Publication Date:
2010-05-15
Description:
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}6NH3]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 andM3. 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 groupP21212, 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 ZrKedge. Also, refined dispersive and anomalous occupancies, on an absolute scale, could be obtained using the programMLPHARE, allowing estimates for the Zn incorporation of approximately 22 and 18 at. % at theM1 andM3 sites to be obtained. In addition,f′ andf′′ values for Ga and Zn at each wavelength could be estimated both fromMLPHAREresults, and by refinement inJANA2006. 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 CCP4'sMLPHAREprogram as well asSHELXLandJANA2006. The results by these methods agree closely, andJANA2006 allowed the ready determination of standard uncertainties on the occupancy parameters, which were forM1 andM3, 20.6 (3) and 17.2 (3) at %, respectively.
Print ISSN:
0108-7681
Electronic ISSN:
1600-5740
Topics:
Chemistry and Pharmacology
,
Geosciences
,
Physics
Permalink