Observation of a valence transition in (Pr,Ca)CoO3 cobaltites: charge migration at the metal-insulator transition

TL;DR

This study uses X-ray absorption spectroscopy to reveal a valence transition in (Pr,Ca)CoO3 cobaltites, showing charge migration at the metal-insulator transition driven by praseodymium valence change.

Contribution

It demonstrates the valence change of praseodymium ions and its role in the metal-insulator transition in cobaltites, providing new insights into charge transfer mechanisms.

Findings

Pr ions are stable at +3 in the metallic state.

Pr valence abruptly shifts towards +4 during localization.

Charge transfer from Pr to Co stabilizes the insulating state.

Abstract

X-ray absorption spectroscopy measurements in Pr0.5Ca0.5CoO3 and (Pr,Y)0.55Ca0.45CoO3 compositions reveal that the valence of praseodymium ions is stable and essentially +3 (Pr [4f 2]) in the metallic state, but abruptly changes when carriers localize approaching the oxidation state +4 (Pr [4f 1]). This mechanism appears to be the driving force of the metal-insulator transition. The ground insulating state of Pr0.5Ca0.5CoO3 is an homogeneous Co3.5-d state stabilized by a charge transfer from Pr to Co sites: 1/2Pr3+ + Co3.5 \to 1/2Pr3+2d + Co3.5-d, with 2d \approx 0.26 e-.