Diffraction Studies of Ordered Phases and Phase Transitions

Kristian Kjær

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    Abstract

    Two investigations are reported here. First, monolayers of CF4 physisorbed on the (001) face of graphite have been studied by means of x-ray diffraction experiments carried out at the electron storage ring DORIS in HAMBURG. The exfoliated graphite substrate UCAR-ZYX was used in order to obtain a large area for adsorption and hence a large sample. Four two-dimensional solid phases of the CF4 films were seen, including a structure which is 2 * 2 commensurate relative to the substrate. On compression (by variation of coverage or temperature), this phase transforms to a uniaxially compressed structure ("stripe" phase). Further, at higher coverages a hexagonal structure was seen, incommensurate relative to the substrate, and at low temperatures and coverages, a complicated structure emerged, giving three close diffraction peaks in the powder pattern. Data are presented characterizing the meltings and commensurate to incommensurate transitions.

    Complementary to the synchrotron x-ray data, a presentation of the theory of synchrotron radiation is given.

    The second investigation was of the ferromagnetic phase transitions in the randomly diluted, dipolar coupled uniaxial ferromagnets LiTb.3Y.7F4 and LiH0.3Y.7F4 by neutron diffraction at the RISØ DR 3 reactor. Crystals were grown, the expected (Scheelite) structure was confirmed and the structural parameters were refined for the Tb compound on the basis of a neutron diffraction structural study. Mounted in a 3He-4He dilution refrigerator, the crystals were cooled to their Curie points of 0.49 and 0.36K for the Tb and the Ho compound, respectively. Just above the phase transition, the critical neutron scattering of either crystal exhibits the asymmetry characteristic of a dipolar coupled uniaxial ferromagnet; however, an anomaly appears at low momentum transfers for the Tb compound only. Further, data are presented for che magnetic Bragg scattering of both crystals.
    Original languageEnglish
    Place of PublicationRoskilde
    PublisherRisø National Laboratory
    Number of pages177
    ISBN (Print)87-550-1016-4
    Publication statusPublished - 1984
    SeriesDenmark. Forskningscenter Risoe. Risoe-R
    Number506
    ISSN0106-2840

    Keywords

    • Risø-R-506

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