FIG. 1. a) Non-substitutional β-Sn, b) substitutional 2-site
cluster equivalent for β-Sn. Blue balls: Ge atoms (lattice
with diamond symmetry); gray balls: Sn atoms, (a):Sn-atom
located at the center of a Ge-divacancy and (b):Sn-atoms in
substitutional positions. t′ represents a hopping between a
Sn-atom and nearest neighbors Ge-atoms. t denoted an intr-
acluster hopping between Sn atoms on substitutional repre-
sentation.