Temperature behaviour of thermal expansion anisotropy and Grüneisen parameters of chalcopyrite silver thiogallate

X-ray powder diffraction measurements on silver thiogallate (AgGaS2) are made in the temperature range between 301 and 772 K, a region in which this compound has a tetragonal chalcopyrite structure. From the analysis of the X-ray diffraction lines, accurate lattice parameter values are determined as a function of temperature. These results allow the evaluation of the thermal expansion coefficients of the corresponding parameters. It is found that the thermal expansion behaviour of AgGaS2 is similar to that of other AgBIIIC2VI compounds having a relatively small expansivity along the c-axis and a large one in the perpendicular direction (αc < αa). The mean values of αa and αc, evaluated in the temperature range 301-772 K, are found to be 12.14 × 10−6 and −11.56 × 10−6 K−1, respectively. The value of αc is negative at room temperature and its numerical value increases with increasing temperature. The Grüneisen parameters γa and γc have been theoretically calculated at different temperatures. It has been found that the values of Grüneisen parameters increase with increasing temperature and exhibit anisotropy between them, namely γa > γc. These results are discussed in terms of tetragonal distortion and thermal expansion of the individual bonds forming the AgGaS2 crystal.