Peculiarities of crystal structures of rare-earth metal sulfides with the period 4 Å

To crystal chemistry of compounds of MHgYX (M = Cu, Ag, Y = S, Se, X = Cl, Br, I)

In the crystal structures MHgYX (M = Cu, Ag, Y = S, Se, X = Cl, Br, I) and also in CuHg2S2I and Ag2HgSI2 the packing of anions X and Y varies from the hexagonal closest packing (CuHgSI, CuHg2S2I, and α-AgHgSI) to a cubic space-centered packing (Ag2HgSI2, CuHgSBr (295 K), β-AgHgSI, and CuHgSBr (358 K). The packing regularity for large (I–) and smaller (S2–) anions is ensured because Hg2+ has linear coordination S—Hg—S (S—S ≈ 4.5 Å), which is comparable with I—I contacts in a joint close packing. S2–(Se2–) are surrounded in a tetrahedral manner and form stable groups (Hg,M)4 Y where the most strong chemical bonds Hg—Y, M—Y, Hg—Hg (∼3.5 Å), Hg—M are concentrated. The chains, layers, and frameworks of (Hg,M)4 Y tetrahedra form a “skeleton” and, thus, determine the structure form and the sizes of elementary cells.