Browsing by Author "Wu, E"

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    Determination of the deuterium order-disorder structure transition in Pd-DO.65 at '50K'
    (Australian and New Zealand Institutes of Physics, 1994-02-09) Kennedy, SJ; Wu, E; Krisi, EH; Gray, EM; Kennedy, BJ
    Anomalies in the heat capacity and the electrical resistance of Palladium Hydride at ~50K were first reported in the 1950's(l). Since then a number of experimental techniques have been applied in attempts to determine the origin of these anomalies. These techniques have included XRD and neutron diffraction. All crystallographic studies have been hampered by the sluggishness of this transition (i.e. it takes several days to complete). Early powder diffraction results suggested a phase transition involving migration of H (or D) atoms from sites of octahedral symmetry to sites of tetrahedral symmetry(2). Later evidence from single crystal neutron diffraction studies indicated an order-disorder transition within the octahedral sites(3). Neither description has found global support because conclusive evidence has not been produced. We have just completed a time resolved powder neutron diffraction study of the structural transition in b-Pd-DO 6 5 at S4K using the new Medium Resolution Neutron Powder Diffractometer (MRPD) at the HIFAR research reactor. Our results show that the latter description of an order-disorder transition of the octahedral sites is correct. The 'ordered' crystal structure can be accurately described with space group Pm3n and by doubling the lattice constant from the high temperature structure (Fm3m). The transition to the ordered phase involves diffusion from specific 'D' sites from nearest 'D' neighbour positions to second nearest 'D' neighbour positions. This eventually produces 97% and 33% occupancy of the favoured and unfavoured sites respectively.