Alan J. Leadbetter

The equilibrium low‐temperature structure of ice

Following the discovery of a first‐order phase transition in annealed KOH‐doped ice at 72 K, which was identified as the order–disorder transition associated with the proton positions, a structural study has now been made. Powder neutron diffraction measurements on KOD‐doped D2O above and below the phase transition, together with x‐ray diffraction measurements, reveal a partial ordering of the hydrogen atoms at low temperatures. The equilibrium structure of ice at low tempertures has orthorhombic symmetry, space group Cmc21, with the same lattice as the high‐temperature Ih (P63/mmc) modification. The structure is polar and there is evidence that the ordered domains are less than about 40 A in dimension.

Effect of preparation method on the structure of amorphous solids in the system AsS

Abstract A brief summary is given of previous neutron and X-ray diffraction data for vapour-deposited and bulk vitreous AsSx. The structure of the vapour-deposited material contains As4S4 molecular fragments. On the other hand an analysis of the first diffraction peak for vitreous AsSx and GeSe2 indicates that the evidence for a conventional layer structure in bulk chalcogenide glasses is much less certain.

The structure of vapour-deposited arsenic sulphides

Abstract High resolution X-ray and neutron correlation functions have been obtained for arsenic sulphide bulk glass and vapour-deposited films by making measurements to high momentum transfers. These data are compared with a vatiety of models and it is concluded that the structure of the films is dominated by the presence of As4S4 molecules in the vapour phase, which results in more As-As bonds than the minimum required by stoichoimetry. The extent to which these As4S4 molecules polymerise in the as-deposited film is unclear, but depends strongly on preparation conditions.

Structure of the smectic D phase

Abstract The so-called ‘smectic’ D phase of 4′-n-octadecyloxy-3′-cyanobiphenyl-4-carboxylic acid (one of only four materials known to exhibit this phase) has been shown unambiguously by X-ray diffraction to be characterized by a primitive cubic space group. The space group is either P23 or Pm3, and the lattice parameter a 0 is 86 A. It is shown that data from previous studies of this phase may be re-intepreted to be consistent with these findings. In view of these conclusions it is clearly inappropriate to refer to the phase as smectic.

Structure of the D phase. II

Abstract Results are presented of a Fourier analysis of quantitative X-ray diffraction data obtained from a D phase single crystal, the structure of which has previously been shown to possess primitive cubic symmetry. The Fourier analysis method determines the distribution of electron density in the crystallographic unit cell from the intensities and phases of the Bragg reflections. Diffraction data provide no information on the phase factors, and the Fourier method is therefore practical only when the structure is centrosymmetric, as is shown to be the case for the D phase, since the phase factors are then either 0 or π. It is shown that there are essentially only four physically reasonable solutions for the electron density distribution. The physical interpretation of these models is discussed in terms of molecular packing in the unit cell.

Structure determination of the low-temperature phase of tertiary butyl cyanide by the constrained profile refinement of the powder diffraction pattern

The application of the constrained powder diffraction profile refinement program, EDINP, to determine the structure of the low-temperature phase (crystal II) of tertiary butyl cyanide (CD3)3CCN from neutron diffraction data is described. The structure has been refined at 5 and 147 K and the effect of the quality of the powder on the final parameters obtained from the refinement has been investigated. This work is the first step in a detailed investigation of structural changes through the order–disorder transition at 233 K.

The phase transition in t-butyl cyanide [(CH3)3CCN]

Extensive neutron scattering measurements have been carried out on t-butyl cyanide over a wide temperature range. Crystal structures and the potential for methyl-group rotation have been determined and will be reported elsewhere; this paper is concerned with the whole-molecule motions as determined by incoherent quasielastic and inelastic scattering measurements. One transition only has been observed at 233 K and the whole-molecule reorientational motions determined between 190 and 280 K (m.p. 292 K). At all temperatures the molecules undergo rapid reorientational motion about their C—C—N axes in a 3-fold potential which is strongly temperature dependent. The transition is associated with an order of magnitude decrease in the rotational correlation time and the onset of a slower, strongly cooperative motion involving translational as well as rotational displacements of the molecules. These probably involve relaxations between different local structures each having essentially the monoclinic structure of the low-temperature phase and related by rotations of π/2 resulting in the average tetragonal symmetry of the high-temperature phase. The neutron results lead to a self-consistent reinterpretation of previous n.m.r. data.

Neutron scattering studies on the hydroquinone clathrate compound of deuterium sulphide

Abstract High resolution powder diffraction experiments were carried out on the clathrate compound [C6D4(OD)2]3(D2S)0.96 at several temperatures. Above the 7.6 K phase transition the molecules are preferentially orientated lying flat in the ab plane of the R 3 structure and are disordered between six equivalent sites. Below the transition the molecular orientation becomes more localised, probably between 2 sites.

Dynamics of molecular reorientations in t-butyl bromide studied by incoherent quasielastic neutron scattering

The whole-molecule motions in the three solid phases of t-butyl bromide have been studied using incoherent quasielastic neutron-scattering measurements. In all three solid phases the molecules undergo rapid reorientational motion about their C—Br axes in a three-fold potential. This was the only observable motion in the lowest-temperature phase (phase III), whilst in phase II an additional motion consisting of ca. 30–60° fluctuations of the dipolar axis was found. In phase I the observed motion was consistent with a model for isotropic rotational diffusion of the molecules. A comparison between the whole-molecule reorientations in t-butyl bromide, chloride and cyanide is made.

Structure of the disordered phase of t-butyl cyanide determined using neutron scattering methods

The structure of the high-temperature phase of t-butyl cyanide has been determined in detail by neutron scattering experiments on single crystals of a fully deuteriated specimen. The structure is tetragonal (P4/n, a= 6.85 A, c= 6.77 A) with two molecules per unit cell. Analysis of the intensities of 82 independent reflections show the structure to be disordered, each molecule having one-quarter occupancy of four positions related by π/2 rotations about the four-fold axes. The diffuse scattering intensity shows clearly that local correlations exist which are closely similar in structure to that of the low-temperature monoclinic phase, with a correlation length of ca. 7 A.New incoherent quasi-elastic scattering measurements provide the basis for a slightly modified analysis of previous results, which are then in complete accord with the above model. The correlation time for rotation of the molecules about their three-fold axes is 10–12s and for the cooperative relaxations among the four local structures the correlation time is [graphic omitted]10–11 s.