PREDICTION OF FERROELECTRICITY IN RECENT INORGANIC CRYSTAL-STRUCTURE DATABASE ENTRIES UNDER SPACE GROUP PBA2

Three new entries in space group Pba2 in the 1995 edition of the Inorg anic Crystal Structure Database have been examined in the light of pre viously developed criteria for the prediction of ferroelectricity. The structural prediction [Abrahams (1989). Acta Cryst. B45, 228-232] tha t seven of the 21 entries presented under this space group in the 1988 edition most Likely corresponded to previously unrecognized ferroelec tric materials is thereby updated. The atomic coordinates of [(NH2)(2) C(NHNH3)]ZrF6, among the new entries, are shown to satisfy the structu ral criteria; the maximum displacement along the polar axis required o f any atom to undergo spontaneous polarization reversal is no larger t han similar to 0.9 Angstrom for a C or N atom and similar to 1.4 Angst rom for a H atom, within the aminoguanidinium(2+) cation. By contrast, all atoms in the two independent ZrF6 anions are within 0.1 Angstrom of an arrangement with zero spontaneous polarization. The characterist ic force constant of the organic group is presently unknown, hence the Curie temperature for the crystal cannot be estimated. In the second new entry, the only atoms in the structure of the super-conductor Ba0. 6K0.4BiO23 at 403 K which depart significantly from a centrosymmetric arrangement are three of the four independent oxygens, one of which is at a site only 6% occupied. If these displacements are not artefacts, then this material is ferroelectric with an estimated Curie temperatu re in the range 410-740 K; a subsequent study, however, reported the s tructure in space group Pbam. The atomic coordinates of the final 1995 entry, (Cl3PNPCl3)(MoOCl4), do not differ significantly from centrosy mmetry, in which case the material is not ferroelectic, apart from une qual occupancy of the Mo-atom sites. If the disorder model is correct and the two Mo sites are unequally occupied, then these sites cannot b ecome equivalent and (Cl3PNPCl3)(MoOCl4) must remain polar without the possibility of becoming ferroelectric.