Charge-density determination in TiAl-Cr and TiAl-V using quantitative convergent-beam electron diffraction

Abstract

Effects of 5 at.% alloying with Cr and V on the electronic structure of TiAl compounds are studied using convergent-beam electron diffraction (CBED) in a transmission electron microscope. The quantitative CBED systematic row method is used to determine low-order structure factors in TiAl-V and TiAl-Cr. This method is based on pixel-by-pixel comparisons and refinement of theoretical and experimental CBED patterns. The quantitative CBED method is developed further; the definition and use of fit factors, the number of beams to include in the calculations and how to treat the background from diffuse scattering are topics that are addressed in particular. The structure factors are determined with an accuracy of typically 0.3%. Charge-density maps for TiAl-V are found to be similar to those for undoped TiAl, indicating strong Ti d-d directional bonding in the Ti(001) plane. TiAl-Cr has a more uniformly distributed charge density, and charge is moved from the Ti-Ti bond to in between second-nearest-neighbour atoms.