Systematic prediction of new ferroelectrics in space group R3. I

Release 2005/1 of the Inorganic Crystal Structure Database contains 158 entries under the space group R3. The first 81 together with 15 related entries include 52 different structure types, 10 of which represent families that have two or more isostructural members. There are 18 types that satisfy the structural criteria for ferroelectricity with a confidence level that necessarily depends upon the reliability of each structural determination. Among them, (LiTaO₃)₉·Ta₂O₅ and the CsCd(NO₂) family are known ferroelectrics. The remaining 16 include the high-intensity higher-order harmonic generator II-BaB₂O₄, not previously recognized as ferroelectric. The other predicted ferroelectrics are Te(OH)₆·NaF, the MgSO₃·6H₂O family, Li₃P₃O₉·3H₂O, SeNbF₉, Al₂(H₂PO₄)₃PO₄·6H₂O, [Cr(OCN₂H₄)₆]·[(Co(NH₃)₂(NO₂)₄]₃·(H₂O)_1.5], NaZnBr₃·5H₂O, III-Ag₃SI, Tl₂S, the Ba₃Yb₄O₉ family, the Fe_10.7Ru_1.3As₅ family, Sr₁₇Ta₁₀S₄₂, Pb₈Sb₃ and the Sc₇I₁₂C family. Six structures reported in the space group R3 most likely exhibit mirror symmetry, with three experimental confirmations. The other 19 structures are most likely nonpolar, with six experimental confirmations. The capacity for physical property prediction by the remaining ten structures is shown to be in doubt. Thus, 46% of the present 52 inorganic structure types initially determined in the space group R3 and 35% of those currently assigned to R3 more likely exhibit an additional inversion center, mirror or glide plane, strongly exceeding the recently reported error rate of ∼ 11% in organic structures.