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  • 1985-1989  (5)
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  • 1
    Electronic Resource
    Electronic Resource
    A molecular-dynamics simulation study of the β phase of nitrogen (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 7877-7887 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structural characteristics of the β phase of solid nitrogen have been studied for large samples by the molecular-dynamics simulation technique. An effective interatomic potential appropriate to this phase was developed including nonbonded and quadrupole terms. Several properties of the crystal structure have been studied as functions of both temperature and pressure. In particular, a detailed investigation of the molecular orientational order has been made as a function of these parameters. At high temperatures the simulation sample is found to have a hexagonal structure with randomly oriented molecules. As the temperature decreases an orientational ordering transition is found to occur at ≈35 K. The effects of increasing pressure are found to be analogous to those of reducing the temperature. The nature of the local transition in the sample is investigated by calculating correlation parameters which specify twinning and tetrahedral ordering. The variation of the latter indicates that tetrahedral ordering increases strongly with decreasing temperature or increasing pressure, suggesting that the low-temperature phase is a condensation of this arrangement.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457255
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Inelastic neutron scattering determination of phonon dispersion curves in the molecular crystal sym-C6F3Cl3 (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 1918-1923 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Phonon dispersion curves have been obtained along the Δ, Σ, and T directions in a single crystal of sym-C6F3Cl3 at 5 K by inelastic neutron scattering measurements. These have been interpreted within the framework of rigid-molecule lattice dynamics. A model intermolecular potential reproduces the overall behavior, but there remains scope for improvement in the model. Other models based on transferable potentials are found to be less satisfactory.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456033
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    Paper (German National Licenses)
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  • 3
    Electronic Resource
    Electronic Resource
    Crystal structures and phase transition of the cyanospinel K2Hg(CN)4 (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 6004-6009 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The crystal structures of the high temperature (paraelastic) and low temperature (ferroelastic) phases of the cyanospinel K2Hg(CN)4 have been determined by neutron powder diffraction. At 298 K the crystal has the spinel structure and the cyanide coordination is ordered in the sequence N–C–Hg–C–N. Below the phase transition (111 K) the structure is trigonal with space group R3¯c. At the transition Hg(CN)4 tetrahedra adjacent along [111] rotate in antiphase as rigid units by ≈7° and the K ion shifts by ≈0.1A(ring). The transition has an ordering wave vector q0≡{1/2 1/2 1/2}. Comparison is made with the structural characteristics of other cyanospinels and it is suggested that the transitions cannot be understood in terms of steric interactions but must be a dynamic effect.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.451514
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    Paper (German National Licenses)
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  • 4
    Electronic Resource
    Electronic Resource
    Neutron diffraction study of the tricritical orientational order/disorder phase transition in calcite at 1260 K (1989)
    Springer
    Physics and chemistry of minerals 16 (1989), S. 503-507 
    Details
    ISSN: 1432-2021
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract Neutron powder diffraction measurements of the temperature dependence of superlattice reflections in calcite have shown that there is a continuous phase transition at 1260 K. The change in space group symmetry $$({\text{R}}\overline {\text{3}} {\text{c}} \to {\text{R}}\overline {\text{3}} {\text{m}})$$ and the halving of the unit cell size on heating indicate that this transition is an orientational order/disorder transition. The intensities of the superlattice reflections show that the temperature dependence of the order parameter, Q, is of the form (T c −T)β, where β is 0.25, indicating that the transition is tricritical. The transition is accompanied by a large contraction along the c axis on cooling, defining a spontaneous strain e3 which is related to the order parameter (and hence temperature) via e3 αQ 2. No evidence for critical lowering of the value of β was found. These measurements confirm that, apart from the detailed critical behaviour, the phase transition in calcite is similar to that observed in NaNO3.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00197019
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    Paper (German National Licenses)
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  • 5
    Electronic Resource
    Electronic Resource
    Structure of oxygen clathrate hydrate by neutron powder diffraction (1986)
    Springer
    Journal of inclusion phenomena and macrocyclic chemistry 4 (1986), S. 235-240 
    Details
    ISSN: 1573-1111
    Keywords: Hydrate ; oxygen ; structure ; structure ; neutron ; diffraction
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Oxygen hydrate was prepared by reacting oxygen with deuteriated ice at high pressure. Its structure was examined with powder neutron diffraction. It was found to crystallize in the cubic space group $$Fd\bar 3m$$ witha=17.070(1) Å. The formation of the type II structure rather than type I can be attributed to the larger Langmuir constant of sorption for the oxygen molecules in the 12-hedral cages. The encaged oxygen molecules are orientationally disordered. A comparison of the oxygen thermal parameters in both cages suggests the existence of local potential minima in the 16-hedral cages.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00657996
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    Paper (German National Licenses)
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