Crystal Structure and Phase Transitions of solid deuterium iodide Ph D thesis John H Clarke 1975 London University The crystal structure of solid deuterium iodide was studied by thermal neutron and X-ray diffraction at three temperatures. At 4.2K the low phase structure is ordered, monoclinic space group C2/c, with 16 molecules in the unit cell. At 84K a two-fold disordered orthorhombic structure, space group Bbcm, is proposed with four molecules in the unit cell, each deuterium atom oscillating between two equivalent positions 180° apart. This middle phase is isomorphous with deuterium bromide. At 137K a twelve-fold disordered cubic structure is proposed, space group Fm3m, with four molecules in the face-centred unit cell. The deuterium atom occupies in turn twelve statistically equivalent positions. This structure is isomorphous with the high phases of deuterium chloride and bromide. Phase transitions between the monoclinic and orthorhombic phases at 77K and the orthorhombic and cubic phases at 127K were studied by neutron diffraction in the temperature intervals 45-80K and 110-130K. Order parameters for these transitions have been calculated from the neutron diffraction data and agreement obtained with those calculated from calorimetric data for the lower transition.