STRUCTURE OF ZNGA2S4, A DEFECT SPHALERITE DERIVATIVE

Digallium zinc tetrasulfide, ZnGa2S4, M(r) = 333.07, tetragonal, I4BAR2m, a = 5.2744 (7), c = 10.407 (1) angstrom, V = 289.51 (9) angstrom 3, Z = 2, D(x) = 3.82 g cm-3, lambda(Mo K-alpha) = 0.71069 angstrom, mu = 146.78 cm-1, F(000) = 312, T = 292 K. ZnGa2S4 crystallizes in the defect stannite structure which is related to sphalerite. Refinement of different models of cation distribution gave R values of 0.041 to 0.043 for 360 independent reflections with \F(o)\ > 3-sigma(F). Of the models considered, the ordered model with Zn only at point set 2(a) and Ga only at point set 4(d) was found to be worse, statistically, than models with some degree of cation disorder. Models with cation disorder are consistent with a previous review in which it was suggested that the displacement of the anion from the ideal (1/4,1/4,1/8) was insufficient in ZnGa2S4 to indicate cation ordering. Disorder amongst the cation sites is also consistent with reported Raman and photoluminescence results. The slight difference observed between the metal-sulfur bond lengths for the two crystallographic cation sites is attributed to the differing effect of the vacancy on these sites rather than to cation ordering.