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

LiNbO₃‑Type InFeO₃: Room-Temperature Polar Magnet without Second-Order Jahn–Teller Active Ions

Great effort has been devoted to developing single-phase magnetoelectric multiferroics, but room-temperature coexistence of large electric polarization and magnetic ordering still remains elusive. Our recent finding shows that such polar magnets can be synthesized in small-tolerance-factor perovskites <i>A</i>FeO₃ with unusually small cations at the <i>A</i>-sites, which are regarded as having a LiNbO₃-type structure (space group <i>R</i>3<i>c</i>). Herein, we experimentally reinforce this finding by preparing a novel room-temperature polar magnet, LiNbO₃-type InFeO₃. This compound is obtained as a metastable quench product from an orthorhombic perovskite phase stabilized at 15 GPa and an elevated temperature. The structure analyses reveal that the polar structure is characterized by displacements of In³⁺ (<i>d</i>¹⁰) and Fe³⁺ (<i>d</i>⁵) ions along the hexagonal <i>c</i>-axis (pseudocubic [111] axis) from their centrosymmetric positions, in contrast to well-known perovskite ferroelectrics (e.g., BaTiO₃, PbTiO₃, and BiFeO₃) where <i>d</i>⁰ transition-metal ions and/or 6<i>s</i>² lone-pair cations undergo polar displacements through the so-called second-order Jahn–Teller (SOJT) distortions. Using density functional theory calculations, the electric polarization of LiNbO₃-type InFeO₃ is estimated to be 96 μC/cm² along the <i>c</i>-axis, comparable to that of an isostructural and SOJT-active perovskite ferroelectric, BiFeO₃ (90–100 μC/cm²). Magnetic studies demonstrate weak ferromagnetic behavior at room temperature, as a result of the canted G-type antiferromagnetic ordering of Fe³⁺ moments below <i>T</i>_N ∼ 545 K. The present work shows the functional versatility of small-tolerance-factor perovskites and provides a useful guide for the synthesis and design of room-temperature polar magnets

Room-Temperature Polar Ferromagnet ScFeO₃ Transformed from a High-Pressure Orthorhombic Perovskite Phase

Multiferroic materials have been the subject of intense study, but it remains a great challenge to synthesize those presenting both magnetic and ferroelectric polarizations at room temperature. In this work, we have successfully obtained LiNbO₃-type ScFeO₃, a metastable phase converted from the orthorhombic perovskite formed under 15 GPa at elevated temperatures. A combined structure analysis by synchrotron X-ray and neutron powder diffraction and high-angle annular dark-field scanning transmission electron microscopy imaging reveals that this compound adopts the polar <i>R</i>3<i>c</i> symmetry with a fully ordered arrangement of trivalent Sc and Fe ions, forming highly distorted ScO₆ and FeO₆ octahedra. The calculated spontaneous polarization along the hexagonal <i>c</i>-axis is as large as 100 μC/cm². The magnetic studies show that LiNbO₃-type ScFeO₃ is a weak ferromagnet with <i>T</i>_N = 545 K due to a canted <i>G</i>-type antiferromagnetic ordering of Fe³⁺ spins, representing the first example of LiNbO₃-type oxides with magnetic ordering far above room temperature. A comparison of the present compound and rare-earth orthorhombic perovskites RFeO₃ (R = La–Lu and Y), all of which possess the corner-shared FeO₆ octahedral network, allows us to find a correlation between <i>T</i>_N and the Fe–O–Fe bond angle, indicating that the A-site cation-size-dependent octahedral tilting dominates the magnetic transition through the Fe–O–Fe superexchange interaction. This work provides a general and versatile strategy to create materials in which ferroelectricity and ferromagnetism coexist at high temperatures

Room-Temperature Polar Ferromagnet ScFeO₃ Transformed from a High-Pressure Orthorhombic Perovskite Phase

Multiferroic materials have been the subject of intense study, but it remains a great challenge to synthesize those presenting both magnetic and ferroelectric polarizations at room temperature. In this work, we have successfully obtained LiNbO₃-type ScFeO₃, a metastable phase converted from the orthorhombic perovskite formed under 15 GPa at elevated temperatures. A combined structure analysis by synchrotron X-ray and neutron powder diffraction and high-angle annular dark-field scanning transmission electron microscopy imaging reveals that this compound adopts the polar <i>R</i>3<i>c</i> symmetry with a fully ordered arrangement of trivalent Sc and Fe ions, forming highly distorted ScO₆ and FeO₆ octahedra. The calculated spontaneous polarization along the hexagonal <i>c</i>-axis is as large as 100 μC/cm². The magnetic studies show that LiNbO₃-type ScFeO₃ is a weak ferromagnet with <i>T</i>_N = 545 K due to a canted <i>G</i>-type antiferromagnetic ordering of Fe³⁺ spins, representing the first example of LiNbO₃-type oxides with magnetic ordering far above room temperature. A comparison of the present compound and rare-earth orthorhombic perovskites RFeO₃ (R = La–Lu and Y), all of which possess the corner-shared FeO₆ octahedral network, allows us to find a correlation between <i>T</i>_N and the Fe–O–Fe bond angle, indicating that the A-site cation-size-dependent octahedral tilting dominates the magnetic transition through the Fe–O–Fe superexchange interaction. This work provides a general and versatile strategy to create materials in which ferroelectricity and ferromagnetism coexist at high temperatures