The detailed crystal structures of deuterated methyl bromide in both its α-and β-phases have been determined at 175 K and 146 K respectively by neutron powder profile measurements. The α-phase has space group Cmc21 while the β-phase has space group Pnma and both have a quasi-two-dimensional character. The major characteristic of the first order transition is a large change in the relative orientation of the two molecules lying in the mirror planes. The transition is successfully interpreted in terms of changes in the topology of the lattice energy surface calculated as a function of temperature from pair-wise interatomic potentials.

The crystal structures of CHBr2Cl and CHBrCl2 have been investigated using neutron powder profile techniques. These structures were found to be similar to that of CHBr3 in its lowest temperature triclinic phase. The degree of disorder of the halogen atoms has been examined and the results indicate that the Br and Cl atoms have a definite preference for some sites over others.

There is ample experimental evidence on the existence of several crystalline phases of C4F8, although they still have been not clearly identified. In this paper we perform a series of molecular dynamics (MD) simulations using a partially flexible molecular model, which takes into account the mixing of the low frequency intramolecular modes and lattice modes. The calculations are carried on in the constant pressure- constant temperature ensemble and the algorithm employed allows volume and symmetry changes of the MD sample as a function of thermodynamic variables. Although several stable crystalline phases are found, their number is still less than found by experiments.

The crystal structures of CHB2Cl and CHBrCl2 have been investigated using neutron powder profile techniques. These structures were found to be similar to that of CHBr3 in its lowest temperature triclinic phase. Whereas CHBr3 has three phases, both CHBr2Cl and CHBrCl2 have only one. The two CHBr3 phases which are absent in these compounds require the molecules to have threefold axes. The degree of disorder of the halogen atoms has been examined and the results indicate that the Br and Cl atoms have definite preferences for some sites over others, so that the molecules on average do not have a threefold axis.

High-resolution neutron powder diffraction was used to study the lattice parameters of three crystals: Rb4LiH3(SO4)(4) (RLHS), deuterated ammonium (NH4)(4)LiH3(SO4)(4) (ALHS), and K4LiH3(SO4)(4) (KLHS) in a temperature range from 5 to 250 K. For RLHS and ALHS a continuous ferroelastic phase transition 4F2 was detected at 134 and 234 K, respectively. KLHS was found to show monoclinic symmetry in the whole temperature region studied. Using the structural data obtained, it was possible to calculate the temperature behavior of spontaneous strain es and its components (e(11)-e(22)) and e(12). This allowed the calculation of the possible temperature changes in the orientation of W' domain walls. The ''pinning energy'' modification of the free energy expansion was introduced to explain the discrepancy between the measured temperature dependence of the order parameter and that expected in the Landau approximation.

The temperature dependence of the rotational and translational motion of methane encaged in a deuterated clathrate hydrate has been studied with incoherent inelastic neutron scattering. The methane molecules were found to behave almost like free rotors even at 6 K. The most interesting observation is that, unlike methane. deposited in rare-gas matrices, the rotational modes do not converge to the classical rotation-diffusion limit at high temperature. The spectral features indicate that the methane interacts strongly with the host lattice vibrations and its motion is damped at high temperature. The methane vibrational density of states obtained from the experiment using a self-consistent iterative procedure to correct for multiexcitation and multiple scattering effect is found to be in good agreement with molecular dynamics calculations.

The low-temperature structures of CBr2Cl2 and CBrCl3 have been determined using powder neutron diffraction. The two phases are isostructural with one another, CBr4 and CCl4, belonging to the C 2/c space group with 32 molecules per unit cell. A preliminary discussion of the high-T phases of these molecules is presented. The high-T phases show patterns consistent with an orientationally disordered (plastic) crystalline phase, consisting of rotating pseudospheres on an FCC lattice. A second disordered phase, between the other two phases, has also been observed in CBr2Cl3. It is still under investigation.

High-resolution neutron powder diffraction was used to study phase transitions in the leucite phases of KAlSi2O6, RbAlSi2O6, CsAlSi2O6, and KFeSi2O6. The temperature-dependent structural behavior involves two mechanisms: relaxation of the tetrahedral framework about channel cations, and slowly changing T-O bond lengths. The high-temperature cubic phase is characterized by a fully-extended tetrahedral framework; thermal expansion occurs by an increase in mean T-O bond lengths. On decreasing temperature, a displacive phase transition to tetragonal symmetry is manifested by an optic instability; twisting of tetragonal prisms of corner-linked (Al,Si)O-4 tetrahedra about [001] leads to collapse of the [111] structural channels and concomitant volume reduction.

The crystal structure of the two phases of chlorodifluoromethane (CHClF2) were determined at 70 K and 10 K using Rietveld refinement of neutron powder profiles. The upper phase is tetragonal P42/n (C4 4 h) with eight molecules in the unit cell occupying general positions. The lattice constants are a = 10•3711(1) Å and c = 5•5915(2) Å. The lower phase is monoclinic P112/n (C2 4 h) with eight molecules in the unit cell with dimensions a = 10•1106(2), b = 10•4830(2), c = 5•5868(2) Å and γ = 90•319(2)°. The asymmetric unit has two molecules in general positions. The phase transition is of a displacive type but there are no large changes in the molecular positions or orientations at the transition.

The crystal structure of dichlorofluoromethane or freon 21 (CHFCl2) has been determined at temperatures ranging from 5 K to 105 K using neutron powder profile techniques. There is only one phase present at all temperatures, with eight molecules per unit cell positioned on general sites in the space group Pbca. Starting positions and orientations of the molecules were determined to high accuracy using an energy minimization process with atom-atom potentials. Coulombic interactions, as represented using atomic charges, were shown to be unimportant in determining the structure.

The structure of fluoroform, CHF3, was determined at 4 . 2, 40 and 70 K using neutron powder profile analysis. There is only one phase with a monoclinic space group P2(1)/c. At the lowest temperature the lattice parameters are: a = 4 . 7561(1) Angstrom, b = 6 . 6280(2) Angstrom, c = 8 . 5232(2) Angstrom beta = 122 . 02(2)degrees. Starting positions and orientations of the four molecules in the unit cell were determined to high accuracy using an energy minimization process with atom-atom potentials. The structure is made up of weakly hydrogen bonded sheets with adjacent sheets related by centres of symmetry.

The structure of perdeuterobenzene adsorbed in zeolitic H-SAPO-37 has been studied by powder neutron diffraction and H-2 NMR. Low temperature (5 K) diffraction measurements reveal that benzene is located both above the six-ring window and in the plane of the 12-ring window and that protons are found only at O(2) where a facial interaction with adsorbed benzene can occur. The results indicate that the acid sites are redistributed under the influence of the sorbate in order to enhance the proton-benzene interactions at the six-ring site. Variable temperature H-2 spin-lattice relaxation measurements show that the diffusion of benzene in H-SAPO-37 is intermediate between those in Na-Y (Si/Al = 1.7) and siliceous Y bur is very similar to those in H-Y and US-Y. We may also infer from the H-2 NMR data that the benzene participates in a facial interaction with the protons at O(2) at low temperatures.

A powder neutron diffraction studyof the structures of D-Y [Fd3m, a = 24.722(1) Angstrom, R(p) = 2.52%, R(wp) = 3.32%] and its complex with benzene [Fd3m, a = 24.712(2) Angstrom R(p) = -3.24%, R(wp) 4.27%] shows that benzene molecules are located above the six-ring window in the supercage and in the plane of the twelve-ring window, and that no redistribution of deuterium atoms occurs upon adsorption of benzene.

The structure of formamide was redetermined at 7 K using neutron powder profile analysis. The space group is P2(1)/n with a = 3.5432(2), b = 8.9512(5), c = 6.9741(4) angstrom and beta = 101.051-degrees. The results are in agreement with earlier X-ray results but a more accurate determination of the hydrogen positions has been made and a lower temperature has reduced anharmonic effects. A simple model using standard exp-6 atom-atom potentials, experimentally determined atomic charges and an exponential attractive potential for the O-D hydrogen bonds gives a good fit to the structural parameters.

The Chalk River Laboratories of AECL Research provides neutron beams for research with the NRU reactor. The NRU reactor has eight reactor loops for engineering test experiments, 30 isotope irradiation sites and beam tubes, six of which feed the neutron scattering instruments. The peak thermal flux is 3 x 1014n cm−2s−1. The neutron spectrometers are operated as national facilities for Canadian neutron scattering research. Since the research requirements for the Canadian nuclear industry are changing, and since the NRU reactor is unlikely to operate much beyond the year 2000, a new Irradiation Research Facility (IRF) is being considered for start-up in the first decade of the next century. An outline is given of this proposed new neutron source.

The structures of the three phases of DBr and DI were determined using neutron powder profile techniques. The highest temperature phases are cubic, Fm3m, with the deuteriums in twelvefold disordered positions about the halogens. The intermediate temperature phases are orthorhombic, Cmca, with the deuteriums in twofold disordered positions about the halogens in the mirror planes. In the lowest temperature phase of DBr, molecular ordering results in zigzag chains of molecules in the mirror planes of an orthorhombic structure with space group Cmc21. Molecular ordering appears in a different way in DI giving distorted diamonds (almost squares), rather than chains, and the rotational sense within the diamonds differs from one plane to the next. DI molecules are parallel or perpendicular to each other but the molecules point 6·27° away from the lines joining the iodines. the structure is triclinic, with space group P1.

The vibrational densities of states in the lattice translation region below 130 cm−1 (3 THz) for structure I xenon hydrate and structure II krypton hydrate have been determined from incoherent inelastic neutron scattering experiments. The main features in the lattice vibrations are found to shift to higher frequencies when compared to ice Ih. The shift is due to repulsive interactions between the water molecules and the guests which help to stabilize the hydrate structure. The experimental observation is in good accord with that predicted from earlier molecular dynamics calculations.

A powder neutron diffraction study of H-SAPO-37 (Fd3; a = 24.804(2) angstrom; R(p) = 2.98%, R(wp) = 3.73%, chi2 = 1.966) reveals Al/P alternation in the framework, with Si substituting for P; protonation, in order of decreasing occupancy, is observed at O(1), O(2) and O(3).

Inelastic neutron scattering measurements on calcite (CaCO3) in its low temperature phase have revealed the existence of an unusual column of inelastic scattering at the wavevector corresponding to the F point of the high temperature Brillouin zone. At the same wavevector there is also a transverse acoustic soft mode and the column of scattering ranges in energy from zero up to the soft mode. The intensity of the anomalous scattering increases rapidly with temperature, and is consistent with an Arrhenius relation the form exp(-T*/T) where T*=1035 K. The authors speculate that this scattering arises from thermal fluctuations of the calcite structure into a different ordered structure, which is related to an ordering instability at the F point. Evidence for this possibility has also been obtained from lattice energy calculations.

We have performed inelastic neutron scattering experiments on calcite in the ordered phase, focusing on a zone boundary F-point that shows X-ray diffuse scattering. At this position we have observed an incipient soft mode along with a column of inelastic scattering ranging in energy between the soft mode energy and zero energy. The intensity of this scattering varies dramatically with temperature, and is described by an Arrhenius relation with an activation energy of 1035 K.