ROLE OF IONICITY IN THE DETERMINATION OF SURFACE ATOMIC GEOMETRIES - GAP, ZNS, AND CUCL (110) SURFACES

First-principles total energy calculations were carried out in order t o determine the local atomic geometry of the (110) surfaces for the is oelectronic series GaP, ZnS, and CuCl. Despite the large change in ion icity across the series (Phillips ionicities: f(GaP) = 0.37, f(ZnS) = 0.62, and f(CuCl) = 0.75), it was found that all three surfaces exhibi t an activationless, bond-rotation relaxation with a rotation angle of approximately 25-degrees or greater. While this result is well known in the case of III-V (110) surfaces, it is in contradiction to a previ ous calculation which predicted that the bond-rotation angle would dec rease with increasing ionicity, going to zero for CuCl. However, our r esults are in qualitative agreement with a recent dynamical low-energy electron diffraction analysis of the CuCl (110) surface. Despite the presence of a large-angle bond rotation for all three surfaces, the lo cal atomic geometries depend on ionicity as follows: both the total en ergy gain for a given bond-rotation angle and the deviation of the sur face anion from its ideal, bulk-terminated position decrease with incr easing ionicity. In addition, the contraction of the top-layer anion-c ation bond, relative to the bulk bond length, increases with increasin g ionicity.