Neutron inelastic scattering and diffraction techniques have been used to study the MnV2O4 spinel system. Our measurements show the existence of two transitions to long-range ordered ferrimagnetic states, the first collinear and the second noncollinear. The lower temperature transition, characterized by development of antiferromagnetic components in the basal plane, is accompanied by a tetragonal distortion and the appearance of a gap in the magnetic excitation spectrum. The low-temperature noncollinear magnetic structure has been definitively resolved. Taken together, the crystal and magnetic structures indicate a staggered ordering of the V d orbitals. The anisotropy gap is a consequence of unquenched V orbital angular momentum.

The formation of CO2 clathrate hydrate was investigated by using time-of-flight neutron powder diffraction at temperatures ranging from 230 to 290 K with a CO2 gas pressure of 900 psi. CO2 clathrate hydrate was prepared in situ from deuterated ice crystals at 230, 243, 253, and 263 K by pressurizing the system with CO2 gas to produce the hydrate in approximately 70% yield. Nearly complete conversion from the hexagonal ice to the sI type CO2 hydrate was observed as the temperature of the sample was slowly increased through the melting point of D2O ice. The conversion of ice into hydrate is believed to be a two-stage process in which an initial fast conversion rate is followed by a slower, diffusion-limited rate. On the basis of a shrinking core diffusion model, an activation energy of 6.5 kcal/mol was obtained from the temperature dependence of the reaction. Our findings suggest that the formation of the hydrate is through a reaction between CO2 and water molecules in the quasi-liquid layer (QLL). The CO2 hydrate remained stable following removal of excess liquid CO2 and subsequent pressurization with helium, allowing for a low-temperature (14 K) structure analysis from powder diffraction data without the presence of solid CO2.

The kinetics of methane hydrate formation was investigated by in-situ time-of-flight neutron powder diffraction. Samples were prepared from deuterated ice particles (< 0.25 mm) and transformed to clathrate hydrate by pressurizing the system with methane gas. The rates of sI methane hydrate formation were measured in-situ under isothermal conditions with a methane pressure of 1000 psi (6.9 MPa). Kinetic data were analyzed in terms of a shrinking core model, including possible contributions of nucleation, methane diffusion, and interface reaction. The data support the hypothesis that methane hydrate formation reaction from ice particles is diffusioncontrolled. The reaction starts quickly at the nucleation stage, which propagates to form a hydrate layer that covers the ice particle. Further reaction is limited by the growth of the hydrate layer and inward diffusion of methane molecules through the hydrate layer to the unreacted ice core. The reaction rate at the interface between hydrate and unreacted ice particle is fast compared to that of methane diffusion. The conversion of ice particle to methane hydrate follows Arrhenius behavior, from which an activation energy of 14.7(5) kcal/ mol was derived. Complete transformation of ice to methane hydrate was achieved through temperature rampingsa nonisothermal procedure that involves slowly increasing the sample temperature through the ice melting point.

The first example of a pressure-induced Jahn-Teller distortion switch has been observed in a single-crystal time-of-flight neutron diffraction study of the Tutton salt ( N D~)~[ C U ( D Z O )~] ( S~~)~ at T = 15 K, P = 1 bar (h) and T = 15 K, P = 1.5 kbar (h). Also studied was the hydrogenous salt ( N H~)~[ C U ( H~O )~] ( S O~)~ at T = 14 K, P = 1.4 kbar (HH). The unit cell parameters are as follows: h, a = 9.451(2) (2), Z = 2, V = 674.5(2) A3, space group = P21/a. Application of 1.5 kbar of pressure to the deuterated crystal produces a decrease in the lengths of the a and b axes by 0.315(2) and 0.451(4) A, respectively, whereas the c axis increases by 0.275( 1) A. Comparison of the 4. and L~H structures shows that the long axis of the Jahn-Teller distortion has switched by 90°, Le., Cu(l)-0(7) = 2.022(2) and Cu(1)-O(8) = 2.310(2) A for 4. vs Cu(1)-0(7) = 2.290(2) and Cu(1)-O(8) = 2.014(2) A for h, so that the DH and HH structures are similar. For HH Cu(1)-0(7) = 2.272(2) and Cu(1)-O(8) = 2.005(2) A. The switching of the long axis of the Jahn-Teller distortion appears to be associated with the rotation of the ND4+ ion with a concomitant change in the hydrogen bonding of the coordinated water molecules with the S042-ions. The resulting adiabatic potential energy surfaces are calculated using tetragonal and orthorhombic strain parameters estimated from the

Recent advances in the design and synthesis of organic synthetic metals have yielded materials that have the highest superconducting transition temperatures (T(c) approximately 13 kelvin) reported for these systems. These materials have crystal structures consisting of alternating layers of organic donor molecules and inorganic anions. Organic superconductors have various electronic and magnetic properties and crystal structures that are similar to those of the inorganic copper oxide superconductors (which have high T(c) values); these similarities include highly anisotropic conductivities, critical fields, and short coherence lengths. The largest number of organic superconductors, including those with the highest T(c) values, are charge-transfer salts derived from the electron donor molecule BEDT-TTF or ET [bis(ethylenedithio)-tetrathiafulvalene]. The synthesis and crystal structures of these salts are discussed; their electrical, magnetic, and band electronic structure properties and their many similarities to the copper oxide superconductors are treated as well.