Temperature–composition phase diagram and optical energy gap values of Cu2(1 –z)CdzIn2Te4 alloys

Abstract

The temperature–composition phase diagram of the system Cu_2(1-z)Cd_zIn₂Te₄ was obtained from X-ray diffraction and differential thermal analysis measurements. At higher temperatures, there is an apparent single phase solid solution across the complete diagram, but the phase changes abruptly from a disordered defect zinc-blende to a partially disordered tetragonal symmetry somewhere in the range 0.85 < z < 1.0. This high-temperature phase shows a eutectoid reaction at z= 0.65 and T= 230 °C giving the two tetragonal terminal phases: α, with space group I2d corresponding to CuInTe₂; and γ, with space group 14 corresponding to CdIn₂Te₄. At lower temperatures, the fields α and γ are separated by a relatively wide two-phase field (α+γ), which extends over the range 0.35 < z < 0.75. The form of this T(z) diagram is compared with those of related systems in which the terminal compounds have similar structure but differ in space group. The values of room-temperature optical energy gap, E_o were determined from optical absorption measurements in the ranges of single-phase behaviour, E_o showed a different linear variation with z in each range.