Perovskite-Type InCoO₃ with Low-Spin Co³⁺: Effect of In–O Covalency on Structural Stabilization in Comparison with Rare-Earth Series

Perovskite rare-earth cobaltites <i>A</i>CoO₃ (<i>A</i> = Sc, Y, La–Lu) have been of enduring interest for decades due to their unusual structural and physical properties associated with the spin-state transitions of low-spin Co³⁺ ions. Herein, we have synthesized a non-rare-earth perovskite cobaltite, InCoO₃, at 15 GPa and 1400 °C and investigated its crystal structure and magnetic ground state. Under the same high-pressure and high-temperature conditions, we also prepared a perovskite-type ScCoO₃ with an improved cation stoichiometry in comparison to that in a previous study, where synthesis at 6 GPa and 1297 °C yielded a perovskite cobaltite with cation mixing on the <i>A</i>-site, (Sc_0.95Co_0.05)­CoO₃. The two perovskite phases have nearly stoichiometric cation compositions, crystallizing in the orthorhombic <i>Pnma</i> space group. In the present investigation, comprehensive studies on newly developed and well-known <i>Pnma A</i>CoO₃ perovskites (<i>A</i> = In, Sc, Y, Pr–Lu) show that InCoO₃ does not fulfill the general evolution of crystal metrics with <i>A</i>-site cation size, indicating that InCoO₃ and rare-earth counterparts have different chemistry for stabilizing the <i>Pnma</i> structures. Detailed structural analyses combined with first-principles calculations reveal that the origin of the anomaly for InCoO₃ is ascribed to the <i>A</i>-site cation displacements that accompany octahedral tilts; despite the highly tilted CoO₆ network, the In–O covalency makes In³⁺ ions reluctant to move from their ideal cubic-symmetry position, leading to less orthorhombic distortion than would be expected from electrostatic/ionic size mismatch effects. Magnetic studies demonstrate that InCoO₃ and ScCoO₃ are diamagnetic with a low-spin state of Co³⁺ below 300 K, in contrast to the case of (Sc_0.95Co_0.05)­CoO₃, where the high-spin Co³⁺ ions on the <i>A</i>-site generate a large paramagnetic moment. The present work extends the accessible composition range of the low-spin orthocobaltite series and thus should help to establish a more comprehensive understanding of the structure–property relation