Role of correlations in (GaSb)1−xGe2xalloys

The semiconducting alloy (GaSb${)}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Ge}}_{2\mathrm{x}}$ is a member of a class of alloys that undergo order-disorder phase transitions between zinc-blende and diamond crystal structures. This transition modifies both the long-range and short-range order of the alloy as a function of composition x. This affects both thermodynamic quantities such as the order parameter, entropy, and phase diagram as well as the electronic structure, e.g., the band gap. These quantities are calculated with use of the Kikuchi approximation, combined with a correlated virtual-crystal approximation. Additionally, calculated bond probabilities are compared with extended x-ray-absorption fine structure (EXAFS) data. Both x-ray diffraction data and EXAFS data are found to be consistent with the existence in (GaSb${)}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Ge}}_{2\mathrm{x}}$ of some ‘‘wrong’’ bonds of Ga—Ga and Sb—Sb. Finally, a novel violation of Vegard’s law for this alloy is predicted.