Several new quaternary sulfides, (\mathrm{K}{2} \mathrm{Y}{4} \mathrm{Sn}{2} \mathrm{~S}{11}) and (\mathrm{BaLnAg \textrm {A } _ { 3 }}) ((L n=E r, Y, G d)), have been synthesized by the reaction of the constituent binary chalcogenides and elements at (1000^{\circ} \mathrm{C}). The crystal structures of (\mathrm{K}{2} \mathrm{Y}{4} \mathrm{Sn}{2} \mathrm{~S}{11}) and BaErAgS (S_{3}) have been determined by single-crystal (\mathrm{X})-ray diffraction techniques. Crystal data: (\mathrm{K}{2} \mathrm{Y}{4} \mathrm{Sn}{2} \mathrm{~S}{11})-space group (D_{4 /}^{\mathrm{8}}-P 4 / n c c, M=1023.88, Z=4), (a=8.587(1), c=27.892(4) \lambda(T=115 \mathrm{~K}), V=2056.7(4) \AA^{3}), (R_{w}\left(F^{2}\right)=0.093) for 1965 observations and 48 variables, (R(F)=) 0.034 for 1319 observations having (F_{0}^{2}>2 \sigma\left(F_{0}^{2}\right) ; \mathrm{BaEr}) (\mathrm{AgS}{3})-space group (C{2 h}^{3}-C 2 / m, M=508.65, Z=4, a=) 17.340(4), (b=4.014(1), c=8.509(2) \AA, \beta=103.23(3)^{\circ},(T=) (115 \mathrm{~K}), V=576.5(2) \AA^{3}, R_{w}\left(F^{2}\right)=0.049) for 1404 observations and 48 variables, (R(F)=0.018) for 1299 observations having (F_{0}^{2}>) (2 \sigma\left(F_{0}^{2}\right)). In both structures the rare-earth atoms have octahedral coordination and the octahedra form slabs through edge- and corner-sharing. These slabs are separated by (\mathrm{K}^{+})or (\mathrm{Ba}^{2+}) cations, and are crosslinked into three-dimensional frameworks by (\mathrm{Sn}{2} \mathrm{~S}{6}) units as edge-sharing (\mathrm{SnS}{4}) tetrahedral pairs in (\mathrm{K}{2} \mathrm{Y}{4} \mathbf{S n}{2} \mathrm{~S}{11}), and by (\mathrm{Ag}{2} \mathrm{~S}{9}) units as corner-sharing trigonal-bipyramidal (\mathrm{AgS}{5}) pairs in (\mathrm{BaEr}) (\mathrm{AgS}{3}). From their powder diffraction patterns, (\mathrm{BaYAgS}{3}) and (\mathrm{Ba}) (\mathrm{GdAgS}_{3}) appear to be isostructural with BaErAgS 3 . 1994 Academic Press. Inc.