Atomic-scale structure of disordered Ga{sub 1{minus}{ital x}}In{sub {ital x}}P alloys
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Extended x-ray-absorption fine-structure experiments have previously demonstrated that for each composition {ital x}, the sample average of all nearest-neighbor {ital A}-{ital C} distances in an {ital A}{sub 1{minus}{ital x}}{ital B}{sub {ital x}}{ital C} semiconductor alloy is closer to the values in the {ital pure} ({ital x}{r_arrow}0) {ital AC} compound than to the composition-weighted (virtual) lattice average. Such experiments do not reveal, however, the distribution of atomic positions in an alloy, so the principle displacement directions and the degrees of correlation among such atomic displacements remain unknown. Here we calculate both structural and thermodynamic properties of Ga{sub 1{minus}{ital x}}In{sub {ital x}}P alloys using an {ital explicit} occupation- {ital and} position-dependent energy functional. The latter is taken as a modified valence force field, carefully fit to structural energies determined by first-principles local-density calculations. Configurational and vibrational degrees of freedom are then treated via the continuous-space Monte Carlo approach. We find good agreement between the calculated and measured mixing enthalpy of the random alloy, nearest-neighbor bond lengths, and temperature-composition phase diagram. In addition, we predict yet unmeasured quantities such as (a) distributions, fluctuations, and moments of first- and second-neighbor bond lengths as well as bond angles, (b) radial distribution functions, (c) the dependencemore » of short-range order on temperature, and (d) the effect of temperature on atomic displacements. Our calculations provide a detailed picture of how atoms are arranged in substitutionally random but positionally relaxed alloys, and offer an explanation for the effects of site correlations, static atomic relaxations, and dynamic vibrations on the phase-diagram and displacement maps. (Abstract Truncated)« less