Valence Shift of Pr Ion from 3+ to 4+ in (Pr1-yYy)0.7Ca0.3CoO3 Estimated by X-Ray Absorption Spectroscopy

The temperature dependence of the X-ray absorption near-edge structure (XANES) spectra at the Pr L-3 edge was measured for the (Pr1-yYy)(0.7)Ca0.3CoO3 samples (y = 0.075 and 0.15), in which a peculiar metal-insulator (MI) transition and a spin-state (SS) transition took place simultaneously at a critical temperature T-MI. The valence states of praseodymium ion were determined using the analyses of XANES spectra. The results suggest that the average valence of the praseodymium ion increases below room temperature from the common value 3.0+, undergoes a steepest change at T-MI, and reaches finally 3.15+ and 3.27+ at 8K for the y = 0.075 and 0.15 samples, respectively. The final valences are consistent with those estimated by the entropy change using the Schottky peak due to the Pr4+ Kramers doublet in the low-temperature specific heat shown in our previous paper [Phys. Rev. B 82 (2010) 165107].