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RESEARCH PRODUCT

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?

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The new quaternary niobium thiophosphates ANb 2 P 2 S 1 2 (A=K, Rb, Cs) have been prepared and characterized. The title compounds were synthesized by reacting Nb metal, A 2 S, P 2 S 5 , and S at 600-700°C in evacuated silica tubes. They crystallize as "stuffed" variants of the tetragonal TaPS 6 structure type in the tetragonal space group I42d with eight formula units per unit cell and lattice constants a=15.923(2) and c=13.238(3) A for CsNb 2 P 2 S 1 2 , a= 15.887(3) and c=13.132(3) A for RbNb 2 P 2 S 1 2 , and a=15.850(2) and c= 13.119(3) A for KNb 2 P 2 S 1 2 . Their structures are based on double helices formed from interpenetrating, noninteracting spiral chains of binuclear [Nb 2 S 1 2 ] cluster units and [PS 4 ] 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 3 1 P 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 ANb 2 P 2 S 1 2 is a consequence of the structure symmetry. Therefore, CsNb 2 P 2 S 1 2 may be considered a chalcogenide analogue of metal phosphate bronzes.