ReC15 forms monoclinic crystals with lattice constants: a=9.24, b=11.54, c= 12.03/~; /~=109.1 ° The space group is P21/c, and the unit cell contains eight ReC15 units. Full-matrix, least-squares refinement of diffractometrically measured X-ray diffraction data resulted in a final R of 9.3% (all reflections). The structure is based on a double-hexagonal closest packing of chlorine atoms, in which rhenium atoms occupy one-fifth of the octahedral holes. Two such octahedra share an edge to form a dimeric Re2Cll0 unit. The Re-CI bond distances involving bridging C1 average to 2.465+0-013/~; the average bond length to nonbridging C1 is 2.244 + 0.012/~. Rhenium atoms in the Re2Cll0 units are each shifted by about 0.20/~ from the centers of the octahedra in directions away from one another. The packing mode of C1 in ReCI5 contrasts with a hexagonal closest packing of C1 in TaC15 and a cubic closest packing of C1 in UC15. Of these, ReC15 exhibits both the highest density and the smallest interatomic distances.

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Reproducible neutron diffraction patterns were obtained from samples of powdered deuterium with para (J = 1) concentrations equal to 3%, 33%, 63%, and 80%. The diffraction patterns were obtained at various temperatures in the range 1.9°–13°K and for the 80% sample at temperatures both above and below the λ anomaly in the heat capacity. The deuterium powdered samples were randomly oriented except for an estimated preferred orientation of 8% or less. All of the samples froze from the liquid to a hexagonal structure with the molecules at the close-packed sites. In the case of the 80% paradeuterium sample a transformation from the hcp structure to a cubic structure occurred at a lower temperature than has been observed for the λ anomaly in the heat capacity. Observations of the transformation between the hexagonal and cubic phases, on warming or cooling, have shown a large hysteresis effect. The cubic-phase diffraction patterns corresponded to space group Pa3 with four molecules per unit cell and a = 5.074 Å. Absolute integrated intensities of the five observed lines were calculated and are in excellent agreement with observed values. The hexagonal-phase lattice parameters for the above samples are presented. An analysis of the integrated intensities of the six observed hexagonal lines indicated a disordered structure. Four of the five structural models under consideration gave an average site symmetry which is equivalent to that produced by a molecule precessing about the z direction with a mean precession angle δ. In general, δ was found to increase with increasing para concentration except for 63% paradeuterium, at 2°K, where a spherical average site symmetry (i.e., complete disorder) was indicated.