Lithium niobium disulfide, Li_0.63NbS₂

Lithium niobium di­sulfide, Li_0.63NbS₂, has been prepared by a metathesis reaction between physically separated solid reactants, i.e. separated reactant metathesis (SRM). Single-crystal data were collected at reduced temperature [150 (2) K], yielding a refined Li content of y = 0.63 (6). The Li content in the crystalline samples was also determined analytically by flame photometry. The compound crystallizes in hexagonal space group P6₃/mmc (No. 194), with Li⁺ ions situated in octahedral sites between NbS₂ layers.

S1. Comment

Layered transition metal dichalcogenides and their alkali metal intercalated derivatives are of interest for both their physical properties and potential technological applications (Dahn et al., 1986; Chen et al., 1993; Starnberg, 2000). Li-intercalated compounds, such as Li_yNbS₂, warrant particular attention, due to their ability to serve as the basis for high performance rechargeable battery materials (Wilson & Yoffe, 1969; Jellinek, 1972; Whittingham, 1978). Accurate determination of the Li stoichiometry, the limit(s) of Li-intercalation and the effects on the crystal structure of the parent dichalcogenide are of prime importance in understanding the nature of the resulting electronic and transport properties.

Despite the importance of ascertaining Li content precisely, the inherent difficulties involved in the detection and quantification of a light ionic guest within the heavier host framework have often prevented accurate determination of occupancy. Previous reports have predicted the limits of Li intercalation in Li_yNbS₂ to lie in the range 0.5 < y < 1.0, and lattice parameters have been found for samples (crystals and powders) ranging from a = 3.331 (1) to 3.348 (1) Å and c = 12.861 (7) to 12.90 (1) Å for y < 0.75 (Omloo & Jellinek, 1970; Barker & Gareh, 1994; Gareh, Barker & Begley, 1995; Gareh, Barker et al., 1995). To date, the lowest Li occupancy reported for a Li_yNbS₂ single-crystal is y = 0.5 in Li_0.5NbS_2.06, obtained by reaction of Li₂CO₃ under a flow of CS₂/argon (Gareh, Barker & Begley, 1995). However, this Li stoichiometry was not quantitatively determined (or refined), but was estimated by comparison with powder data to be a minimum of y = 0.5. The earlier study (Omloo & Jellinek, 1970) proposes a model on the basis only of observed intensities in the X-ray powder pattern.

For the present work, the single-crystal structure of Li_yNbS₂ [y = 0.63 (6)] has been determined using data collected at reduced temperature [150 (2) K] in a stream of cold nitrogen gas. The compound indexed as an hexagonal cell with lattice parameters of a = 3.3477 (8) and c = 12.875 (4) Å, consistent with what is expected for lithium occupancies in the range 0.5 < y < 1.0. The fractional occupancy of the Li-atom position was allowed to vary freely, converging to 0.63 (6) with physically meaningful anisotropic displacement parameters.

This refined Li occupancy is in excellent agreement with flame photometry experiments (Corning 400 flame photometer) performed on crystals of Li_yNbS₂ dissolved in dilute nitric acid solution. Analysis yielded an Li occupancy of y = 0.51 (11). Furthermore, preliminary results from refinement of powder neutron diffraction (PND) data taken at 1.8 K on the POLARIS instrument at the ISIS facility, Rutherford-Appleton Laboratory, Oxfordshire, for a bulk sample of Li_yNbS₂ from the same reaction vessel, yielded an occupancy for Li of y = 0.58 (1). Details of PND investigations of intercalated dichalcogenides will be published elsewhere.

The co-ordination and structural environments of Li, Nb and S in Li_0.63NbS₂ are illustrated in Figs. 1 and 2. Li_0.63NbS₂ has a layered structure, consisting of Li^I ions situated in octahedral sites between NbS₂ layers. The coordination sphere of the Li atoms is octahedral, forming edge-sharing layers in the ab plane (Fig. 1). S—Li—S bond angles of 82.37 and 97.63° (for coordination of Li to S ions in the ab plane and along c, respectively) indicate that the Li octahedra are distorted by an elongation along one of the threefold axes in the c direction. Nb ions are trigonal-prismatically coordinated to S and are edge-sharing (Fig. 2) along the a and b directions. The Nb—S bond distance of 2.4886 (9) Å in Li_0.62NbS₂ is in good agreement with that in the parent NbS₂ structure [2.472 (7) Å; Reference?]. S ions are arranged in slabs, surrounding Nb in an hexagonal close-packed array. Each S ion is coordinated to three Nb and three Li ions.

S2. Experimental

Single crystals of Li_0.63NbS₂ were prepared by the separated reactant metathesis reaction between an oxide precursor (LiNbO₃) and excess chalcogen source material (Y₂S₃). The reactants were weighed out in the approximate ratio of 1:2 (LiNbO₃:Y₂S₃) and placed into separate alumina crucibles inside a silica ampoule, which was then evacuated to approximately 10⁻⁵ Torr (1 Torr = 133.322 Pa) and sealed. The silica ampoule was reacted inside a muffle furnace at 1073 K for 7 d, then slowly cooled to room temperature at the rate of 10 K h⁻¹. Metallic grey platelets of Li_0.63NbS₂ were found growing within the inner crucible, which originally contained LiNbO₃. Opening of the ampoule and all manipulations of the crystals were carried out under a nitrogen atmosphere in a nitrogen-filled dry glove box. Single crystals were extracted by hand and placed under moisture-free perfluoropolyether oil (RS3000; Riedel de Hahn) for protection from the atmosphere during analysis.

Computing details

Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2000); data reduction: SAINT and SHELXTL (Bruker, 1997); program(s) used to solve structure: SIR92 (Altomare et al., 1993); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ATOMS (Dowty, 1998) and SHELXTL (Bruker, 1997); software used to prepare material for publication: SHELXL97 and PLATON (Spek, 2002).

Figures

	Fig. 1. An Li-centred polyhedral representation of Li0.63NbS2, depicting layers of edge-sharing LiS6 octahedra which stack along the c axis. Nb atoms (large grey spheres) are surrounded by an hexagonal close-packed array of S ions.
	Fig. 2. An Nb-centred polyhedral representation of Li0.63NbS2, depicting layers of edge-sharing NbS6 trigonal prisms which stack along the c axis. Li ions (mid-grey spheres) are situated in octahedral holes between NbS2 layers.

lithium niobium disulfide

Crystal data

Li0.63NbS2	Dx = 4.290 Mg m^−3
Mr = 161.40	Mo Kα radiation, λ = 0.71073 Å
Hexagonal, P63/mmc	Cell parameters from 294 reflections
Hall symbol: -p 6c 2c	θ = 3.2–27.4°
a = 3.3477 (8) Å	µ = 6.08 mm^−1
c = 12.875 (4) Å	T = 150 K
V = 124.96 (6) Å^3	Tablet, metallic grey
Z = 2	0.09 × 0.07 × 0.03 mm
F(000) = 150

Data collection

Bruker SMART1000 CCD area-detector diffractometer	79 independent reflections
Radiation source: normal-focus sealed tube	76 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.020
ω scans	θmax = 27.4°, θmin = 3.2°
Absorption correction: multi-scan (SHELXTL; Bruker 2001)	h = −4→4
Tmin = 0.762, Tmax = 0.832	k = −2→3
383 measured reflections	l = −16→4

Refinement

Refinement on F^2	6 restraints
Least-squares matrix: full	Primary atom site location: structure-invariant direct methods
R[F^2 > 2σ(F^2)] = 0.018	Secondary atom site location: difference Fourier map
wR(F^2) = 0.044	w = 1/[σ^2(Fo^2) + (0.0215P)^2 + 0.2484P] where P = (Fo^2 + 2Fc^2)/3
S = 1.42	(Δ/σ)max < 0.001
79 reflections	Δρmax = 0.68 e Å^−3
9 parameters	Δρmin = −0.36 e Å^−3

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. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Nb	0.0000	0.0000	0.2500	0.0063 (3)
S	0.3333	0.6667	0.12824 (9)	0.0071 (4)
Li	0.0000	0.0000	0.0000	0.009 (5)	0.63 (6)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Nb	0.0053 (3)	0.0053 (3)	0.0081 (4)	0.00267 (16)	0.000	0.000
S	0.0062 (5)	0.0062 (5)	0.0090 (6)	0.0031 (2)	0.000	0.000
Li	0.011 (8)	0.011 (8)	0.006 (6)	0.005 (4)	0.000	0.000

Geometric parameters (Å, º)

Nb—S	2.4886 (9)	S—Li	2.5420 (9)
Nb—Li	3.2188 (10)	Li—Li^i	3.3477 (8)
Nb—Nb^i	3.3477 (8)
S^i—Nb—S	84.54 (4)	Nb—S—Li	79.55 (2)
S^ii—Nb—S	134.299 (17)	Li—S—Li^vii	82.37 (4)
S^iii—Nb—S	78.09 (5)	S^viii—Li—S	180.00 (4)
S—Nb—Li^iv	129.04 (2)	S—Li—S^i	82.37 (4)
S—Nb—Li	50.96 (2)	S—Li—S^ix	97.63 (4)
Li^iv—Nb—Li	180.0	S^viii—Li—Nb	130.51 (2)
S—Nb—Nb^i	132.268 (18)	S—Li—Nb	49.49 (2)
S—Nb—Nb^v	47.732 (18)	Nb—Li—Nb^viii	180.0
Li—Nb—Nb^i	90.0	S—Li—Li^i	131.183 (18)
S—Nb—Nb^vi	90.0	S—Li—Li^v	48.817 (18)
Nb^i—Nb—Nb^v	180.0	S^x—Li—Li^v	90.0
Nb^i—Nb—Nb^vii	120.0	Nb—Li—Li^v	90.0
Nb^v—Nb—Nb^vii	60.0	Li^v—Li—Li^i	180.0
Nb^v—S—Nb	84.54 (4)	Li^v—Li—Li^x	120.0
Nb^v—S—Li	134.782 (8)	Li^i—Li—Li^x	60.0

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