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

TL;DR

This study investigates the metal-insulator transition in Pr-based cobaltites, revealing a valence shift in praseodymium ions and its correlation with electronic and magnetic properties, challenging previous explanations based solely on cobalt spin states.

Contribution

It demonstrates that the metal-insulator transition involves a significant Pr$^{3+}$/Pr$^{4+}$ valence shift, not just cobalt spin state changes, providing new insights into the transition mechanism.

Findings

Pr$^{4+}$ ions are present in the insulating phase, evidenced by Schottky peaks.

The transition temperature varies with yttrium doping, at 64K and 132K.

Pure Pr$_{0.7}$Ca$_{0.3}$CoO$_3$ remains metallic down to low temperatures.

Abstract

The magnetic, electric and thermal properties of the ($Ln_{1-y}$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.