presence of Pr 4+ ions at low temperatures and indicates simultaneously the intermediate spin to low spin crossover of Co species on lowering the temperature. The study thus definitively confirms the synchronicity of the electron transfer between Pr 3+ ions and Co 3+/4+ O 3 subsystem and the transition to the low-spin, less electrically conducting phase. The large extent of the transfer is evidenced by the good quantitative agreement of the determined amount of the Pr 4+ species, obtained either from the temperature dependence of the XANES spectra or via integration of the magnetic entropy change over the Pr 4+ related Schottky peak in the low-temperature specific heat. These results show that the average valence of Pr 3+ /Pr 4+ ions increases (in concomitance with the decrease of the formal Co valence) below T MI for (Pr 0.925 Y 0.075 ) 0. 7 Ca 0.3 CoO 3 up to 3.16+ (the doping level of the CoO 3 subsystem decreases from 3.30+ to 3.20+), for (Pr 0.85 Y 0.15 ) 0.7 Ca 0.3 CoO 3 up to 3.28+ (the decrease of doping level from 3.30+ to 3.13+) and for Pr 0.5 Ca 0.5 CoO 3 up to 3.46+ (the decrease of doping level from 3.50+ to 3.27+). Copyright EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg 2013