Analysis of orientationally disordered structures. II. Examples: solid CD₄,p-D₂andMD₄Br

Abstract

In a number of examples a new method of analysing orientational structures of molecular crystals is illustrated. An application to solid heavy methane demonstrates a close analogy to magnetic structures. In the case of solid p-D2, structure factors are calculated on the basis of the rotational wave functions of the individual molecules and a comparison with neutron diffraction data is performed. Finally several solid phases of ND4Br, representative of the ammonium halides, are investigated and a better understanding of the orientational order in the NaCl-type phase is achieved.

The structures of crystalline hydrogen bromide

“…Using a shell model to describe the hydrogen scattering-length density requires 5 symmetry-adapted functions for / <4 (Press, 1973;Prandi, 1981): K ou Κ 2Λ23 , and tf 4 . 567 are given in Table 1 in the stereographic projections of Figure 6.…”

The determination of the effective rotational potential in the plastic phase of C2Cl6from neutron Bragg scattering data

“…Press and Hfiller [2,3] expanded the probability density function of finding a scatterer on a sphere around the molecular centre into a series of symmetry adapted surface harmonics. However the scattering length density distribution obtained by this procedure may have negative values in some regions of space, as can be seen, for example, from our previous investigation of the plastic phase of hexachloroethane [4] or from the analysis of CBr 4 and SF 6 [5].…”

Orientational disorder in the cubic phase of ΚΝO2

“…Orientationally disordered molecular crystals are often composed of nearly spherical, rigid molecules like CD 4 , or NH 4 in some ammonium halides (Adachi et al, 1968;Press, 1973). It is by no means necessary that the averaged site symmetry which is very often mbm or 43m and the molecular symmetry are identical.…”