High-pressure synthesis and research progress of multi-order perovskite oxides

<sec><p indent="0mm">Multi-order perovskite oxide is derived from ABO₃ perovskite oxides. In this kind of ordered structure, three-quarters of the A sites are replaced by transition metal Aʹ cations, and A and Aʹ sites are orderly arranged in a 1:3 ratio, resulting in the formation of the A-site ordered quadruple perovskite oxide AAʹ₃B₄O₁₂. Furthermore, when half of the B sites are replaced by another different ion Bʹ, meanwhile the B and Bʹ sites are arranged in a rocksalt-type order, the A- and B-site ordered quadruple perovskite oxide AAʹ₃B₂Bʹ₂O₁₂ thus forms. Because of the ability to accommodate versatile magnetic ions in various cation sites, as well as the framework constructed by corner-sharing A′O₄ square-planar and B/BʹO₆ octahedra units, multi-order perovskite oxides hold abundant magnetic and electric interactions, giving rise to interesting physical properties such as high dielectric constant, zero/negative thermal expansion, charge ordering, high-temperature magnetism, colossal magnetoresistivity, ferroelectricity, magnetoelectric multiferroicity, half-metallicity and electrocatalysis. These features make multi-order perovskite oxides significant for fundamental research and promising for practical applications. Nevertheless, studies on multi-order perovskite oxides are quite limited on account of the size mismatch between the small Aʹ cations and the B cations. A high-pressure technique is usually indispensable to synthesize such compounds. </sec><sec> In this review, we first make a brief introduction to the structural characteristics of multi-order perovskite oxides and the high-pressure synthesis technique employed. Then we focus on the recent progress of multi-order perovskite oxides in three different fields of magnetoelectric multiferroic, half-metallicity, and electrocatalysis: (1) the first observation of magnetoelectric multiferroicity in a cubic perovskite LaMn₃Cr₄O₁₂. The magnetic structure breaks the spatial inversion symmetry, thus giving rise to spin-induced ferroelectricity. Especially, when the A site is replaced by Bi³⁺ with 6<italic>s</italic>² lone pair electrons, both strong magnetoelectric coupling and large electrical polarization are realized in BiMn₃Cr₄O₁₂. Furthermore, in TbMn₃Cr₄O₁₂, the nontrivial topologic Roman surface was experimentally observed for the first time. In another case, in CaFe₃Ti₄O₁₂, the displacement of the magnetic Fe²⁺ ion gives rise to the ferroelectricity. The antiferromagnetic ground state of Fe²⁺ can be readily tuned by external magnetic fields toward ferromagnetic corrections. As a result, large magnetoelectric coupling effects are realized in this displacement-type ferroelectric. (2) The realization of ferrimagnetic half-metallicity in LaCu₃Fe₂Re₂O₁₂ with a high Curie temperature up to <sc>710 K</sc> accompanied with a wide majority-spin gap of <sc>2.3 eV</sc> and a large saturated magnetization of 8 <italic>μ</italic>_B f.u.⁻¹. Half-metallicity was also observed in the osmium-based multi-order perovskite oxides NaCu₃Fe₂Os₂O₁₂, and the Fermi surface can be well-tuned by changing the A site cation. (3) Enhanced activity of oxygen evolution reaction (OER) in CaCu₃Ir₄O₁₂ owing to the strong 3<italic>d</italic>-2<italic>p</italic>-5<italic>d</italic> orbital hybridizations via the corner-shared Cu-O-Ir framework. The special orbital symmetry as well as the delicate 3<italic>d</italic>-5<italic>d</italic> levels enhance orbital overlap, thereby promoting charge transfer. The OER procedure of CaCu₃Co₂Ru₂O₁₂ was traced by <italic>in-situ</italic> X-ray absorption spectroscopy, and the lattice-oxygen oxidation mechanism is responsible for its high OER activity. Finally, we make perspectives on multi-order perovskite oxides and their potential applications in future technologies, such as logic elements, spintronics devices, and electrocatalysis. </sec>