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 A 2 S/P 2 S 5 (A = K, Rb, Cs) produced the quaternary compounds A 2 CuP 3 S 9 (A = K, Rb) and Cs 2 Cu 2 P 2 S 6 . The reaction of Cu with a molten mixture of K 2 S/P 2 S 5 /S produced the quaternary compound K 3 CuP 2 S 7 . A 2 CuP 3 S 9 (A = K, Rb); I, II, respectively) crystallizes in the space group P 2 1 / 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) Å 3, ρ calc = 2.298 g/cm 3 . Cs 2 Cu 2 P 2 S 6 ( III ) crystallizes in the space group P 2 1 / 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) Å 3, ρ calc = 3.478 g/cm 3 . K 3 CuP 2 S 7 ( IV ) crystallizes in the space group P 2 1 / 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) Å 3, ρ calc = 2.35 g/cm 3 . The structure of I − II consists of novel one-dimensional [CuP 3 S 9 ] n 2 n - chains separated by A + cations. The chains run along the crystallographic b -axis. The Cu + cation is in tetrahedral coordination with the cyclic [P 3 S 9 ] 3 - anion, a new polythiophosphate unit. The structure of III has one-dimensional [Cu 2 P 2 S 6 ] n 2 n - chains separated by Cs + cations. The chains consists of alternating [Cu 2 ] 2+ dimers linked by the ethane-like [P 2 S 6 ] 4 - units. The structure of IV is also one-dimensional, with [CuP 2 S 7 ] n 3 n - chains separated by K + ions. The Cu + ions are in a slightly distorted trigonal planar environment with the [P 2 S 7 ] 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 K 2 CuP 3 S 9 are also discussed.