Determination of zinc incorporation in the Zn-substituted gallophosphate ZnULM-5 by multiple wavelength anomalous dispersion techniques

M. Helliwell, J. R. Helliwell, V. Kaucic, N. Zabukovec Logar, S. J. Teat, J. E. Warren, E. J. Dodson

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    Abstract

    The location of isomorphously substituted zinc over eight crystallographically different gallium sites has been determined in a single-crystal study of the gallophosphate ZnULM-5, Ga(16-x) Zn x (PO4)14(HPO4)2(OH) 2F7, [H3N{CH2}6NH 3]4, 6H2O, in an 11 wavelength experiment, using data from Station 9.8, SRS Daresbury. The measurement of datasets around the K edges of both Ga and Zn, as well as two reference datasets away from each absorption edge, was utilized to selectively exploit dispersive differences of each metal atom type in turn, which allowed the major sites of Zn incorporation to be identified as the metal 1 and 3 sites, M1 and M3. The preferential substitution of Zn at these sites probably arises because they are located in double four-ring (D4R) building units which can relax to accommodate the incorporation of hetero atoms. As the crystal is non-centrosymmetric, with space group P21212, it was also possible to use anomalous differences to corroborate the results obtained from the dispersive differences. These results were obtained firstly from difference Fourier maps, calculated using a phase set from the refined structure from data measured at the Zr K edge. Also, refined dispersive and anomalous occupancies, on an absolute scale, could be obtained using the program MLPHARE, allowing estimates for the Zn incorporation of approximately 22 and 18 at. % at the M1 and M3 sites to be obtained. In addition, f′ and f′′ values for Ga and Zn at each wavelength could be estimated both from MLPHARE results, and by refinement in JANA2006. The fully quantitative determinations of the dispersive and anomalous coefficients for Ga and Zn at each wavelength, as well as metal atom occupancies over the eight metal atom sites made use of the CCP4s MLPHARE program as well as SHELXL and JANA2006. The results by these methods agree closely, and JANA2006 allowed the ready determination of standard uncertainties on the occupancy parameters, which were for M1 and M3, 20.6 (3) and 17.2 (3) at %, respectively. © 2010 International Union of Crystallography. Printed in Singapore-all rights reserved.
    Original languageEnglish
    Pages (from-to)345-357
    Number of pages12
    JournalActa Crystallographica Section B: Structural Science
    Volume66
    Issue number3
    DOIs
    Publication statusPublished - 2010

    Keywords

    • 11 X-ray wavelengths
    • D4R structural building blocks
    • MAD technique
    • Zinc substitution

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