Redetermination of AgNb2PS10 revealing a silver deficiency

In comparison with a previous crystallographic study [Goh et al. (2002 ▶). J. Solid State Chem. 168, 119–125] of the title compound, silver diniobium tris(disulfide) tetrathiophosphate(V), that reports a full occupation of the silver position and isotropic displacement parameters for the atoms, the current redetermination reveals a silver deficiency with a site-occupation factor of 0.88 (1) and reports all atoms with anisotropic displacement parameters. The structure of Ag0.88Nb2PS10 is composed of ∞ 1[Nb2PS10] chains, which are built up from pairs of distorted bicapped trigonal-prismatic [NbS8] polyhedra forming [Nb2S12] dimers and of tetrahedral [PS4] groups. These chains are connected via the statistically disordered Ag+ ions, forming double layers. Adjacent layers are stacked solely through van der Waals forces into a three-dimensional structure. Short and long Nb—Nb distances [2.880 (1) and 3.770 (2) Å, respectively] alternate along the chain and S2 2− and S2− anionic species are observed.

Redetermination of AgNb 2 PS 10 revealing a silver deficiency J. Do and H. Yun

Comment
In comparison with the previous study of AgNb 2 PS 10 (Goh et al., 2002), both lattice parameters and atomic coordinates of the current redetermination are the same within their standard deviations. However, our investigation indicated that there is a deficiency of Ag atoms in the title compound with a site occupation factor (s.o.f.) of 0.88 (1). This observation is consistent with crystal structure refinements from crystals obtained from other reaction batches. Therefore we assume that the crystal originally investigated by Goh et al. (2002) shows the same behaviour. In general, non-stoichiometry in multinary niobium thiophosphates is not uncommon and has been observed in one of our previous studies (Kwak & Yun, 2008). and Nb2 atoms are surrounded by 8 S atoms in a bicapped trigonal-prismatic fashion. Two prisms are sharing a rectangular face to form the [Nb 2 S 12 ] unit. This unit shows an approximate 2-fold rotational symmetry and the rotation axis bisects the short Nb1-Nb2 distance and the (S-S) 2sides of the rectangular face shared by each trigonal prism. The [Nb 2 S 12 ] unit is bound to each other to form the infinite [Nb 2 S 9 ] chains by sharing the S-S prism edge. One of the S atoms at the prism edge and two other capping S atoms are bound to the P atom and an additional S atom (S1) is attached to the P atom to complete the [PS 4 ] tetrahedral coordination. The P-S distances (2.048 (4)-2.065 (3) Å) are in good agreement with P-S distances found in related phases (Brec et al., 1983). The S1 atom is the only sulfur atom that is not coordinated to any of the Nb atoms causing the short P-S1 distance (2.009 (4) Å) as well as the large ADP of the S1 atom (Do & Yun, 1996).
Along the chains, the Nb atoms associate in pairs with Nb-Nb interactions alternating in the sequence of one short and one long distances. Although the short distance (2.880 (1) Å) is typical of Nb 4+ -Nb 4+ bonding interactions (Angenault et al., 2000), the long distance (3.770 (2) Å) implies that there is no significant Nb-Nb interaction and such an arrangement is consistent with the highly resistive and diamagnetic nature of the compound.
supplementary materials sup-2 Experimental Ag 0.88 Nb 2 PS 10 was prepared by the reaction of the elements Nb, P, and S with an elemental ratio of 2:1:10 in the eutectic mixture of AgCl/LiCl (Kojima, 99.5%). The starting materials, Nb powder (CERAC 99.8%), P powder (CERAC 99.5%), and S powder (Aldrich 99.999%) were placed in a silica glass tube. The mass ratio of reactants and halide flux was 1:2. The tube was evacuated to 0.133 Pa, sealed, and heated to 973 K where it was kept for 7d. Afterwards, the tube was cooled at a rate of 4 K/h to room temperature. Black needle-shaped crystals were isolated from the flux by leaching out with water.
The crystals are stable in water and air. Electron microprobe analysis of the crystals established their homogeneity and the presence of Ag, Nb, P and S. No other element was detected.

Refinement
Large anisotropic displacement parameters (ADPs) of the silver atom were found when the structure was refined with the stoichiometric model AgNb 2 PS 10 , (Goh et al., 2002). The deficient nature of the Ag site was checked by refining the occupancy of Ag while that of the other atoms were fixed. With the non-stoichiometric model (Ag x Nb 2 PS 10 ), the occupation factor of the Ag site was reduced significantly from 1 to 0.88 (1) and the reliability factor (wR2 = 0.1089) was improved in comparison with full occupation of the silver position (wR2 = 0.1341). In addition, the anisotropic displacement parameters in the disordered model became plausible. As no evidence was found for ordering of the Ag site, a statistically disordered structure was assumed. With the composition established, the data for the compound were corrected for absorption with the use of the analytical method (de Meulenaer & Tompa, 1965). The highest residual electron density is 0.96 Å from the S1 site and the deepest hole is 0.73 Å from the Ag site. No additional symmetry, as tested by PLATON (Spek, 2009), was detected in this structure. Structure data were finally standardized by means of the program STRUCTURE TIDY (Gelato & Parthé, 1987).    (4) Nb1-S8 iv 2.573 (3) P-S10 2.065 (3) Nb1-S10 iv 2.659 (2) S1 i -Ag-S9 ii 131.39 (11) S7-Nb2-S5 ii 47.83 (9) S1 i -Ag-S2 iii 113.11 (9) S3 ii -Nb2-S2 viii 48.11 (8)