Magnetism and the spin state in cubic perovskite CaCoO3 synthesized under high pressure

Abstract

Cubic $\mathrm{SrCo}{\mathrm{O}}_{3}$ with an intermediate spin state can only be stabilized by high pressure and high temperature (HPHT) treatment. It is metallic and ferromagnetic with the highest Curie temperature of the transition-metal perovskites. The chemical substitution by Ca on Sr sites would normally lower crystal symmetry from cubic to orthorhombic as seen in the perovskite family of $\mathrm{Ca}M{\mathrm{O}}_{3}$ ($M={M}^{4+}$ of transition metals, $\mathrm{G}{\mathrm{e}}^{4+}, \mathrm{S}{\mathrm{n}}^{4+}$, and $\mathrm{Z}{\mathrm{r}}^{4+}$) at room temperature. This structural change narrows the bandwidth, so as to further enhance the Curie temperature as the crossover to the localized electronic state is approached. We report a successful synthesis of the perovskite $\mathrm{CaCo}{\mathrm{O}}_{3}$ with a HPHT treatment. Surprisingly, $\mathrm{CaCo}{\mathrm{O}}_{3}$ crystallizes in a simple cubic structure that remains stable down to 20 K, the lowest temperature in the structural study. The new perovskite has been thoroughly characterized by a suite of measurements including transport, magnetization, specific heat, thermal conductivity, and thermoelectric power. Metallic $\mathrm{CaCo}{\mathrm{O}}_{3}$ undergoes two successive magnetic transitions at 86 K and 54 K as temperature decreases. The magnetization at 5 K is compatible with the intermediate spin state ${t}^{4}{e}^{1}$ of $\mathrm{C}{\mathrm{o}}^{4+}$ at the octahedral site. The thermal expansion of the Co-O bond length indicates that the population of high spin state ${t}^{3}{e}^{2}$ increases for $Tg100\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The shortest Co-O bond length in cubic $\mathrm{CaCo}{\mathrm{O}}_{3}$ is responsible for delocalizing electrons in the ${\ensuremath{\pi}}^{*}$-band and itinerant-electron ferromagnetism at $Tl54\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. A comprehensive comparison between $\mathrm{SrCo}{\mathrm{O}}_{3}$ and $\mathrm{CaCo}{\mathrm{O}}_{3}$ and the justification of their physical properties by first-principles calculation have also been made in this report. Partially filled ${\ensuremath{\pi}}^{*}$ and ${\ensuremath{\sigma}}^{*}$ bands would make $\mathrm{CaCo}{\mathrm{O}}_{3}$ suitable to study the Hund's coupling effect in a metal.