Metal-insulator transition and the Pr3+/Pr4+ valence shift in (Pr1-yYy)0.7Ca0.3CoO3

The magnetic, electric, and thermal properties of the (Ln(1-y)Y(y))(0.7)Ca0.3CoO3 perovskites (Ln=Pr, Nd) were investigated down to very low temperatures. The main attention was given to a peculiar metal- insulator (M-I) transition, which is observed in the praseodymium based samples with y=0.075 and 0.15 at TM-I=64 K and 132 K, respectively. The study suggests that the transition, reported originally in Pr0.5Ca0.5CoO3, is not due to a mere change in cobalt ions from the intermediate to the low- spin states but is associated also with a significant electron transfer between Pr3+ and Co3+/Co4+ sites, so that the praseodymium ions occur below TM-I in a mixed Pr3+/Pr4+ valence. The presence of Pr4+ ions in the insulating phase of the yttrium doped samples (Pr1-yYy)(0.7)Ca0.3CoO3 is evidenced by Schottky peak originating in Zeeman splitting of the ground-state Kramers doublet. The peak is absent in pure Pr0.7Ca0.3CoO3 in which metallic phase, based solely on nonKramers Pr3+ ions, is retained down to the lowest temperature.