Identification of electronic state in perovskite CaCr O3 by high-pressure studies

CaCrO3 is at the crossover from localized to itinerant electronic behavior, and interpretation of its electronic state has remained controversial. It is a metal from an optical study. However, the collinear type-C antiferromagnetic spin ordering below TN≈90K is characteristic of localized electron magnetism. We have performed many runs of high-pressure synthesis. CaCrO3 crystals can be found in some batches. We have used specific-heat measurement as a diagnostic tool to probe the electronic states near the Fermi energy. An electronic bandwidth is broadened by applying high pressure. The magnetization measurement under pressure reveals a dTN/dP<0. The crystal structural change corresponding to the pressure-induced electron structural change has been monitored by in situ neutron diffraction under high pressure. The t22 d-electron configuration on octahedral site Cr4+ is orbitally threefold degenerate. Local site distortions are argued to show that in CaCrO3 the crossover from localized to itinerant 3d electrons does not result in a charge-density wave in which segregation of the interatomic interactions results in the stabilization of molecular clusters, but in an intraatomic orbital ordering that stabilizes a half-filled localized-electron xy orbital and a 1/4-filled c-axis π∗ band. Local structural changes under pressure reveal a weakening of long-range magnetic order is associated with a smooth Mott-Hubbard transition of the xy electrons.