An inorganic double helix sheathing alkali metal cations:: ANb2P2S12 (A=K, Rb, Cs), a series of thiophosphates close to the metal-nonmetil boundary -: Chalcogenide analogues of transition-metal phosphate bronzes?

The new quaternary niobium thiophosphates ANb(2)P(2)S(12) (A = K, Rb, Cs) have been prepared and characterized. The title compounds were synthesized by reacting Nb metal, A(2)S, P2S5, and S at 600-700degreesC in evacuated silica tubes. They crystallize as "stuffed" variants of the tetragonal TaPS6 structure type in the tetragonal space group I (4) over bar 2d with eight formula units per unit cell and lattice constants a = 15.923(2) and c = 13.238(3) Angstrom for CsNb2P2S12, a = 15.887(3) and c = 13.132(3) Angstrom for RbNb2P2S12, and a = 15.850(2) and c = 13.119(3) Angstrom 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 P-31 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(12) is a consequence of the structure symmetry. Therefore, CsNb2P2S12 may be considered a chalcogenide analogue of metal phosphate bronzes.