KNb1.75V0.25PS10

The title compound, potassium diniobium vanadium phosphorus decasulfide, KNb1.75V0.25PS10, was obtained by reaction of the elements with a eutectic mixture of KCl/LiCl. It is isostructural with the quaternary KNb2PS10, but the Nb sites are occupied by statistically disordered Nb (87.5%) and V (12.5%) atoms. The structure is composed of anionic ∞ 1[M 2PS10]− chains (M = Nb/V) separated from each other by K+ ions. The chain is composed of [MS8] distorted bicapped trigonal prisms and [PS4] tetrahedra. There are no interchain bonding interactions. The crystal used for the X-ray analysis was a racemic twin.

1 1 [M 2 PS 10 ] À chains (M = Nb/V) separated from each other by K + ions. The chain is composed of [MS 8 ] distorted bicapped trigonal prisms and [PS 4 ] tetrahedra. There are no interchain bonding interactions. The crystal used for the X-ray analysis was a racemic twin.
The structure of KNb 2 -x V x PS 10 is isostructural with the quaternary KNb 2 PS 10 and detailed description of the structure is given previously (Do & Yun, 1996) ].
The M atoms associate in pairs with M-M interactions alternating in the sequence of one short (2.8851 (3) Å) and one long (3.7590 (3) Å) distances. The short distance is typical of Nb 4+ -Nb 4+ bonding interactions (Angenault et al., 2000).
There are no interchain bonding interactions except the van der Waals forces and the K + ions in this van der Waals gap stabilize the structure through the electrostatic interactions ( Figure 2

Refinement
The refinement of the model with occupational disorder on the M site caused significant decrease of the R-factor (wR2 = 0.042) in comparison if the full occupation by either metal had been considered (wR2 > 0.05). Also the displacement parameters in the disordered model became plausible. The disordered atoms were supposed to have the same displacement parameters. The nonstoichiometry of the K site was checked by refining the occupancy of K while those of the other atoms were fixed. With the nonstoichiometric model, the parameter remained the same. The large anisotropic displacement parameters for alkali metals are also found in the related compounds such as KNb 2 PS 10 (Do & Yun, 1996). The highest residual electron density is 0.86 Å from the M2 site and the deepest hole is 0.85 Å from the M1 site.