Magnetism and the spin state in cubic perovskite CaCo O3 synthesized under high pressure

Cubic SrCoO3 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 CaMO3 (M=M4+ of transition metals, Ge4+, Sn4+, and Zr4+) 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 CaCoO3 with a HPHT treatment. Surprisingly, CaCoO3 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 CaCoO3 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 t4e1 of Co4+ at the octahedral site. The thermal expansion of the Co-O bond length indicates that the population of high spin state t3e2 increases for T>100K. The shortest Co-O bond length in cubic CaCoO3 is responsible for delocalizing electrons in the π∗-band and itinerant-electron ferromagnetism at T<54K. A comprehensive comparison between SrCoO3 and CaCoO3 and the justification of their physical properties by first-principles calculation have also been made in this report. Partially filled π∗ and σ∗ bands would make CaCoO3 suitable to study the Hund's coupling effect in a metal.