BONDING EFFECTS IN GaAs: MEASUREMENTS OF THE 222/222 BIJVOET RATIO. Andrew W. Stevenson, CSIRO Division of Materials Science & Technology, Private Bag 33, Clayton South MDC, Victoria 3169, Australia

Various X-ray diffraction studies of bonding effects in GaAs have been reported. Early investigations [e.g. De Marco, J. J. & Weiss, R. J. (1964). Phys. Lett. 13, 209-210] concentrated on the amount and direction of electron charge transfer. Several attempts at accurately producing electron-density maps have been made.

Stevenson [Acta Cryst. (1994). A50, 621-632] has recently collected and analysed an accurate and extensive X-ray integrated-intensity data set, for an extended-face-crystal specimen, to yield detailed information on thermal vibrations and bonding effects in GaAs. In particular, the structure-factor magnitudes for the very weak 200, 222 and 222 reflections, for which h+k+l=4n+2, clearly showed the presence of bonding effects. A least-squares refinement of the full GaAs data set yielded values for four bonding parameters. These occur in additional atomic-scattering-factor terms which describe the spherical expansion or contraction of the bonded atom relative to an isolated atom, and an antisymmetric component corresponding to nearest-neighbour bonding. The atomic-scattering-factor expressions reflect the atomic-site symmetry for the zinc blende structure and were derived by Moss [PhD thesis (1977). Univ. of Melbourne, Australia]. In the present study, and continuing on from the earlier study, wavelength-dependent measurements of the 222/222 Bijvoet ratio are reported and interpreted in terms of the presence of bonding effects. This Bijvoet ratio is zero in the absence of anharmonic thermal vibrations and bonding effects, and the contribution from the former is relatively small here. Given that extinction effects are also very small for these reflections, such measurements provide a valuable opportunity to study bonding effects directly.