Crystallography and optical energy gap values for AgGa(Se1−zTez)2 alloys

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Abstract
Polycrystalline samples of AgGa(Se1−zTez)2 alloys were prepared by a melt‐and‐anneal technique, annealing at 650 °C for up to 4 months being required to give good equilibrium conditions. It was confirmed that single phase solid solution occurs at all values of z and values of lattice parameters a, c, and c/a were determined from Guinier–Haegg powder photographs. Line intensities were obtained from diffractometer measurements and, hence, values of the anion displacement u determined as a function of z. Comparison of these experimental data has been made with the expressions proposed by Abrahams and Bernstein and by Jaffe and Zunger. The values of u from the former equation show rather poor agreement, particularly at the AgGaSe2 end of the composition range. However, the equations of Jaffe and Zunger, used with Phillips radii, give very good agreement for both u and a but poorer agreement for c and c/a. The optical energy gap E0 is found to show a parabolic variation with z. Comparison with seven similar alloy systems shows that the E0 versus composition curve is bowed for mixed‐anion alloys but practically linear for mixed‐cation alloys.