Crystal structure of the mixed-metal thiophosphate Nb1.18V0.82PS10

The mixed-metal thiophosphate Nb1.18V0.82PS10 has been prepared by the use of a halide flux and the crystal structure has been analyzed by single-crystal diffraction methods.


Chemical context
Ternary group 5 metal thiophosphates, M 2 PS 10 (M = V, Nb) have been reported to have low-dimensional structures with partially filled d orbitals which can accommodate electrons. Therefore, they are of potential importance as cathode materials for high-energy density lithium batteries (Rouxel, 1986). While both are composed of the same linear chains, i.e. 1 1 [M 2 PS 10 ], V 2 PS 10 has a chain structure (Brec et al., 1983a) and Nb 2 PS 10 adopts a layered structure (Brec et al., 1983b). To understand the cause of different dimensionality between these phases, we have conducted research on the synthesis of the mixed phases, (Nb/V) 2 PS 10 . We report here ISSN 2056-9890 Figure 1 A view of the [M 2 S 12 ] dimer unit (M = Nb or V) and its neighbouring tetrahedral [PS 4 ] group. Open circles are S atoms, filled circle are M atoms and gray circles are P atoms. Displacement ellipsoids are drawn at the 60% probability level. the synthesis and structural characterization of a mixedmetallic thiophosphate, namely Nb 1.18 V 0.82 PS 10 .

Structural commentary
The title compound, Nb-rich Nb 1.18 V 0.82 PS 10 , is isostructural with Nb 2 PS 10 and detailed descriptions of this structural type have been given previously (Brec et al., 1983b). The usual [M 2 S 12 ] (M = Nb, V) dimeric units (Yun et al., 2003) built up from two bicapped trigonal prisms and tetrahedral [PS 4 ] units (Yu & Yun, 2011) share S atoms ( Fig. 1) to construct an 1 1 [M 2 PS 10 ] chain along the a axis. These chains are linked through the disulfide bonds between [PS 4 ] units in adjacent chains to form layers parallel to the ab plane (Fig. 2). These layers then stack on top of each other to complete the threedimensional structure with van der Waals gaps shown in Fig. 3. There is no bonding interaction, only van der Waals forces, between the layers.
The M sites occupied by statistically disordered Nb (59%) and V (41%) are surrounded by eight S atoms in a bicapped trigonal prismatic fashion and the average M-S bond length [2.51 (6) Å ] in the title compound is between those of Nb 2 PS 10 [2.54 (6) Å ; Brec et al., 1983b] and V 2 PS 10 [2.46 (7) Å ; Brec et al., 1983a]. The M atoms associate in pairs, with MÁ Á ÁM interactions alternating in the sequence of one short [2.855 (1) Å ] and one long distance [3.728 (1) Å ]. This M-M distance, which is longer than that of V 2 PS 10 [2.852 (2) Å ] and shorter than that of Nb 2 PS 10 [2.869 (1) Å ], is indicative of a d 1 -d 1 interaction. The long distance implies that there is no significant bonding (Angenault et al., 2000), which is consistent with the highly resistive nature of the crystal since no intermetallic bond can be set. The P-S distances in the tetrahedral [PS 4 ] unit are in good agreement with those found in other thiophosphates (Brec et al., 1983b). There is no terminal S atom in this unit and this is responsible for the absence of the rather short P-S distances (< 2.0 Å ) found in V 2 PS 10 (Brec et al., 1983a) and other related compounds, such as KNb 2 PS 10 (Do & Yun, 1996).
The classical charge balance of the title compound can be This study does not provide conclusive results on the different dimensionality between Nb 2 PS 10 and V 2 PS 10 and thus we believe that further studies to search for V-rich phases are necessary.

Synthesis and crystallization
The compound Nb 1.18 V 0.82 PS 10 was prepared by the reaction of the elements Nb, V, P and S by the use of the reactive alkali metal halides. A combination of the pure elements, Nb powder (CERAC 99.8%), V powder (CERAC 99.5%), P powder (CERAC 99.95%) and S powder (Aldrich 99.999%) were mixed in a fused-silica tube in an Nb:V:P:S molar ratio of 1:1:1:10 with KCl. The mass ratio of the reactants and the halides flux was 2:1. The tube was evacuated to 0.133 Pa, sealed and heated gradually (100 K h À1 ) to 650 K, where it was kept for 12 h. The tube was cooled to 473 K at a rate of 4 K h À1 and then quenched to room temperature. The excess halides were removed with distilled water and black needle shaped crystals were obtained. The crystals are stable in air and water. A qualitative X-ray fluorescence analysis of selected crystals indicated the presence of Nb, V, S and P. The final composition of the title compound was determined by single-crystal X-ray diffraction.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1 View of the M 2 PS 10 layers showing the two-dimensional nature of the compound. Atoms are as marked in Fig. 1.

Figure 3
The structure of Nb 1.18 V 0.82 PS 10 , viewed down the c axis.
decrease of the R factor (wR2 = 0.103) in comparison with the case where full occupation by either metal had been considered (wR2 > 0.176). No evidence was found for ordering of this site and thus a statistically disordered structure is assumed. Also the displacement parameters in the disordered model became plausible. The disordered atoms were supposed to have the same displacement parameters. The Nb:V ratios on both M sites are almost the same, i.e. 59:41. The program STRUCTURE TIDY (Gelato & Parthé, 1987) (Sheldrick, 2015), DIAMOND (Brandenburg, 1999) and WinGX (Farrugia, 2012 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2013 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: WinGX (Farrugia, 2012).