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Electronic charge densities are calculated as a function of position in the unit cell for several diamond and zinc-blende semiconductors using wave functions derived from pseudopotential band-structure calculations. The covalent bonding charge is also calculated for these crystals and is plotted against the ionicity scales of Phillips and Van Vechten and of Pauling. It is shown that an extrapolation to zero covalent bonding charge yields a critical value of the ionicity which separates four-fold coordinated and six-fold coordinated diatomic crystals. This value is in agreement with the empirical value obtained by Phillips and Van Vechten.
Walter et al. (Mon,) studied this question.