Structure and negative thermal expansion in Cu(In1-xGax)(S,Se)2

Compound semiconductors like Cu(In1-xGax)(S,Se)2 are due to their superior optical and electronical properties successfully used as absorber layers in thin film solar cells. These compounds adopt the chalcopyrite type structure, which is characterized by a tetrahedrally coordination of the cations by anions and vice versa. One of the most intriguing properties of solids whose crystal structures are based on tetrahedrally coordinated ions is their negative thermal expansion. To verify this concept, in-situ neutron diffraction experiments in a temperature range from 290 K to 1.5 K were performed on Cu(In1-xGax)(S,Se)2 powder samples. The lattice parameters and linear thermal expansion coefficients were determined by Rietveld-Analysis of the neutron diffraction data applying the chalcopyrite structure as structural model and using the FullProf suite software package. Negative thermal expansion coefficients were observed for the Cu(In1-xGax)Se2 compounds, whereas the critical temperature, where α(a,c) becomes negative, varies with chemical composition.