Synthesis and Crystal Structure of K3AsSe4 and K4As2Se5 Containing [AsSe4]3- Anions and aNovel [As2Se5]4- Isomer Featuring an As--As Bond

Two novel selenoarsenates, K3AsSe4 and K4As2Se5, were synthesized by the mild solvothermal reaction of potassium carbonate and arsenic and selenium powder in the presence of superheated ethylenediamine and structurally characterized by single-crystal X-ray diffraction. K3AsSe4 crystallizes in the orthorhombic space group Pnma (no. 62) with a = 9.4169(4), b = 10.9695(5), c = 9.7568(8) Å, Z = 4 and V = 1007.86(10) Å3 and contains [AsSe4] anions with pentavalent arsenic. K4As2Se5 crystallizes in the monoclinic space group P21/n (no. 14) with a = 7.6167(2), b = 22.8733(4), c = 8.4687(2) Å, β = 107.241(2)◦, Z = 4 and V = 1409.14(5) Å3, and shows isolated [As2Se5] anions. The most prominent feature of the latter anions is the presence of arsenic in two different formal oxidation states (+4/+2) due to the presence of an As–As bond.