We describe the polymorphic order-disorder transition in the chalcopyrite-type semiconductor ${\mathrm{Cu}}_{0.5}$${\mathrm{In}}_{0.5}$Se through a Monte Carlo simulation of a generalized Ising Hamiltonian whose interaction energies are determined from ab initio total-energy calculations. The calculated transition temperature (${\mathit{T}}_{\mathit{c}}$=1125±10 K) compares well with experiment (${\mathit{T}}_{\mathit{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.

• Received 23 September 1991

DOI:https://doi.org/10.1103/PhysRevB.45.2533