SB...SB AND BI...BI INTERACTIONS IN CS8M4(P2SE6)5 (M=SB, BI)

Cs8M4(P2Se6)5 (M = Sb, Bi) were synthesized by reacting Bi (Sb) with a molten mixture of Cs2Se/P4Se10/Se at 460-degrees-C. The structure features weak M...M interactions and a [P2Se6]4-ligand in three unique bonding modes. The [M4(P2Se6)5]n8n- layers are composed of [M4(P2Se6)4] zigzag chains that are linked via [P2Se6]4- ligands to form a staircase layered framework. The layers are interdigitated to form tunnels along the b direction. Cs+ ions are found in the tunnels and within the layers. The dark red crystals of Cs8Sb4(P2Se6)5 (I) (75% yield) and the black crystals of the isostructural Cs8Bi4(P2Se6)5 (II) (90% yield) are stable in air and water. Compound I crystallizes in the monoclinic space group P2(1)/n (No. 14) with a = 15.489(3) angstrom, b = 11.505(2) angstrom, c = 17.772(3) angstrom, beta = 95.59(2)-degrees, V = 3152 angstrom3, Z = 2, and d(c) = 4.455g/cm3, with F(o)2 > 3sigma(Fsigma2) = 3544, number of variables = 236, and 2theta(max) = 50-degrees. The final R/R(w) = 3.2/4.9. Compound II is X-ray isomorphous with a = 15.752(4) angstrom, b = 11.523(2) angstrom, c = 17.916(2) angstrom, beta = 95.20(2)-degrees, V = 3241 angstrom3, Z = 2, and d(c) = 4.694 g/cm3, with F(o)2 > 3sigma(Fsigma2) = 2598, number of variables = 236, and 2theta(max) = 50-degrees. The final R/R(w) = 5.4/6.7. The optical band gaps of I and II were determined by optical spectroscopy to be 1.58 and 1.44 eV, respectively. Both compounds melt congruently at 532 (I) and 497-degrees-C (II).