Metal-insulator transition and the Pr${}^{3+}$/Pr${}^{4+}$ valence shift in (Pr${}_{1-y}$Y${}_y$)${}_{0.7}$Ca${}_{0.3}$CoO${}_3$

Abstract

The magnetic, electric and thermal properties of the ($L{n}_{1-y}$Y${}_y$)${}_{0.7}$Ca${}_{0.3}$CoO${}_3$ perovskites ($Ln$~=~Pr, Nd) were investigated down to very low temperatures. The main attention was given to a peculiar metal-insulator transition, which is observed in the praseodymium based samples with $y=0.075$ and 0.15 at $T_{M-I}=64$ and 132~K, respectively. The study suggests that the transition, reported originally in Pr${}_{0.5}$Ca${}_{0.5}$CoO${}_3$, is not due to a mere change of cobalt ions from the intermediate- to the low-spin states, but is associated also with a significant electron transfer between Pr${}^{3+}$ and Co${}^{3+}$/Co${}^{4+}$ sites, so that the praseodymium ions occur below $T_{M-I}$ in a mixed Pr${}^{3+}$/Pr${}^{4+}$ valence. The presence of Pr${}^{4+}$ ions in the insulating phase of the yttrium doped samples (Pr${}_{1-y}$Y${}_y$)${}_{0.7}$Ca${}_{0.3}$CoO${}_3$ is evidenced by Schottky peak originating in Zeeman splitting of the ground state Kramers doublet. The peak is absent in pure Pr${}_{0.7}$Ca${}_{0.3}$CoO${}_3$ in which metallic phase, based solely on non-Kramers Pr${}^{3+}$ ions, is retained down to the lowest temperature.

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