The mixed-metal tris(disulfide) thiophosphate, KNb1.77Ta0.23PS10

The title compound catena-poly[potassium [tri-μ-disulfido-μ-tetrathiophosphato-di[niobate(IV)/tantalate(IV)(0.885/0.115)]]], has been obtained through the reaction of the elements with KCl. The title compound is isostructural with KNb2PS10, with the Nb sites occupied by statistically disordered Nb (88.5%) and Ta (11.5%) atoms. The structure is composed of anionic ∞ 1[M 2PS10]− chains along [100] (M = Nb/Ta) and K+ ions. This chain is built up from distorted bicapped trigonal prisms [MS8] and [PS4] tetrahedra. There are no interchain bonding interactions, except for electrostatic and van der Waals forces. The S2 2− and S2− anionic species and the M 4+–M 4+ pair [M—M = 2.8939 (3) Å] are observed. The classical charge balance is represented by [K+][M 4+]2[PS4 3−][S2 2−]3.


Comment
A number of monovalent metal Nb thiophosphates have been investigated. Among them are NaNb 2 PS 10 (Goh et al., 2002), KNb 2 PS 10 (Do & Yun, 1996) As a result of efforts to find new phases in this family, we have found a mixed-metallic phase,. In this paper we report the synthesis and structure of another mixed-metallic quintenary thiophosphate, KNb 1.77 Ta 0.23 PS 10 .
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).
The structural studies of the three different crystals from the same reaction tube implied that the stoichiometry of each metal can vary, KNb 2 -x Ta x PS 10 , 0.18≤x≤0.26 and they seem to form a random substitutional solid solution. However Ta analogue of this phase, ATa 2 PS 10 has never been synthsized and thus the maximum x should be small. Finally, the classical charge balance of this phase can be represented by

Experimental
The title compound, KNb 1.77 Ta 0.23 PS 10 was prepared by the reaction of the elemental with the use of the reactive alkali metal halides-flux technique. A combination of the pure elements, Nb powder (CERAC 99.8%), Ta powder (CERAC 99.9%), P powder(Aldrich 99.9%), and S powder (Aldrich 99.999%) were mixed in a fused silica tube in a molar ratio of Nb: Ta: P: S = 1:1:1:10 with KCl (CERAC 99.9%). The mass ratio of the reactants and the alkali metal halides flux was 1:1. The tube was evacuated to 0.133 Pa, sealed and heated gradually (70 K/h) to 1073 K, where it was kept for 72 h. The tube was cooled to 473 K at 6 K/h and then was quenched to room temperature. The excess halides were removed with distilled water and black needle-shaped single crystals were obtained. The crystals are stable in air and water. Qualitative analysis of these crystals using XRF showed the presence of K, Nb, Ta, P, and S.

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 > 0.077). Also the displacement parameters in the disordered model became plausible. The disordered atoms were supposed to have the same displacement parameters. 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 0.40 Å from the M2 site and the deepest hole is 0.64 Å from the M1 site.

catena-Poly[potassium [tri-µ-disulfido-µ-tetrathiophosphato-di[niobate(IV)/tantalate(IV)(0.885/0.115)]]]
Crystal data Special details Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.