Perovskite based materials show many interesting physical, chemical and mineral properties such as ferromagnetism, ferroelectricity, piezoelectricity, ion conductivity, photocatalysis and superconductivity, etc. The most popular perovskite is in the simple form of ABO₃, in which A-site is often occupied by large size ions such as alkali metal, alkaline earth or lanthanide cations with 12 fold coordination while the 6-fold coordinated B-site is often occupied by small size ions such as transition metals, in order to meet the tolerance factor relationship and the principle of electric neutrality. Most of the known perovskite and their derivatives are derived from simple perovskite. With the advancement of high-pressure synthesis technology, multi-ordered perovskites with several simple perovskite units in a single unite cell becomes possible to synthesize. For example, if 3/4 of A-site of simple perovskite is replaced by another ion, a new A-site ordered quadruple perovskite AA'₃B₄O₁₂ will be derived, in which both A'-site and B-site can accommodate transition metal ions, resulting in multiple interactions such as A'-A, A'-B, B-B, etc., which cannot be realized in simple perovskite, and then inducing intriguing physical properties, such as charge transfer, magnetoelectric coupling and high dielectric properties, etc. Ferroelectricity is the most well-known functions of perovskite compound. Though large numbers of displacive type ferroelectric materials possess simple perovskite structures, such as BaTiO₃, PbTiO₃ or Pb(Ti, Zr)O₃, it is seldom observed in multi-ordered perovskite compounds for the absence of centrosymmetric to centrosymmetric phase transition. Finding a bridge between simple perovskite and A-site-ordered perovskite in structure is a challenge in material design and synthesis.

Prof. Jin's group, from Institute of Physics CAS/Beijing National Laboratory for Condensed Matter Physics, has worked on perovskite and its related materials by high-pressure synthesis for long time. With new techniques, they designed and fabricated a variety of new functional materials containing perovskite structural units (Nature 375, 301 (1995); Phys. Rev. B 61, 778 (2000); Phys. Rev. Lett. 96, 46408 (2006); App. Phys. Lett. 91,172502 (2007); J. Solid State Chem. 182, 327 (2009); PNAS 105, 7115 (2008); PNAS 116, 12156 (2019); Angew. Chem. 59, 8240 (2020)).

Recently, Prof. Jin's group in collaboration with Prof. Weng Hongming have made important progress on high pressure synthesis of A-site-ordered perovskite ferroelectricity. Under the guidance of Prof. Jin and Associate Prof. Runze Yu, Dr. Zhao Jianfa has successfully fabricated a new A-site quadruple ordered perovskite PbHg₃Ti₄O₁₂ (PHTO) by using advanced high pressure synthesis technology, and built a structural bridge connecting simple perovskite and A-site-ordered perovskite, as shown in Figure 1. It is found that PHTO is located in the middle region between simple perovskite and A-site ordered perovskite. Moreover Hg is 8-coordinated, which is just between the simple perovskite (coordination number of A is 12) and the conventional A-site-ordered perovskite (coordination number of A' is 4). This indicates that PHTO is a new structure which connects simple perovskite and conventional A-site ordered quadruple perovskite. Further study shows that the dielectric constant of PHTO has a ferroelectric peak at 250 K (Fig. 2a, b), and an obvious hysteresis loop is observed below the ferroelectric transition temperature (Fig. 2c, d). Synchrotron X-ray diffraction of variable temperature shows that a phase transition from centrosymmetric structure (Im-3) to non centrosymmetric structure (Imm2) takes place at about 250 K, and then ferroelectric phase transition is induced. First principles calculations by Prof. Weng Hongming's team show that PHTO is a direct band gap semiconductor with a band gap of about 1.70 eV, and the ferroelectric distortion is dominated by Ti-O phonon mode anomaly (Fig. 3). This is the first time to find a noncentrosymmetric structural phase transition and displacive ferroelectricity in A-site ordered quadruple perovskite AA'₃B₄O₁₂, which also provides a new research direction for finding ferroelectric materials with high Curie temperature.

The related research results are published in Nature Communications 12, 747 (2021) (https://www.nature.com/articles/s41467-020-20833-6). The research benefited from the collaboration of Dr. Zhiwei Hu of Max Planck Institute in Germany, Professor Greenblatt of Rutgers University in the United States, Dr. Yang Ren of Argonne National Laboratory and Qingzhen Huang of National Institute of Standards and Technology in the United States. This work is supported by the Ministry of Science and Technology, National Natural Science Foundation of China and Beijing Natural Science Foundation.