Richard C. Ward

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.

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 symmetry of the methyl hindering potential and the temperature independence of the tunnel frequency in 4-methylpyridine

Abstract The barrier to methyl rotation is exceptionally small in solid 4-methylpyridine, resulting in unusual properties. Earlier inelastic neutron scattering work concluded that the hindering potential has sixfold symmetry. This question is re-examined and a simpler spectrum found, indicative of the more normal threefold potential. A single tunnel peak at 0.5 meV broadens without shifting as the temperature is raised from 4 to 15 K. The absence of shift is discussed in the context of two different explanations for the shift normally observed with more strongly hindered methyl groups.

Quasielastic neutron scattering investigation of motional processes associated with two dynamic hydrogen populations in the bronze H1.68MoO3

Abstract Variable-temperature incoherent quasielastic neutron scattering (QENS) measurements on instruments of differing elastic energy resolutions and differing ranges of elastic scattering vector magnitude allow characterisation of two (slowly exchanging) dynamic hydrogen populations in the interlayer region in H 1.68 MoO 3 and motional processes associated with them. One population (present in hydroxyl groups) undergoes rapid self-diffusion ( D t (295 K)=4×10 −6 cm 2 s −1 ). The second population (in coordinated H 2 O molecules) has a higher activation barrier to H-atom self-diffusion and takes part in fourfold reorientational motion ( E A =26 kJ mol −1 , τ res 0 ≈10 −14 s).

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.

Structural changes associated with the phase transition in t-butyl cyanide

The structure of the low-temperature phase of t-[2H9]butyl cyanide [(CD3)3CCN] has been determined and refined as a function of temperature between 5 and 232 K (Ttr= 233 K) using powder diffraction data.Although the structure may be refined using a fully ordered model, arguments are given to support an alternative and equally successful refinement involving the onset of disorder similar to that already established in the high-temperature phase. The order parameter is then determined from the unit-cell angle β and its temperature dependence is in reasonable accord with simple mean field behaviour with Tc= 245 K; the transition is shown to be of the order-disorder type.No structural changes have been found near the previously reported transition at 213 K.

Ordered and disordered structures of (CD3ND3)2[SnCl6]

High resolution powder diffraction experiments have been performed on (CD3ND3)2[SnCl6] at various temperatures. In the high temperature R3m phase, the deuterium is equally distributed between two mirror-related positions. Below the 155 K transition (space group R3) only one of these sites is occupied. Implications of this result for other experiments are discussed.

Cooperative dynamics in the disordered phase of tertiary butyl cyanide

The cooperative dynamics in the high temperature disordered phase of tertiary butyl cyanide have been investigated using coherent inelastic neutron scattering techniques. Diffuse scattering peaks have been observed near Brillouin zone centres corresponding to Bragg reflections which are systematically absent in the disordered tetragonal phase but present in the low temperature ordered monoclinic phase and show clearly that local correlations exist which are closely similar in structure to that of the monoclinic phase. The extent of correlation of the local monoclinic structure within the disordered phase is approximately 2 unit cells and the correlation time for the cooperative relaxations among the four local structures is approximately 10-12s. The uniaxial C 3 molecular rotations present in both phases are strongly coupled with the fluctuations in the local structure and there is a suggestion that the two motions become decoupled on approaching the transition. Transverse and longitudinal acoustic phonon...

Resolution of dynamic processes in proton-conducting 12-tungstophosphoric acid 14-hydrate by a combination of quasielastic neutron-scattering and n.m.r. techniques

Combination of variable temperature incoherent quasielastic neutron-scattering (q.e.n.s.) and 1H n.m.r. relaxation time measurements allows the characterisation of dynamic processes associated with proton transport in 12-tungstophosphoric acid 14-hydrate; q.e.n.s. identifies 180°-flips of H2O and enables n.m.r. data to be interpreted in terms of this reorientation and H+ self-diffusion [Dt(295 K)∼ 10–7 cm2 s–1].