The electron density in the $A^{\mathrm{III}}{B}^{\mathrm{VI}}$ crystal GaS was experimentally determined by x-ray diffraction. The resulting density distribution varies markedly from the distribution predicted by the pseudopotential model proposed by Schlüter et al. The real bonds observed in the x-ray diffraction determined distribution are curved and are located not only between Ga-Ga and Ga-S nearest neighbors as predicted by the pseudopotential model, but also between Ga and S next-nearest neighbors in the same elementary cell, between Ga and S atoms in adjoining elementary cells, and between S-S interlayer neighbors in the same elementary cell. The two atoms Ga and S are highly polarized. These findings allow a better understanding of the bondings and the discrepancies between our experimental findings, and the present pseudopotential model should permit an improvement in a future theoretical model for bonding in the $A^{\mathrm{III}}{B}^{\mathrm{VI}}$ crystals.

• Received 21 July 1980

DOI:https://doi.org/10.1103/PhysRevB.25.4081