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

Abstract

X-ray powder diffraction measurements on silver thiogallate (AgGaS 2) 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 AgGaS 2 is similar to that of other Ag B III C 2 VI compounds having a relatively small expansivity along the c-axis and a large one in the perpendicular direction ( α c < α a ). The mean values of α a and α c , 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 α c is negative at room temperature and its numerical value increases with increasing temperature. The Grüneisen parameters γ a and γ c 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 γ a > γ c . These results are discussed in terms of tetragonal distortion and thermal expansion of the individual bonds forming the AgGaS 2 crystal.