X-ray diffraction data, ellipsometry data, Raman-scattering data, and theory combine to provide strong evidence of a zinc-blende–diamond order-disorder transition with $x_c$≃0.3 that affects the crystal, electronic, and vibrational structures of (GaAs${)}_{1\mathrm{-}\mathrm{x}}$(${\mathrm{Ge}}_2$${)}_{\mathrm{x}}$ metastable, substitutional, crystalline alloys. It is argued, based on analyses of extended x-ray-absorption fine-structure data for (GaSb${)}_{1\mathrm{-}\mathrm{x}}$(${\mathrm{Ge}}_2$${)}_{\mathrm{x}}$, that the number of anion-anion bonds in these ($A^{\mathrm{III}}$-$B^{{\mathrm{V})}_{1\mathrm{-}x}/B(C_2\mathrm{IV}{)}_x}$ alloys is significant, and requires a theory that goes beyond the mean-field approximation.

• Received 14 October 1986

DOI:https://doi.org/10.1103/PhysRevB.39.657