A₂CuP₃S₉ (A = K, Rb), Cs₂Cu₂P₂S₆, and K₃CuP₂S₇: New Phases from the Dissolution of Copper in Molten Polythiophosphate Fluxes

The reaction of Cu with a molten mixture of A₂S/P₂S₅ (A = K, Rb, Cs) produced the quaternary compounds A₂CuP₃S₉ (A = K, Rb) and Cs₂Cu₂P₂S₆. The reaction of Cu with a molten mixture of K₂S/P₂S₅/S produced the quaternary compound K₃CuP₂S₇. A₂CuP₃S₉ (A = K, Rb); I, II, respectively) crystallizes in the space group P2₁/n (no. 14) for I: a = 6.8331-(6) Å, b = 23.562(2) Å, c = 9.5415(7) Å, β = 100.242(7)°, Z = 4, V = 1511.8(4) ų, ρ_calc = 2.298 g/cm³. Cs₂Cu₂P₂S₆ (III) crystallizes in the space group P2₁/c (no. 14) with a = 9.538-(3) Å, b = 12.793(4) Å, c = 10.429(4) Å, β = 103.76(2)°, Z = 4, V = 1236.1(7) ų, ρ_calc = 3.478 g/cm³. K₃CuP₂S₇ (IV) crystallizes in the space group P2₁/a (no. 14) with a = 13.113(5) Å, b = 6.702(3) Å, c = 15.141(7) Å, β = 115.13(1)°, Z = 4, V = 1315(1) ų, ρ_calc = 2.35 g/cm³. The structure of I-II consists of novel one-dimensional [CuP₃S₉]_n^2n- chains separated by A⁺ cations. The chains run along the crystallographic b-axis. The Cu⁺ cation is in tetrahedral coordination with the cyclic [P₃S₉]^3- anion, a new polythiophosphate unit. The structure of III has one-dimensional [Cu₂P₂S₆]_n^2n- chains separated by Cs⁺ cations. The chains consists of alternating [Cu₂]²⁺ dimers linked by the ethane-like [P₂S₆]^4- units. The structure of IV is also one-dimensional, with [CuP₂S₇]_n^3n- chains separated by K⁺ ions. The Cu⁺ ions are in a slightly distorted trigonal planar environment with the [P₂S₇]^4- units. Differential thermal analysis, far-IR, Raman spectroscopy, and optical spectroscopic data are reported. The results of a band structure calculation at the DFT-LMTO level for K₂CuP₃S₉ are also discussed.