The structures of deuterated ices VI, VII, and VIII have been studied under their conditions of stability by neutron powder diffraction. The mode of ordering of (tetragonal) ice VIII is clearly established, and no evidence is found of partial ordering as the temperature is raised. Ice VII is accurately cubic with D2O molecules disordered around their center of mass; there is no evidence of partial ordering at any temperature. The water molecule geometry is normal in both phases, and the hydrogen‐bonded first neighbor in ice VIII is confirmed to be more distant than the first nonbonded neighbor. The transition temperature between the two phases occurs at 263±2 K, some 11 K lower than expected. Hydrogen bond lengths in both phases are equal at the transition. Although the ice VI data are not as good, we can see no evidence of the antiferroelectric ordering proposed by Kamb from work on recovered samples. Our results are consistent with thermodynamic measurements indicating disorder in ice VI at 193 K. We co...

Structure and hydrogen ordering in ices VI, VII, and VIII by neutron powder diffraction

Single-particle rotations in molecular crystals

We present the results of a high-resolution neutron diffraction experiment with a fully deuterated methane hydrate type I at temperatures of 2, 100, and 150 K. Precise crystallographic parameters of the ice-like D 2 O lattice and the thermal parameters of the encaged methane molecules have been obtained. The parameters of the host lattice differ only slightly from values found for hydrates with asymmetric guests included, which leads to the conclusion that the host lattice of structure I is only a little adaptive. At low temperatures (2 K) the methane molecules in both types of cages present in structure I occupy positions in the center of the cages. At higher temperatures the thermal parameters in both types of cages reflect the surrounding cage geometries or more precisely the translational potentials of the cages. The orientational scattering length density of the CD 4 molecules has been analyzed in terms of a multipole expansion with symmetry adapted functions [Press and Huller, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. A29, 252 (1972); Press, ibid. A29, 257 (1972)]. In both types of cages we found only small modulations of a spherically symmetric scattering density accounting for almost free rotations of the methane molecules. The large and asymmetric cage leads to a somewhat more pronounced modulation of the orientational density than in the small dodecahedral cage. The orientational probability distribution function (PDF) remains nearly unchanged from 2 to 150 K. At 200 K we observed the time-resolved decomposition of the hydrate structure I into hexagonal ice Ih.

The structure of deuterated methane–hydrate

The structure of succinonitrile in its orientationally disordered phase was reexamined through extended X-ray diffraction measurements. It was solved by using both the analytic procedures of symmetry-adapted functions and a Frenkel model assuming discrete orientations. A possible translation-rotation coupling was included in this latter case via an offset vector e. The study confirms that the nitrogen atoms are localised along the fourfold axis of the cubic cell but evidences a strong offset of the centre of mass for gauche conformations. It is shown that this can be explained by steric hindrance between some configurations of neighbouring molecules.

https://iopscience.iop.org/article/10.1088/0953-8984/2/33/002/pdf

Structure of succinonitrile in its plastic phase

A molecular dynamics (MD) calculation has been carried out for a model of solid methane at a state condition close to the triple point. We have used systems of 32 and 108 molecules interacting via atom–atom potentials. Equilibrium properties such as the configurational energy, pressure, and specific heat have been evaluated and compared where possible with experimental data. The simulated static structure factor has been analyzed assuming that rotational and translational motion can be decoupled and that the rotational structure factor can be developed as a power series in Kubic harmonics. This description is shown to yield a reasonably faithful representation of the structure in the plastic phase. The dynamical structure factor S(Q,ω) has been calculated for selected values of the momentum transfer h/Q. These results have been used to estimate the velocity of sound and hence the elastic constants. The overall agreement of the dynamical properties with experiment is not particularly good, indicating the need for a more refined intermolecular potential.

Molecular dynamics simulation of the plastic phase of solid methane

The concept of a 'rotational form factor' analysis of molecular crystalline structures is generalized by allowing correlations between the orientation of a molecule and its centre-of-mass position. This coupling results in correction terms to the well known rotational form factor. The method is illustrated with the example of (i) tetrahedral molecules and (ii) dumbells at a site of cubic symmetry (m3m) and the ro1e of the site symmetry as well as the magnitude of the mean-squared amplitude (u2) of the centre-of-mass motion are investigated.

A. Hüller

Papers

Analysis of orientationally disordered structures. IV. Correlations between orientation and position of a molecule