Role ofA-site cations in the metal-insulator transition inPr0.5Ca0.5CoO3−γ(γ≈0)

Abstract

Remarkable structural changes take place during the metal to insulator first-order transition in ${\text{Pr}}_{0.50}{\text{Ca}}_{0.50}{\text{CoO}}_{3}$: a contraction of the unit-cell volume, the bending of the Co-O-Co bond angle, and a decrease in (Pr,Ca)-O bond distance. However, the Co-O bond length and the volume of the ${\text{CoO}}_{6}$ octahedra remain almost unaltered during the transition. In view of these data and the absence of transition in ${({\text{Nd}}_{2/3}{\text{La}}_{1/3})}_{0.5}{\text{Ca}}_{0.5}{\text{CoO}}_{3}$, we review the current spin-state transition picture, emphasizing that an active role is played by the $A$-site Pr cations that could be related with the effect of Pr in 123 superconducting cuprates.