Crystal structure of the mixed-metal thio­phosphate Nb_1.18V_0.82PS₁₀

The mixed-metal thio­phosphate, Nb_1.18V_0.82PS₁₀ (niobium vanadium phospho­rus deca­sulfide), has been prepared though solid state reactions using an alkali-metal halide flux. The title compound is isostructural with two-dimensional Nb₂PS₁₀. [M₂S₁₂] (M = Nb or V) dimers built up from two bicapped trigonal prisms and tetra­hedral [PS₄] units share sulfur atoms to construct ¹_∞[M₂PS₁₀] chains along the a axis. These chains are linked through the di­sulfide bonds between [PS₄] units in adjacent chains to form layers parallel to the ab plane. These layers then stack on top of each other to complete the three-dimensional structure with van der Waals gaps. The M sites are occupied by 59% of Nb and 41% of V and the average M—S and M—M distances in the title compound are in between those of V₂PS₁₀ and Nb₂PS₁₀. The classical charge balance of the title compound can be represented by [(Nb/V)⁴⁺]₂[P⁵⁺][S²⁻]₃[S⁻]₇.

1. Chemical context

Ternary group 5 metal thio­phosphates, M₂PS₁₀ (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_∞[M₂PS₁₀], V₂PS₁₀ has a chain structure (Brec et al., 1983a) and Nb₂PS₁₀ 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)₂PS₁₀. We report here the synthesis and structural characterization of a mixed-metallic thio­phosphate, namely Nb_1.18V_0.82PS₁₀.

2. Structural commentary

The title compound, Nb-rich Nb_1.18V_0.82PS₁₀, is isostructural with Nb₂PS₁₀ and detailed descriptions of this structural type have been given previously (Brec et al., 1983b). The usual [M₂S₁₂] (M = Nb, V) dimeric units (Yun et al., 2003) built up from two bicapped trigonal prisms and tetra­hedral [PS₄] units (Yu & Yun, 2011) share S atoms (Fig. 1) to construct an _∞¹[M₂PS₁₀] chain along the a axis. These chains are linked through the di­sulfide bonds between [PS₄] 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 three-dimensional structure with van der Waals gaps shown in Fig. 3. There is no bonding inter­action, only van der Waals forces, between the layers.

Figure 1 A view of the [M2S12] dimer unit (M = Nb or V) and its neighbouring tetra­hedral [PS4] 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.

Figure 2 View of the M2PS10 layers showing the two-dimensional nature of the compound. Atoms are as marked in Fig. 1.

Figure 3 The structure of Nb1.18V0.82PS10, viewed down the c axis.

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 Nb₂PS₁₀ [2.54 (6) Å; Brec et al., 1983b] and V₂PS₁₀ [2.46 (7) Å; Brec et al., 1983a]. The M atoms associate in pairs, with M⋯M 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 V₂PS₁₀ [2.852 (2) Å] and shorter than that of Nb₂PS₁₀ [2.869 (1) Å], is indicative of a d¹–d¹ 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 [PS₄] 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 V₂PS₁₀ (Brec et al., 1983a) and other related compounds, such as KNb₂PS₁₀ (Do & Yun, 1996).

The classical charge balance of the title compound can be represented by [(Nb/V)⁴⁺]₂[P⁵⁺][S²⁻]₃[S⁻]₇. This study does not provide conclusive results on the different dimensionality between Nb₂PS₁₀ and V₂PS₁₀ and thus we believe that further studies to search for V-rich phases are necessary.

3. Synthesis and crystallization

The compound Nb_1.18V_0.82PS₁₀ 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⁻¹) to 650 K, where it was kept for 12 h. The tube was cooled to 473 K at a rate of 4 K h⁻¹ 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.

4. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. The refinement of the model with occupational disorder on the M sites caused significant 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) was used to standardize the positional parameters. A difference Fourier synthesis calculated with phase based on the final parameters shows that the highest residual electron density (1.04 e Å⁻³) is 1.40 Å from the M1 site and the deepest hole (−1.06 e Å⁻³) is 0.79 Å from the M2 site.

Table 1
Experimental details

Crystal data
Chemical formula	Nb1.18V0.82PS10
Mr	502.97
Crystal system, space group	Orthorhombic, P21212
Temperature (K)	290
a, b, c (Å)	12.8472 (4), 13.6212 (4), 7.1972 (3)
V (Å^3)	1259.47 (8)
Z	4
Radiation type	Mo Kα
μ (mm^−1)	3.42
Crystal size (mm)	0.20 × 0.02 × 0.02
Data collection
Diffractometer	Rigaku R-AXIS RAPID S
Absorption correction	Multi-scan (ABSCOR; Higashi, 1995)
Tmin, Tmax	0.503, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	11897, 2757, 2053
Rint	0.081
(sin θ/λ)max (Å^−1)	0.639
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.045, 0.103, 1.13
No. of reflections	2757
No. of parameters	121
Δρmax, Δρmin (e Å^−3)	1.04, −1.06
Absolute structure	Flack (1983)
Absolute structure parameter	0.64 (13)
Computer programs: RAPID-AUTO (Rigaku, 2006), SHELXS2013 (Sheldrick, 2008), SHELXL2013 (Sheldrick, 2015), DIAMOND (Brandenburg, 1999) and WinGX (Farrugia, 2012).

Supporting information

S1. Chemical context

Ternary group 5 metal thio­phosphates, M₂PS₁₀ (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_∞[M₂PS₁₀], V₂PS₁₀ has a one-dimensional chain structure (Brec et al., 1983a) and Nb₂PS₁₀ adopts a two-dimensional 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)₂PS₁₀. We report here the synthesis and structural characterization of a mixed-metallic thio­phosphate, namely Nb_1.18V_0.82PS₁₀.

S2. Structural commentary

The title compound, Nb-rich Nb_1.18V_0.82PS₁₀, is isostructural with Nb₂PS₁₀ and detailed descriptions of this structural type have been given previously (Brec et al., 1983b). The usual bicapped trigonal prismatic dimers [M₂S₁₂] (Yun et al., 2003) and tetra­hedral [PS₄] units (Yu & Yun, 2011) share S atoms (Fig. 1) to build an ¹_∞[M₂PS₁₀] chain along the a axis. These chains are linked through the di­sulfide bonds between [PS₄] 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 three-dimensional 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 distance [2.513 (2) Å] in the title compound is between those of Nb₂PS₁₀ [2.54 (6) Å; Brec et al., 1983b] and V₂PS₁₀ [2.46 (7) Å; Brec et al., 1983a]. The M atoms associate in pairs, with M···M inter­actions alternating in the sequence of one short [2.855 (1) Å] and one long distance [3.728 (1) Å]. This M—M bond distance, which is longer than that of V₂PS₁₀ [2.852 (2) Å] and shorter than that of Nb₂PS₁₀ [2.869 (1) Å], is indicative of the d¹–d¹ inter­metallic bond. The long distance implies that there is no significant bonding inter­action (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 [PS₄] 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 V₂PS₁₀ (Brec et al., 1983a) and other related compounds, such as KNb₂PS₁₀ (Do & Yun, 1996).

The classical charge balance of the title compound can be represented by [(Nb/V)⁴⁺]₂[P⁵⁺][S^2-]₃[S^-]₇. This study does not provide conclusive results on the different dimensionality between Nb₂PS₁₀ and V₂PS₁₀ and thus we believe that further studies to search for V-rich phases are necessary.

S3. Synthesis and crystallization

The compound Nb_1.18V_0.82PS₁₀ was prepared by the reaction of the elemental 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^-1) to 650 K, where it was kept for 12 h. The tube was cooled to 473 K at a rate of 4 K h^-1 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.

S4. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. The refinement of the model with occupational disorder on the M sites caused significant 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) was used to standardize the positional parameters. A difference Fourier synthesis calculated with phase based on the final parameters shows that the highest residual electron density (1.04 e Å^-3) is 1.40 Å from the M1 site and the deepest hole (-1.06 e Å^-3) is 0.79 Å from the M2 site.

Related literature

For the structure of V₂PS₁₀, see: Brec et al.(1983a). For the structure of Nb₂PS₁₀, see: Brec et al.(1983b). For the structure of [M₂S₁₂], see: Yun et al. (2003). For the structure of [PS₄], see: Yu & Yun, (2011).

Computing details

Data collection: RAPID-AUTO (Rigaku, 2006); cell refinement: RAPID-AUTO (Rigaku, 2006); data reduction: RAPID-AUTO (Rigaku, 2006); program(s) used to solve structure: SHELXS2013 (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).

Figures

	Fig. 1. A view of the bicapped trigonal biprismatic dimer [M2S12] unit (M = Nb or V) and its neighbouring tetrahedral [PS4] groups. 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.
	Fig. 2. View of the M2PS10 layers showing the two-dimensional nature of the compound. Atoms are as marked in Fig. 1.
	Fig. 3. The structure of Nb1.18V0.82PS10, viewed down the c axis.

Niobium vanadium phosphorus decasulfide

Crystal data

Nb1.18V0.82PS10	F(000) = 969
Mr = 502.97	Dx = 2.653 Mg m^−3
Orthorhombic, P21212	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2 2ab	Cell parameters from 8753 reflections
a = 12.8472 (4) Å	θ = 3.2–27.5°
b = 13.6212 (4) Å	µ = 3.42 mm^−1
c = 7.1972 (3) Å	T = 290 K
V = 1259.47 (8) Å^3	Needle, black
Z = 4	0.2 × 0.02 × 0.02 mm

Data collection

Rigaku R-AXIS RAPID S diffractometer	2757 independent reflections
Radiation source: Sealed X-ray tube	2053 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.081
ω scans	θmax = 27.0°, θmin = 3.2°
Absorption correction: multi-scan (ABSCOR; Higashi, 1995)	h = −16→14
Tmin = 0.503, Tmax = 1.000	k = −17→17
11897 measured reflections	l = −9→9

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.045	w = 1/[σ^2(Fo^2) + (0.0386P)^2 + 1.1529P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.103	(Δ/σ)max < 0.001
S = 1.13	Δρmax = 1.04 e Å^−3
2757 reflections	Δρmin = −1.06 e Å^−3
121 parameters	Absolute structure: Flack (1983)
0 restraints	Absolute structure parameter: 0.64 (13)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refined as a 2-component inversion twin.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
Nb1	0.08230 (7)	0.74603 (8)	0.06349 (16)	0.0193 (3)	0.5889
V1	0.08230 (7)	0.74603 (8)	0.06349 (16)	0.0193 (3)	0.4111
Nb2	0.20780 (7)	0.24841 (8)	0.06671 (16)	0.0183 (3)	0.5856
V2	0.20780 (7)	0.24841 (8)	0.06671 (16)	0.0183 (3)	0.4144
S1	0.0601 (2)	0.11893 (17)	0.1005 (4)	0.0233 (6)
S2	0.0647 (2)	0.37577 (18)	0.1107 (4)	0.0258 (6)
S3	0.0727 (2)	0.03201 (19)	0.5538 (4)	0.0285 (6)
S4	0.1930 (2)	0.22558 (19)	0.4241 (4)	0.0284 (6)
S5	0.2221 (2)	0.6344 (2)	0.1672 (4)	0.0277 (7)
S6	0.3292 (2)	0.10963 (19)	0.0949 (4)	0.0284 (7)
S7	0.3483 (2)	0.3569 (2)	0.1743 (4)	0.0292 (7)
S8	0.4267 (2)	0.27029 (19)	0.5806 (4)	0.0284 (6)
S9	0.5644 (2)	0.18846 (18)	0.1448 (4)	0.0275 (6)
S10	0.7986 (2)	0.11500 (19)	0.0879 (5)	0.0275 (7)
P	0.0585 (2)	0.15464 (19)	0.3772 (3)	0.0236 (6)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Nb1	0.0164 (5)	0.0218 (5)	0.0197 (6)	0.0011 (5)	0.0007 (4)	−0.0009 (6)
V1	0.0164 (5)	0.0218 (5)	0.0197 (6)	0.0011 (5)	0.0007 (4)	−0.0009 (6)
Nb2	0.0145 (5)	0.0214 (5)	0.0192 (6)	−0.0012 (5)	0.0009 (4)	0.0013 (6)
V2	0.0145 (5)	0.0214 (5)	0.0192 (6)	−0.0012 (5)	0.0009 (4)	0.0013 (6)
S1	0.0206 (12)	0.0257 (12)	0.0238 (14)	0.0001 (14)	0.0004 (14)	0.0006 (10)
S2	0.0200 (12)	0.0271 (13)	0.0303 (15)	0.0006 (14)	0.0024 (14)	−0.0042 (10)
S3	0.0256 (13)	0.0319 (13)	0.0281 (14)	−0.0039 (13)	−0.0051 (13)	0.0078 (12)
S4	0.0221 (12)	0.0388 (16)	0.0242 (14)	−0.0074 (13)	−0.0013 (13)	0.0023 (13)
S5	0.0234 (14)	0.0301 (15)	0.0296 (17)	0.0041 (13)	0.0029 (12)	0.0047 (13)
S6	0.0231 (14)	0.0288 (14)	0.0333 (19)	−0.0023 (14)	0.0017 (13)	0.0054 (13)
S7	0.0267 (15)	0.0326 (15)	0.0283 (17)	−0.0050 (14)	0.0009 (12)	−0.0067 (14)
S8	0.0233 (13)	0.0386 (15)	0.0233 (14)	−0.0064 (13)	−0.0030 (13)	0.0026 (12)
S9	0.0248 (13)	0.0334 (14)	0.0243 (14)	−0.0014 (14)	−0.0004 (13)	0.0023 (11)
S10	0.0236 (14)	0.0258 (14)	0.0332 (18)	−0.0002 (13)	−0.0011 (14)	0.0069 (13)
P	0.0223 (13)	0.0283 (13)	0.0202 (14)	0.0001 (15)	0.0013 (12)	0.0036 (10)

Geometric parameters (Å, º)

Nb1—S10^i	2.440 (3)	Nb2—S5^v	2.461 (3)
Nb1—S7^ii	2.450 (3)	Nb2—S10^vi	2.462 (3)
Nb1—S6^ii	2.458 (3)	Nb2—S9^vi	2.540 (3)
Nb1—S5	2.469 (3)	Nb2—S2	2.547 (3)
Nb1—S9^ii	2.533 (3)	Nb2—S4	2.598 (3)
Nb1—S2^iii	2.537 (3)	Nb2—S1	2.602 (3)
Nb1—S8^iv	2.585 (3)	P—S1	2.050 (3)
Nb1—S1^iii	2.608 (3)	P—S3	2.107 (3)
Nb1—Nb2^ii	2.8549 (13)	P—S4	2.008 (4)
Nb2—S7	2.458 (3)	P—S8^vii	2.002 (4)
Nb2—S6	2.459 (3)
S8^vii—P—S4	117.17 (16)	S8^vii—P—S3	112.75 (17)
S8^vii—P—S1	106.08 (17)	S4—P—S3	101.85 (16)
S4—P—S1	105.61 (17)	S1—P—S3	113.40 (15)

Symmetry codes: (i) −x+1, −y+1, z; (ii) −x+1/2, y+1/2, −z; (iii) −x, −y+1, z; (iv) −x+1/2, y+1/2, −z+1; (v) −x+1/2, y−1/2, −z; (vi) x−1/2, −y+1/2, −z; (vii) x−1/2, −y+1/2, −z+1.