Prediction of ferroelectricity in recent Inorganic Crystal Structure Database entries under space group Pba2

Abstract

Three new entries in space group Pba2 in the 1995 edition of the Inorganic Crystal Structure Database have been examined in the light of previously developed criteria for the prediction of ferroelectricity. The structural prediction [Abrahams (1989). Acta Cryst. B45, 228–232] that seven of the 21 entries presented under this space group in the 1988 edition most likely corresponded to previously unrecognized ferroelectric materials is thereby updated. The atomic coordinates of [(NH2)2C(NHNH3)]ZrF6, among the new entries, are shown to satisfy the structural criteria; the maximum displacement along the polar axis required of any atom to undergo spontaneous polarization reversal is no larger than ~ 0.9 Å for a C or N atom and ~ 1.4 Å for a H atom, within the aminoguanidinium (2+) cation. By contrast, all atoms in the two independent ZrF6 anions are within 0.1 Å of an arrangement with zero spontaneous polarization. The characteristic 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 superconductor Ba0.6K0.4BiO23 at 403 K which depart significantly from a centrosymmetric arrangement are three of the four independent O atoms, 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 temperature in the range 410–740 K; a subsequent study, however, reported the structure in space group Pbam. The atomic coordinates of the final 1995 entry, (Cl3PNPCl3)(MoOCl4), do not differ significantly from centrosymmetry, in which case the material is not ferroelectic, apart from unequal occupancy of the Mo-atom sites. If the disorder model is correct and the two Mo sites are unequally occupied, then these sites cannot become equivalent and (Cl3PNPCl3)(MoOCl4) must remain polar without the possibility of becoming ferroelectric.