Crystallographic structure and phase analysis of CuInSe2 by microdiffraction

We investigate melt-grown Cu_0.93In_1.02Se₂ bulk crystals of nominal chalcopyrite structure by transmission electron microscopy and differential thermo-analysis. The crystal is composed of two coherent phases, namely a matrix phase exhibiting thiogallate crystal structure with average composition Cu:In:Se = 1:1:2, and a second phase with needle-shaped cross-sections (average diameter 80 nm) of similar, but not fully understood crystal structure with average composition Cu:In:Se = 2:3:5. Heat treatment at temperatures above and below the solid-state phase transition temperature of 810°C and subsequent quenching to room temperature causes the two phases to disappear and to form pure chalcopyrite CuInSe₂. This confirms the congruent melting point at a melt composition of Cu_0.93In_1.02Se₂ and gives strong evidence, that the two phases result from an In-supersaturation of the chalcopyrite single crystal at temperatures below 800°C.