The use of anomalous scattering for the determination of crystal structures-KMnO₄

Abstract

The decrease in the scattering factor when the incident radiation is close to the absorption edge of an atom has been utilized for determining the structure of the centrosymmetric crystal KMnO₄. Multiple film zero-layer Weissenberg photographs were taken with CuK_α, CoK_α and FeK_α radiations, all of which are quite close to the K-absorption edge of the Mn atom. After reducing these data to the same relative scale, an Anomalous Difference Patterson (A.D.P.) map was computed with (|F_Cu|² – |F_Fe|²) as coefficient and it was found to have striking resemblance to the Isomorphous Difference Patterson map made with (|F_Cu ^KMnO4|² – |F_Cu ^KClO4|₂) as coefficient. The structure was recovered from the A.D.P. diagram by means of the Buerger Vector shift method. An interesting offshoot of this technique is that the position of the excited atom could be directly determined by making a Patterson with (|F_Cu| – |F_Fe|)² as coefficient. From the knowledge of the position of the anomalously scattering atom and also its scattering factors for the different radiations, it was possible to assign the signs of as many as 50% of the reflexions. Commencing with these the structure was determined and the atomic parameters were refined. The final R-factor for the (hol) and (hko) projections were respectively 0·145 and 0·153. The parameters found are appreciably different from those reported earlier.