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 Kα rotating anode, from a crystal of a nickel-containing aluminophosphate, NiAl₃P₄O₁₈C₄H₂₁N₄ (NiAPO). These data sets, along with an existing Cu Kα rotating anode data set, allowed the calculation of several f′ 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 Kα data to SR (synchrotron radiation), Cu Kα to SR and Mo Kα to Cu Kα, 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 Å from the refined Ni-atom site and was obtained from the f′ difference map calculated with the coefficients |F_{Mo Kα} − 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, Kaučič & Helliwell (1993), Acta Cryst. B49, 420–428]. The techniques described are generally applicable to other systems containing anomalous scatterers in chemical crystallography.