Charge transfer, valence, and the metal-insulator transition in Pr0.5Ca0.5CoO3

Generalized gradient approximation (GGA) + U electronic-structure calculations evidence that the recently observed simultaneous metal-insulator and spin-state transition in perovskite cobaltite Pr0.5Ca0.5CoO3 is accompanied by a charge transfer between Pr and Co cations from 0.5 Pr3+ + Co3.5+ toward 0.5 Pr4+ + Co3+. The metal-insulator transition is a result of the interplay between covalency of the Pr-O bond and increasing stability of the insulating low-spin configuration with decreasing valency of Co cation. Our calculations explain why this type of transition is encountered in the Pr-based system only.