Atomic and electronic structure of ideal and reconstructed alpha-Sn(111) surface

We have carried out an ab initio study of alpha-Sn (111), with the aim of predicting and understanding its structure, reconstructions, and electronic states. We consider a variety of structural possibilities, and optimize them by moving atoms according to Hellmann-Feynman forces. Our results indicate that the unreconstructed surface is highly unstable, while a variety of reconstructions compete for the true ground state. Extrapolated trends from diamond to Si to Ge are well borne out, with a 2 x 1 pi-bonded chain reconstruction prevailing in the absence of adatoms, and a c(4x2) or (2x2) basic adatom-restatom unit reconstruction otherwise. Accompanying surface bucklings are in both cases larger than in Si and Ge, with consequently large ionic charge transfers predicted. Search for a beta-Sn-like metallic state of the surface turned out to be inconclusive.