Browsing by Author "Hoppe U."

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    An X-ray and neutron scattering study of the structure of zinc vanadate glasses
    (2001-01-01) Hoppe U.; Kranold R.; Gattef E.; Neuefeind J.; Keen D.A.
    The short-range order of vitreous V2O5 and of three (ZnO)x(V2O5)1-x glasses with x = 0.2, 0.4, and 0.5 is studied by X-ray and neutron diffraction experiments where the change of the contrast allows to resolve the V-O and Zn-O correlations. The V-O and the Zn-O first-neighbor peaks are approximated by several Gaussian functions. In case of vitreous V2O5 two obvious V-O distances exist which are related with VO4 and VO5 units. With ZnO additions the V-O coordination number decreases from 4.4 in vitreous V2O5 to 4.0 in the metavanadate glass where the strongest decrease of the fraction of VO5 units is found for glasses of x < 0.2. Dominantly, the VO5 groups are linked with the neighboring units by corners. The Zn-O coordination numbers of the modified glasses are about five with closest distances of ≅ 0.200 nm.
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    Structure of zinc phosphate glasses of 75 and 80 mole% ZnO content studied by X-ray diffraction and reverse Monte Carlo simulations
    (2005-01-01) Hoppe U.; Dimitriev Y.; Jóvari P.
    X-Ray diffraction, using high-energy photons from a synchrotron, was used to extend the investigation of (ZnO)x(P2O 5)1-x glasses to samples of ZnO content close to x = 0.8 which were obtained by roller-quenching. The isolated PO4 tetrahedra are surrounded by ZnO polyhedra, where Zn-O coordination numbers of ∼ 4.5 are determined. The small increase of NZnO ∼ 4 at metaphosphate composition (x = 0.5) to ∼ 4.5 is not sufficient to explain the strong increase of the packing density beyond the minimum at x = 0.5. The medium-range order was analyzed on the basis of partial SPP(Q) and S ZnZn(Q) factors obtained from Reverse Monte Carlo simulations of glasses with 0 ≤ x ≤ 0.8. The positions of the first peaks in these factors, the number densities of P and Zn atoms and knowledge of definite P-P and Zn-Zn distances were used to check the applicability of simple models such as the dense packing of uniform P- and Zn-centered spherical environments for glasses with x = 0.8 and 0.5, the packing of corrugated sheets for vitreous P2O5 and the packing of phosphate chains for Zn metaphosphate glass. © 2005 Verlag der Zeitschrift für Naturforschung, Tübingen.
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    Structure of zinc phosphate glasses of 75 and 80 mole% ZnO content studied by X-ray diffraction and reverse Monte Carlo simulations
    (2005-01-01) Hoppe U.; Dimitriev Y.; Jóvari P.
    X-Ray diffraction, using high-energy photons from a synchrotron, was used to extend the investigation of (ZnO)x(P2O 5)1-x glasses to samples of ZnO content close to x = 0.8 which were obtained by roller-quenching. The isolated PO4 tetrahedra are surrounded by ZnO polyhedra, where Zn-O coordination numbers of ∼ 4.5 are determined. The small increase of NZnO ∼ 4 at metaphosphate composition (x = 0.5) to ∼ 4.5 is not sufficient to explain the strong increase of the packing density beyond the minimum at x = 0.5. The medium-range order was analyzed on the basis of partial SPP(Q) and S ZnZn(Q) factors obtained from Reverse Monte Carlo simulations of glasses with 0 ≤ x ≤ 0.8. The positions of the first peaks in these factors, the number densities of P and Zn atoms and knowledge of definite P-P and Zn-Zn distances were used to check the applicability of simple models such as the dense packing of uniform P- and Zn-centered spherical environments for glasses with x = 0.8 and 0.5, the packing of corrugated sheets for vitreous P2O5 and the packing of phosphate chains for Zn metaphosphate glass. © 2005 Verlag der Zeitschrift für Naturforschung, Tübingen.