First-principles calculation of the order-disorder transition in chalcopyrite semiconductors

We describe the polymorphic order-disorder transition in the chalcopyrite-type semiconductor Cu0.5{In}0.5$Se through a Monte Carlo simulation of a generalized Ising Hamiltonian whose interaction energies are determined from ab initio total-energy calculations. The calculated transition temperature (${T}c=1125±10 K) compares well with experiment ({T}c$=1083 K). Unlike the analogous phenomena in isovalent III-V alloys, we find that the transition is dominated by electronic compensation between donor and acceptor states, leading to strong correlations in the disordered phase, and a decrease in the optical band gap upon disordering.