Inorganic structures in space group P31m; coordinate analysis and systematic prediction of new ferroelectrics

The 62 entries listed in ICSD release 2009/1 under polar space group P31m correspond to 31 families of inorganic crystal structures, some with only one member. Coordinate analysis reveals, over a wide confidence range, 11 of these families as ferroelectric candidates. One includes the well known improper ferroelectric GASH (guanidinium aluminum sulfate hexahydrate), [(C(NH₂)₃)Al(SO₄)₂(H₂O)₆], another the previously predicted ferroelectric CsNO₃ phase II. Those remaining include K₃Nb₃B₂O₁₂, the minerals schairerite, galeite and lizardite 1T, LaNi₅D₆ and γ-CaNi₅D_6.1, Ca(OCl)₂Ca(OH)₂, [N(CH₃)₄]₂Mo₃S₁₃, Li₁₇Ag₃Sn₆ and Cs₃As₅O₉. Candidate selection is based upon detecting an approach by the reported atomic arrangement to the symmetry of a corresponding nonpolar supergroup. A further 13 families are typified by their reduced predictive properties, with four others likely to remain polar at higher temperatures and the remaining three noted as having a unit cell larger than reported or a misassigned space group. The primary sources of uncertainty in structurally based predictions of ferroelectricity are the reliability of the underlying structural determination and the upper limit assigned to the cationic displacement magnitudes required to achieve supergroup symmetry.