Determination of the site of incorporation of cobalt in CoZnPO-CZP by multiple-wavelength anomalous-dispersion crystallography

Data were collected from a crystal of CoZnPO-CZP (sodium cobalt-zinc phosph ate hydrate, Na-6[Co0.2Zn0.8PO4](6).6H(2)O} using synchrotron radiation at ELETTRA at the inflection point and 'white line' for both the cobalt and zi nc K edges, and at 1.45 Angstrom, a wavelength remote from the K edges of b oth metals. The data were processed using the programs DENZO and SCALE-PACK . The CCP4 program suite was used for the scaling of data sets and the subs equent calculation of dispersive difference Fourier maps. Optimal scaling w as achieved by using a subset of reflections with Little or no contribution from the metal atoms (i.e. which were essentially wavelength independent i n their intensities) and using weights based on the sigma's to obtain an ov erall scale factor in each case. Phases were calculated with SHELXL97 based on the refined structure using a much higher resolution and complete Cu K alpha data set. An occupancy of 100% by zinc at the two metal-atom sites wa s assumed. The dispersive difference Fourier map calculated for zinc gave t wo peaks above the background of similar heights at the expected metal-atom sites. The peak height at the Zn1 site was a little higher than at the Zn2 site. The dispersive difference Fourier map calculated for cobalt gave jus t one peak above the background, at the Zn1 site, and only a small peak at the Zn2 site, thus indicating that incorporation of cobalt takes place main ly at one site. Refinement of the zinc occupancies using MLPHARE reinforces this conclusion. The chemical environment of each site is discussed.