SUPERCRITICAL AMMONIA-SYNTHESIS AND CHARACTERIZATION OF 4 NEW ALKALI-METAL SILVER ANTIMONY SULFIDES - MAG(2)SBS(4) AND M(2)AGSBS(4) (M=K,RB)

Four new semiconducting quaternary antimony sulfides, KAg2SbS4 (I), K2 AgSbS4 (II), RbAg2SbS4 (III), and Rb2AgSbS4 (IV) were prepared in supe rcritical ammonia at 160 degrees C in 4 days from the appropriate mole ratios of K2S4, K2CO3, or Rb2CO3 with Ag, Sb2S3, and S-8. Both silver -rich materials, I and III, crystallize in structure types previously observed in BaAg2GeS4 and SrCu2SnS4, respectively. However, it was det ermined that the space group of the latter compound was inaccurately r eported in the literature. Novel structure types were obtained for II and IV. The structures of I-III contain three-dimensional anionic fram eworks, either [Ag2SbS4](-) or [AgSbS4](2-), built from a variety of e dge- or vertex-shared antimony sulfide and silver sulfide tetrahedra. The alkali metal cations in I and II reside in two orthogonal, interse cting channels, and in three isolated one-dimensional channels in III. Compound IV has a layered structure, with a double layer of rubidium cations separating the neighboring anionic frameworks. It is noteworth y that I-III crystallize in noncentrosymmetric space groups. The struc tural diversity resulting from the condensation of quite regular SbS4 and distorted AgS4 tetrahedra in these four compounds and their optica l band gaps are discussed. Crystal data: (I), red polyhedron, tetragon al space group, <I(4)over bar 2m>, a = 6.886(1) Angstrom, c = 8.438(2) Angstrom, V = 400.16(9) Angstrom(3), Z = 2, and R(wR) = 0.0397(0.0335 ); (II), yellow plate, orthorhombic space group, Pnn2, a = 10.348(2) A ngstrom, b = 10.522(2) Angstrom, c = 7.946(2) Angstrom, V = 865.2(3) A ngstrom(3), Z = 4, and R(wR) = 0.0377(0.0392); (III), yellow-orange ne edle, trigonal space group, P3(2)21, a = 6.630(1) Angstrom, c = 16.700 (2) Angstrom, V = 635.8(2) A(3) Z = 3, and R(wR) = 0.0244(0.0310); (IV ), yellow polyhedron, monoclinic space group, P2(1)/n, a = 8.229(3) An gstrom, b = 10.857(2) Angstrom c = 10.346(2) Angstrom, beta = 91.55(2) degrees, V = 924.0(4) Angstrom(3), Z = 4,and R(wR) = 0.0377(0.0405). ( C) 1996 Academic Press, Inc.