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

The simple ABO and A-site-ordered AA'BO 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 PbHgTiO, the only known quadruple perovskite that transforms from high-temperature centrosymmetric paraelectric phase to low-temperature non-centrosymmetric ferroelectric phase. The coordination chemistry of Hg is changed from square planar as in typical A-site-ordered quadruple perovskite to a rare stereo type with 8 ligands in PbHgTiO. Thus PbHgTiO appears to be a combinatory link from simple ABO perovskites to A-site-ordered AA'TiO 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'BO-type A-site-ordered perovskites, and opens a direction to search for ferroelectric materials.