Identification of electronic state in perovskiteCaCrO3by high-pressure studies

$\mathrm{CaCr}{\mathrm{O}}_{3}$ 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 ${T}_{N}\ensuremath{\approx}\phantom{\rule{0.16em}{0ex}}90\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ is characteristic of localized electron magnetism. We have performed many runs of high-pressure synthesis. $\mathrm{CaCr}{\mathrm{O}}_{3}$ 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 $d{T}_{N}/dP<0$. The crystal structural change corresponding to the pressure-induced electron structural change has been monitored by in situ neutron diffraction under high pressure. The ${t}_{2}^{2}$ d-electron configuration on octahedral site $\mathrm{C}{\mathrm{r}}^{4+}$ is orbitally threefold degenerate. Local site distortions are argued to show that in $\mathrm{CaCr}{\mathrm{O}}_{3}$ 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 ${\ensuremath{\pi}}^{*}$ 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.