A combinatory ferroelectric compound bridging simple ABO₃ and A-site-ordered quadruple perovskite

The simple ABO₃ and A-site-ordered AA′₃B₄O₁₂ perovskites represent two types of classical perovskite functional materials. There are well-known simple perovskites with ferroelectric properties, while there is still no report of ferroelectricity due to symmetry breaking transition in A-site-ordered quadruple perovskites. Here we report the high pressure synthesis of an A-site-ordered perovskite PbHg₃Ti₄O₁₂, the only known quadruple perovskite that transforms from high-temperature centrosymmetric paraelectric phase to low-temperature non-centrosymmetric ferroelectric phase. The coordination chemistry of Hg²⁺ is changed from square planar as in typical A-site-ordered quadruple perovskite to a rare stereo type with 8 ligands in PbHg₃Ti₄O₁₂. Thus PbHg₃Ti₄O₁₂ appears to be a combinatory link from simple ABO₃ perovskites to A-site-ordered AA′₃Ti₄O₁₂ perovskites, sharing both displacive ferroelectricity with former and structure coordination with latter. This is the only example so far showing ferroelectricity due to symmetry breaking phase transition in AA′₃B₄O₁₂-type A-site-ordered perovskites, and opens a direction to search for ferroelectric materials.