Interaction parameters and a quenched-disorder phase diagram for (GaAs)1-xGe2x alloys.

Abstract

We obtain the interaction parameters for the spin-1 Ising representation of (GaAs${)}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{2\mathit{x}}$ metastable alloys, based on the total energies of a set of ordered structures calculated by the first-principles self-consistent pseudopotential method. We then use these parameters to reexamine the thermodynamic consequences of a recently proposed quenched-disorder model for the observed zinc-blende-to-diamond phase transition in (GaAs${)}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{2\mathit{x}}$. In this model, Ge atoms are distributed at random at all lattice sites. We present a complete phase diagram of the quenched-disorder model in the pair approximation of the cluster-variation method. In view of our set of first-principles interaction parameters, we conclude that, like previously proposed bulk thermodynamic models, the quenched-disorder model does not lead to a phase transition that agrees with the experimental result.