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 °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(P₂- Se₆)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 P2₁/n (No. 14) with a = 15.489(3) Å, b = 11.505(2) Å, c = 17.772(3) Å, β = 95.59(2)°, V = 3152 ų, Z = 2, and d_c = 4.455g/cm³, with F_o ² > 3σ (Fσ²)=3544, number of variables = 236, and 2θ_max = 50°. The final R/R_w = 3.2/4.9. Compound II is X-ray isomorphous with a = 15.752(4) Å, b = 11.523(2) Å, c = 17.916(2) Å, β = 95.20(2)°, V = 3241 ų, Z = 2, and d_c=4.694 g/cm³, with F_o ² > 3σ(Fσ²) = 2598, number of variables = 236, and 2θ_max = 50°. The final R/R_w = 5.4/6.7. The optical band gaps of I and II were determined by optical spectroscopy to be melt congruently at 532 (I) and 497 °C (II).