QUANTITATIVE CONVERGENT BEAM ELECTRON DIFFRACTION APPLIED TO BONDING STUDIES R. Høier, Department of Physics, Norwegian University of Science and Technology, N-7034 Trondheim, Norway.

Methods for quantifying convergent beam electron diffraction patterns have been developed considerably over the last years. By multiparameter least square fitting of observed and calculated patterns we may today determine phases in non-centrosymmetric crystals and magnitudes of individual structure factors with an accuracy down to 0.1 degree and 0.1 %, respectively. The methods used to achieve this high accuracy are demanding both numerically and experimentally. Various strategies are being tested out, based on non-systematic, systematic row or zone axis diffraction, so far all of them on materials with relatively small unit cells.

Methodologically one of the main challenges is related to the numerical computations where the time consuming part is associated with solving a large non-Hermitic eigenvalue problem. This is done repeatedly a large number of times and the standard algorithm in use is very unfavorable for vectorization. We have optimized the diagonalization by determining efficient beam selection criteria. It is also found that the computational efficiency may be strongly increased by using parallel computers. Among the crystal parameters the Debye-Waller factor is still a problem. It is in general anisotropic and atom dependent and difficult to refine in the numerical procedures in use. This fact is in particular clear when experimentally determined bonding distributions are compared with excising computed ones that assume zero thermal vibrations.

Bonding has been investigated in the intermetallic material TiAl, undoped and doped with different other atom types. This material is interesting for high temperature applications, but has an unfortunate brittle-ductile transition at moderate temperatures. It has been found experimentally that the transition temperature is modified by the doping, and e.g. Mn is found to be beneficial in this respect. To understand this effect electron bond distributions of TiAl alloyed with Mn and other atoms have been investigated experimentally by convergent beam diffraction, electron energy loss fine-structure studies and theoretically by LAPW charge density calculations. Influence of dopant type is determined and experimentally consistent differences are seen for Mn, Cr and V.