The surface of ZnS sphalerite

Sphalerite (or zincblende ) has a lattice with zinc atoms at the corners and face centers of a unit cube and sulfur atoms at the centers of four out of the eight smaller cubes into which the large cube can be divided. Both zinc and sulfur are in regular tetrahedral coordination. The cleavage surface is the (110) surface, and this nonpolar face of ZnS (and other zinc-blende-structure binary compounds) is by far the best understood of all semiconductor surfaces (Kahn, 1983). 

Detailed LEED studies of ZnS (and other zincblende-structure compounds, including ZnTe and CdTe) have established that the (110) surface is reconstructed by movement of the Zn atoms inward (towards the bulk solid) and of the S atoms outward. Displacements of the cation and anion in the uppermost layer by 0.5 A and in the second layer by 0.1 A compared to the bulk are involved (Duke, 1983). Harrison (1976, 1980) has explained this in terms of an electronic structure model by conversion of half-occupied dangling-bond hybrid orbitals at both Zn and S on the 

Vaughan et al. (1987) propose that the rapid surface oxidation of bor-nite, and the products of this reaction, may result from the surface reconstruction that leads to Cu+ and Fe being in approximately trigonal planar coordination. Calculations using the multiple-scattering Xa cluster method (MS-SCF-Aa) on Cu+ in tetrahedral and triangular planar coordination with sulfur (on CuS/ and CuSj - clusters see Tossell, 1978b  

The examples discussed in the previous material serve to illustrate the complexity of surface properties of minerals and related materials, the difficulties of adequate characterization, and the important role to be played by calculations of electronic structure. 

The oxides containing cations in octahedral coordination, such as MgO and TiOz, seem to suffer little or no reconstruction of the crystal structure at the surface, but there are major changes in electronic structure. These effects have important implications for surface reactivity, especially when oxygen vacancy defects are considered and particularly in transition-metal oxides. The example of TiOj was discussed above more complex behavior is shown by species such as TijOj and NiO (Henrich, 1987). The latter, like other transition metals, shows increasing /-orbital 

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