The low-temperature phase separation in Pr0.5Ca0.5CoO3

Abstract The perovskite cobaltite Pr 0.5 Ca 0.5 CoO 3 has been characterized by the magnetic and electric transport measurements, and structurally studied by neutron diffraction. The sample is a metallic-like conductor in a broad temperature range. On cooling, it undergoes at 75 K an incomplete transition to the ferromagnetic phase of enhanced metallic character. The residual paramagnetic regions exhibit at about 30 K a metal–insulator (M–I) transition, which is interpreted as a change from the t 2g 5 σ *0.5 cobalt states with collective e g electrons to a mixture of localized Co 3+ ( t 2g 6 ) and Co 4+ ( t 2g 5 ) low-spin states. The new insulating phase is of the same orthoperovskite type but distinguished by about 2% smaller unit cell volume compared to the ferromagnetic phase. The lattice contraction is caused by significant shortening of Pr,Ca–O bonding distances while the Co–O bond lengths remain nearly constant.