hide
Free keywords:
-
Abstract:
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.