COMPARISON OF EXPERIMENTAL AND THEORETICAL ELECTRONIC CHARGE-DISTRIBUTIONS IN GAMMA-TIA1

Using critical voltage electron diffraction, Fox has recently determin ed the lowest seven X-ray structure factors of gamma-TiAl (L1(0) struc ture). We present here a comparison of these accurately measured (0.15 %) structure factors with first-principles local density calculations, finding an agreement within 0.7% and an r.m.s. error of 0.013 e/atom. While such measurements are limited to the first few structure factor s rho(G) (where G is the crystal momentum), theory is able to obtain r ho(G) for arbitrarily high G. If we construct charge density deformati on maps by Fourier summations up to the lowest measured G, the calcula ted and experimental density deformation maps agree very closely. Howe ver, if we include in the theoretical density deformation map high G v alues (outside the range accessible to experiment), qualitatively diff erent bonding patterns appear, in particular between Ti atoms. Systema tic study of the total, valence, and deformation charge densities as w ell as comparison with result for NiAl in the hypothetical L1(0) struc ture elucidate the bonding patterns in these transition metal aluminid es.