Synthesis of the quaternary one-dimensional triple-chain materials K₃Cu₃M₂Q₈ (M=Nb, Ta; Q=S, Se): The structure of K₃Cu₃Nb₂S₈

The one-dimensional triple-chain materials K₃Cu₃Nb₂S₈, K₃Cu₃Nb₂Se₈, K₃Cu₃Ta₂Se₈, and K₃Cu₃Ta₂S₈ have been prepared by the reactive flux method in which Cu and Nb or Ta were reacted with K₂Q₅/Q (Q=S, Se) at 850°C. K₃Cu₃Nb₂S₈ has been characterized by single-crystal X-ray diffraction techniques. It crystallizes with four formula units in space group C_2h⁶-C2/c of the monoclinic system in a cell of dimensions a=23.446(3), b=5.545(3), c=14.204(3) Å, β=120.87(3)° at -120°C. It is composed of one-dimensional _∞¹[Cu₃Nb₂S₈^3- chains isolated from one another by K^- cations. These chains are triple in nature, and consist of successive corner-sharing CuS₄-CuS₄ tetrahedra and edge-sharing CuS₄-NbS₄ tetrahedra. This edge-sharing results in Nb-Cu distances of 2.772(2)-2.781(1) Å. K₃Cu₃ Nb₂Se₈ with cell dimensions a=23.453(4), b=5.592(12), c=14.223(3) Å, β=120.7(3)° at -120°C (λ(CuKα₁)=1.54056 Å); K₃Cu₃Ta₂Se₈ with cell dimensions a=23.493(9), b=5.538(7), c=14.204(12) Å, β=120.5(9)° at -165°C (λ(MoKα₁)=0.70930 Å); and K₃Cu₃Ta₂S₈ with cell dimensions a=23.450(6), b=5.541(3), c=14.110(6) Å, β=120.47(3)° at -165°C (λ(MoKα₁)=0.70930 Å) appear to be isostructural with K₃Cu₃Nb₂S₈. K₃Cu₃Nb₂S₈ has a resistivity greater than 10⁵ mΩ cm at room temperature.