Structures of Solid Deuterium above and below the λ Transition as Determined by Neutron Diffraction

Abstract

Reproducible neutron diffraction patterns were obtained from samples of powdered deuterium with para $\text{(J\hspace{0.17em}=\hspace{0.17em}1)}$ 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 $\lambda$ 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 $\lambda$ 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 $\text{Pa3}$ with four molecules per unit cell and $\text{a\hspace{0.17em}=\hspace{0.17em}5.074 Å}$ . 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 $\delta$ . In general, $\delta$ 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.