Caesium nitrate (II) at 296 K

Structure of the high-temperature phase of caesium nitrate - the importance of high-resolution data

A single-crystal structure determination of the cubic phase of CsNO₃ based on data collected at 439 K up to sinθ_max/λ = 0.995000 Å^-1, i.e. to an unprecedentedly high-θ value, is reported. The structure has been refined in Pm3m (Z = 1). Analysis of the difference electron-density maps revealed that the most appropriate model is the twelve-orientation model with the Cs, N, O1 and O2 atoms situated on the Wyckoff positions 1a, 6f, 6f and 24l, respectively, rather than the eight-orientation aragonite model with the Cs, N and O atoms situated on the Wyckoff positions 1a, 8g and 24m, respectively. Both models, however, show close similarities if the large anisotropic displacement parameters of the O atoms in the eight-orientation aragonite model are taken into account. The reason for this is shown to lie in the smeared electron density around the positions of the disordered [NO₃]^- anion.

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(NH(2))(3))Al(SO(4))(2)(H(2)O)(6)], another the previously predicted ferroelectric CsNO(3) phase II. Those remaining include K(3)Nb(3)B(2)O(12), the minerals schairerite, galeite and lizardite 1T, LaNi(5)D(6) and gamma-CaNi(5)D(6.1), Ca(OCl)(2)Ca(OH)(2), [N(CH(3))(4)](2)Mo(3)S(13), Li(17)Ag(3)Sn(6) and Cs(3)As(5)O(9). 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.