Anomalous dispersion analyses of the Ni-atom site in an aluminophosphate test crystal including the use of tuned synchrotron radiation

Data were collected close to the Ni K edge, using synchrotron radiation at the National Synchrotron Light Source, and using a Mo K alpha rotating anode, from a crystal of a nickel-containing aluminophosphate, NiAl3P4O18C4H21N4 (NiAPO). These data sets, along with an existing Cu K alpha rotating anode data set, allowed the calculation of several fl difference-Fourier maps exploiting the difference in f' for Ni between the various wavelengths. These differences are expected to be 7.8, 4.5 and 3.3 e for Mo K alpha data to SR (synchrotron radiation), Cu K alpha to SR and Mo K alpha to Cu K alpha respectively. The phases were calculated either excluding the Ni atom or with Al at the Ni-atom site. The f' difference-Fourier maps revealed peaks at the Ni-atom site, whose height and distance from the refined Ni-atom position depended on the f' difference and the phase set used. The largest peak was located at a distance of only 0.025 Angstrom from the refined Ni-atom site and was obtained from the f' difference map calculated with the coefficients \F-Mo K alpha-F-SR\, using phases calculated with Al at the Ni-atom site. In all cases, it was found that these phases gave optimal results without introducing bias into the maps. The results confirm and expand upon earlier results [Helliwell, Gallois, Kariuki, Kaucic: & Helliwell (1993), Acta Cryst. B49, 420-428]. The techniques described are generally applicable to other systems containing anomalous scatterers in chemical crystallography.