Temperature dependence of the tetragonal distortion in I–III–VI2 compounds

On the basis of free energy considerations with anharmonic contributions included a model is developed for the temperature dependence of the tetragonal distortion Δ of the I–III–VI₂ compounds. Agreement with experimental data can be only achieved if compounds with Δ < 0.05 and Δ > 0.05 are considered separately. It is suggested that this result is related to the structural order-disorder behaviour of the I–III–VI₂ compounds.