An Inorganic Double Helix Sheathing Alkali Metal Cations: ANb2P2S12 (A=K, Rb, Cs), A Series of Thiophosphates Close to the Metal–Nonmetal Boundary—Chalcogenide Analogues of Transition‐Metal Phosphate Bronzes?

Abstract

The new quaternary niobium thiophosphates ANb₂P₂S₁₂ (A=K, Rb, Cs) have been prepared and characterized. The title compounds were synthesized by reacting Nb metal, A₂S, P₂S₅, and S at 600–700 °C in evacuated silica tubes. They crystallize as “stuffed” variants of the tetragonal TaPS₆ structure type in the tetragonal space group I$\bar 4$ 2d with eight formula units per unit cell and lattice constants a=15.923(2) and c=13.238(3) Å for CsNb₂P₂S₁₂, a=15.887(3) and c=13.132(3) Å for RbNb₂P₂S₁₂, and a=15.850(2) and c=13.119(3) Å for KNb₂P₂S₁₂. Their structures are based on double helices formed from interpenetrating, noninteracting spiral chains of binuclear [Nb₂S₁₂] cluster units and [PS₄] thiophosphate groups. The cavities and tunnels, which are formed by the helical chains, are filled with A⁺ ions. Temperature‐dependent conductivity studies reveal thermally activated electrical transport behavior. This result is consistent with the observation of a temperature‐dependent contribution to the ³¹P MAS‐NMR shift, suggesting that the delocalized s‐electron spin density increases with increasing temperature. These findings are supported by the results of tight‐binding band structure calculations which reveal that the unusual electrical transport behavior of ANb₂P₂S₁₂ is a consequence of the structure symmetry. Therefore, CsNb₂P₂S₁₂ may be considered a chalcogenide analogue of metal phosphate bronzes.