Electron crystallography without limits? Crystal structure of Ti45Se16 redetermined by electron diffraction structure analysis

The crystal structure of the metal-rich compound Ti45Se16 was redetermined from selected-area electron diffraction film data. The structure was solved by quasi-automatic direct methods using a data set of quantified h0l elect ron diffraction intensities. Improved atomic coordinates were obtained from a subsequent least-squares refinement on the basis of the kinematical appr oximation. The compound crystallizes in the monoclinic space group C2/m wit h lattice parameters a = 36.534, b = 3.453, c = 16.984 Angstrom, beta = 91. 73 degrees. The structure contains 23 titanium and 8 selenium atoms per asy mmetric part of the unit cell. The refined atomic coordinates agree on aver age within 0.18 Angstrom with the previously determined structure from high -resolution electron-microscopy images. The precision of the determined ato mic coordinates obtained in this study is better than 0.05 Angstrom. The st ructure of Ti45Se16 is the eighth metal-rich structure that has been solved by direct methods from two-dimensional selected-area electron diffraction data using the quasi-kinematical approximation. The present investigation p roves again that direct methods with electron diffraction data work extreme ly reliably provided that the structure in question is composed of elements of nearly equal scattering power and that data covering the most significa nt parts of the unit-cell transform up to atomic resolution are available. Moreover, a method was developed that allows the estimation of the average crystal thickness from the effective atomic potential in the refined struct ure.