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The crystal structure of deuterated methane hydrate (structure I, space group: Pm3n) was investigated by neutron powder diffraction at temperatures of 7.7–185 K. The scattering amplitude density distribution was examined by a combination of Rietveld method and maximum entropy method (MEM). The distribution of the D atoms in both D2O and CD4 molecules was clarified from three-dimensional graphic images of the scattering amplitude density. The MEM results showed that there were low-density sites for the D atom of D2O in a particular location within the D2O cage at low temperatures. The MEM provided more reasonable results because of the decrease in the R factor that is attainable by this method. Accordingly, the low-density sites for the D atom of D2O probably exist within the D2O cage. This suggests that a spatial disorder of the D atom of D2O occurs at these sites and that hydrogen bonds between D2O molecules become partially weakened. With regard to the CD4 molecules, there were high-density sites for the D atom of CD4, and the density distribution of the C and D atoms was observed separately in the scattering amplitude density image. Consequently, the C-D bonds of CD4 were not observed clearly because the CD4 molecules had an orientational disorder. The D atoms of CD4 were displaced from the line between the C and O atoms, and were located near the face center of the polygon in the cage. Accordingly, the D atoms of CD4 were not bonded to specific O atoms. This result is consistent with the hydrophobicity of the CD4 molecule. We also report the difference between the small and the large cages in the density distribution map and the temperature dependence of the scattering amplitude density. [ABSTRACT FROM AUTHOR]

Abstract: Neutron diffraction studies were performed on the spinel oxide system Mg x Co1−x Cr x Fe2−x O4 (x =0.0, 0.25, 0.50, 0.75 and 1.0) as a function of temperature. Presence of magnetic long-range order at low temperatures was confirmed from the evolution of Bragg reflections with the decrease of temperature. Magnetic structural properties were determined from the Rietveld analysis of neutron diffraction data recorded in the temperature range 10–810K. Sublattice and net magnetic moments for various compositions at various temperatures were found out from the analysis. The transition temperatures were also determined for all the compositions. A decreasing ferrimagnetic order with increasing substitution was revealed from the decreasing ordered sublattice moments. Huge diffuse signals around the 111 Bragg diffraction line were observed in the neutron patterns of the composition x =1.0 below the Curie temperature indicating the existence of magnetic spin clusters. Hysteresis and low field dc magnetization measurements were done on the compositions x =0.75 and 1.0. Thermo-magnetic irreversibility in the zero-field cooled and field cooled magnetization together with a shallow transition below Curie temperature was also observed. The various observations suggest a canted ferrimagnetic ordering in the composition range x ≤0.75 and a semi-spin glass like behavior at x =1.0. A super-paramagnetic behavior below the paramagnetic state is also suggestive from the observed features. [Copyright &y& Elsevier]

Abstract: Temperature-dependent neutron diffraction studies were done on the spinel oxide system Mg x Co1− x Cr x Fe2− x O4 (x=0.0, 0.25, 0.50, 0.75 and 1.0). Neutron diffraction data recorded in the temperature range 10–810K were analyzed in the Rietveld method. Ordered sublattice magnetic moments of the five compositions of the spinel series at various temperatures were deduced from the analysis. A decreasing ordered moment with increasing substitution (x) was found indicating a gradual weakening of ferrimagnetic order. Huge diffuse signals around the 111 Bragg diffraction peak were found in the neutron patterns of the composition x=1.0 below the Curie temperature manifesting a build-up of short-range magnetic spin clusters. The observed features suggest a canted ferrimagnetic structure in the composition range x⩽0.75 and a semi-spin glass-like transition at x=1.0. [Copyright &y& Elsevier]

Abstract: To examine the nature of thermal motions of the CH4 molecules in the methane deuterohydrate (8CH4·46D2O), the scattering length density distribution was observed by the maximum entropy method (MEM) using neutron powder diffraction data measured in the temperature range of 7–185K. We drew the scattering-length density distribution as three dimensional graphic images and used the same isosurface level for all temperatures. The negative scattering length density, corresponding to the H atoms of CH4, was observed only in the large cage. The positive scattering length density attributed to the C atom of CH4 was observed at the center of each cage. With an increase in temperature, the negative and positive scattering length densities in the large cage disappear. The positive scattering length density remains at the center of the small cage regardless of temperature. These results strongly indicate that the motions of CH4 depend on the cage size and geometry. [Copyright &y& Elsevier]

Abstract: We measured neutron diffraction profiles of powdered KOD-doped D2O ice to investigate the formation and growth of ice XI, a deuterium-ordered phase of ice, from ice Ih, a disordered phase. During the measurements, the ice was annealed at 57K for about 17h, and then warmed up and kept at 68K for about 55h, which are below the transition temperature . The diffraction profiles for the doped ice at 57K did not change with time and were close to those for ice Ih with the deuterium-disordered arrangement. However, when the temperature increased from 57 to 68K, the diffraction profiles changed. We carried out the Rietveld analysis with the parameters for a two-phase model to obtain the structural evidence of the appearance and the growth of ice XI. The analysis provided evidence of ice XI as space group Cmc21 at 68K. The temporal increase of the mass fraction of ice XI showed that nucleation of ice XI began just after warming up to 68K. Then, ice XI grew for several days in the doped ice. This study provides first structural observations of the growth of ice XI. [Copyright &y& Elsevier]

Neutrondiffraction patterns for deuterated poly-N,N,-dimethylacrylamide (PDMAA) hydrogels were measuredfrom 10 to 300 K to investigate the structure and properties of waterin the gels. Diffraction peaks observed below 250 K indicate the existenceof ice in the hydrogels. Some diffraction peaks from the ice are atlower diffraction angles than those in ordinary hexagonal ice (Ih).These shifts in peaks indicate that the lattice constants of the aand caxes in the ice are about 0.29and 0.3% higher than those in ice Ih, respectively. The results showthat bulk low-density ice can exist in PDMAA hydrogels. The distortionsin the lattice structure of ice imply significant interactions betweenwater molecules and the surrounding polymer chains, which play animportant role in the chemical and mechanical properties of the hydrogel. [ABSTRACT FROM AUTHOR]