DETERMINATION OF BOND CHARGE DENSITY AND TEMPERATURE FACTORS FOR NiAl BY CONVERGENT BEAM ELECTRON DIFFRACTION. A. L. Weickenmeier, W. Nuchter, J. Mayer, Max-Planck-Institut fur Metallforschung, Seestrasse 92, 70174 Stuttgart, Germany

Quantitative convergent beam electron diffraction (CBED) is in-creasingly appreciated as a very powerful tool to determine bond charge densities and temperature factors (TF) of very small (spot size 20 nm or less) areas of crystalline specimens. Using the Bloch wave formalism, CBED patterns are computed with extremely high accuracy as a function of the TFs and structure factors (SF, Fourier coefficients of the crystal potential). To derive the TFs and SFs the simulations are fitted to experimental patterns. Once a sufficient number of SFs has been obtained the potential is synthesized and by means of Poisson's equation converted to the total charge density. Using the measured TFs for the given temperature the total charge density of a corresponding 'crystal made from neutral atoms is constructed and subtracted yielding the bond charge density. Since the bond charge density is only a small fraction of the total charge density (of order 1 percent) maximum accuracy in experiment and data analysis is required.

The material we investigated is the ordered intermetallic phase NiAl, which is of technical interest as a high performance material at elevated temperatures. Since the room temperature brittleness is usually attributed to a partially covalent bonding, an experimental determination of the bond charge density will yield important information on the mechanical behavior.

With our energy filtering Zeis EM912 Omega transmission electron microscope equipped with a cooled YAG scintillator CCD-camera we have examined stochiometric NiAl at liquid nitrogen temperature. Specimens were prepared in different crystallographic orientations and investigated under various incident beam directions and for different thicknesses. Patterns were taken showing either high or low order Bragg reflections to extract either TFs or SFs. The analysis for the TFs shows that the variance of the results is of order 5 percent for the mean vibration amplitude which is readily explained by experimental errors and uncertainties. The accuracy of SFs obtained so far is in the order of a tenth of a percent. First results on bond charge density clearly reveal a covalent bond between Ni and Al atoms.