Valence transition in (Pr,Ca)CoO_3 cobaltites: Charge migration at the metal-insulator transition

摘要

X-ray absorption spectroscopy measurements in Pr_(0.5)Ca_(0.5)CoO_3 and (Pr,Y)_(0.55)Ca_(0.45)CoO_3 compositions reveal that the valence of praseodymium ions is stable and essentially +3 (Pr [4 f ~2]) in the metallic state, but abruptly changes when carriers localize approaching the oxidation state +4 (Pr [4 f~1]). This mechanism appears to be the driving force of the metal-insulator transition. The ground insulating state of Pr_(0.5)Ca_(0.5)CoO_3 is a homogeneous Co3-5"* state stabilized by a charge transfer from Pr to Co sites: 1/2 Pr~(3+) +Co~(3.5) → 1/2 pr~(3+2δ) + Co~(3.5-δ) , with 2δ ≈ 0.26e-.