Cs₃AgAs₄Se₈ and CsAgAs₂Se₄: Selenoarsenates with infinite ¹(∞)[AsSe₂]^- chains in different Ag⁺ coordination environments

Two quaternary silver selenoarsenates Cs₃AgAs₄Se₈ (I) and CsAgAs₂Se₄ (II) have been discovered by methanothermal reaction of Li₃AsSe₃ with AgBF₄ in the presence of the respective alkali metal sources Cs₂CO₃ and CsCl. Orange crystals of Cs₃AgAs₄Se₈ (I) were formed after reaction at 120 °C for 72 h, whereas red CsAgAs₂Se₄ (II) was obtained under slightly different conditions at 140 °C for 70 h. Both compounds possess novel two-dimensional (2D) polyanions consisting of infinite ¹(∞)[AsSe₂]^- chains that are interconnected by Ag⁺ ions in different coordination patterns. In I, a double layer of ¹(∞)[AsSe₂]^- chains is bridged by distorted trigonal planar coordinated Ag⁺ atoms to form a ²(∞)[AgAs₄Se₈]^3- layer with a thickness of about 11.3 Å, The nonbonding Ag···Ag distances are about 4.220 Å, and large cavities within the layers accommodate for three of the four crystallographic Cs⁺ cations. The double amount of Ag⁺ atoms per AsSe₂ chain unit in II leads to simple layers ²(∞)[AgAs₂Se₄]^- {=[Ag₂As₄Se₈]^2-} in which the Ag⁺ atoms are arranged in rows between the ¹(∞)[AsSe₂]^- chains, with alternating Ag···Ag distances of 3.053(3) and 3.488(3) Å. Hereby the ¹(∞)[AsSe₂]^- polyanions show a disorder within the central (-As-Se(b))- chain (b = bridging), while the positions of the terminal Se atoms (Se(t)) remain unaffected. The thermal, optical, and spectroscopic properties of the compounds are reported. Both I and II melt with decomposition and are wide band gap semiconductors with values of 2.07 and 1.79 eV, respectively. Raman spectroscopic data show typical band patterns expected for infinite [AsSe₂]^- chains. Crystal Data: Cs₃AgAs₄Se₈ (I), monoclinic, C2/c, a = 25.212(2) Å, b = 8.0748(7) Å, c = 22.803(2) Å, β = 116.272(2)°, Z = 8; CsAgAs₂Se₄ (II), monoclinic, P2₁/n, a = 10.9211(1) Å, b = 6.5188(2) Å, c = 13.7553(3) Å, β = 108.956(1)°, Z = 4.