Li₁₅Al₃Si₆ (Li_14.6Al_3.4Si₆), a compound displaying a heterographite-like anionic framework

The title compound, lithium aluminium silicide (15/3/6), crystallizes in the hexagonal centrosymmetric space group P6₃/m. The three-dimensional structure of this ternary compound may be depicted as two interpenetrating lattices, namely a graphite-like Li₃Al₃Si₆ layer and a distorted diamond-like lithium lattice. As is commonly found for LiAl alloys, the Li and Al atoms are found to share some crystallographic sites. The diamond-like lattice is built up of Li cations, and the graphite-like anionic layer is composed of Si, Al and Li atoms in which Si and Al are covalently bonded [Si-Al = 2.4672 (4) Å].

S1. Comment

Lithium alloys have been the subject of considerable interest in electrochemistry as possible replacements for lithium metal as the negative electrode in lithium batteries (Winter et al., 1998; Winter & Besenhard, 1999; Huggins, 1999). Silicon-based lithium-alloying materials are also particularly interesting. Silicon is, after oxygen, the second most common element on earth. Aluminium and silicon are very cheap compared with other metal candidates, and their electronegativities and low weight would provide batteries with high potential and mass capacities.

With a view to some future electrochemical work, we decided to reinvestigate the ternary LiAlSi system, on the basis of the phase diagram established at 523 K (Kevorkov et al., 2001) using differential thermal analysis and X-ray powder diffractometry. This work led us to the discovery of a new ternary compound with the idealized formula Li₁₅Al₃Si₆, the stoichiometry of which has been confirmed by atomic absorption spectrophotometry of single crystals. Here, we present the structure of Li₁₅Al₃Si₆.

The atomic packing within the hexagonal unit cell of Li₁₅Al₃Si₆ may be formally described as interpenetrating anionic and cationic lattices, a graphite-like Li₃Al₃Si₆ layer and a distorted diamond-like lithium lattice (Fig. 1). The diamond-like lattice is built up of Li1 atoms that have a distorted tetrahedral environment, with Li1—Li1 distances ranging from 2.688 (11) to 2.861 (6) Å and angles ranging from 97.3 (3) to 118.4 (4)° (Table 1). Within the graphite-like layer, some covalent Al—Si bonding is observed, with Al1 being linked to three Si atoms [Al—Si 2.4672 (4) Å]. Owing to the Al/Li occupational disorder found at crystallographic sites 2a and 2c (Table 2), each Si atom is connected to three neighbours, Al1, Al/Li2 and Al/Li3, with interatomic distances of 2.4672 (4), 2.5667 (4) and 2.5162 (4) Å, respectively.

S2. Experimental

Initially, our goal was to synthesize the known compound Li₁₂Al₃Si₄, so melts of the elements were prepared in this stoichiometry. Silicon was used as a very pure powder, and the surfaces of the pure aluminium and lithium were scraped before use to remove any oxide film. The alloy was prepared in a tantalum tube weld-sealed in an argon atmosphere. This tube was protected from air by a silica jacket sealed under vacuum. The mixture was heated for 10 h at 1223 K in a vertical furnace and shaken several times for homogenization. It was then cooled at the rate of 6 K h⁻¹ for crystal growth. The product of the reaction appeared to be not quite homogeneous, but contained predominantly black and well crystallized material. A few black crystals were selected and analysed by atomic absorption flame spectrometry to establish their composition. Analysis led to an Li/Al/Si ratio of 1/0.223 (2)/0.41 (1), corresponding to a mean formula of Li_14.63Al_3.26Si₆. The compound could then be re-prepared following this stoichiometry and was obtained in practically 100% yield, as confirmed by the X-ray powder pattern (m.p. 1097 K). Crystals were selected inside a glove box filled with purified argon under a microscope. They were then inserted into Lindemann glass capillaries, avoiding any contact with air and moisture, and checked for singularity by preliminary oscillation and Weissenberg X-ray photographs. The best diffracting crystal was used for the intensity measurements.

S3. Refinement

The highest residual density and the deepest hole in the final difference Fourier map were located near Si and Al1 sites, respectively.

Computing details

Data collection: CrysAlis CCD (Oxford Diffraction, 2002); cell refinement: CrysAlis RED; data reduction: CrysAlis RED (Oxford Diffraction, 2002); program(s) used to solve structure: SHELXS86 (Sheldrick, 1985); program(s) used to refine structure: CRYSTALS (Watkin et al., 2001); molecular graphics: DIAMOND (Brandenburg, 2001).

Figures

Fig. 1. A representation of the atomic packing of Li15Al3Si6 in the hexagonal cell (2 × 2 × 2 unit cells), emphasizing the two-dimensional heterographitic-like (Si, Al, Li) and three-dimensional diamond-like (Li) sublattices.

Lithium aluminium silicide (15/3/6)

Crystal data

Al3.39Li14.61Si6.0	Dx = 1.501 Mg m^−3
Mr = 361.32	Mo Kα radiation, λ = 0.71073 Å
Hexagonal, P63/m	Cell parameters from 451 reflections
a = 7.549 (1) Å	θ = 0–31°
c = 8.097 (1) Å	µ = 0.67 mm^−1
V = 399.61 (9) Å^3	T = 293 K
Z = 1	Plate, black
F(000) = 171.90	0.32 × 0.20 × 0.12 mm

Data collection

Oxford Xcalibur CCD area-detector diffractometer	Rint = 0.049
Graphite monochromator	θmax = 31.2°, θmin = 4.0°
ω scans	h = −10→7
3693 measured reflections	k = −10→10
451 independent reflections	l = −11→11
274 reflections with I > 2σ(I)

Refinement

Refinement on F	24 parameters
Least-squares matrix: full	Primary atom site location: structure-invariant direct methods
R[F^2 > 2σ(F^2)] = 0.036	Chebychev polynomial with 3 parameters (Carruthers & Watkin, 1979), 0.0509, 7.70, -1.98
wR(F^2) = 0.049	(Δ/σ)max = 0.001
S = 1.10	Δρmax = 0.62 e Å^−3
274 reflections	Δρmin = −1.01 e Å^−3

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Si	−0.00879 (7)	0.66238 (6)	0.2500	0.0099
Al1	−0.3333	0.3333	0.2500	0.0211
Li1	−0.0064 (3)	0.6635 (3)	−0.0840 (5)	0.0211
Al2	0.0000	1.0000	0.2500	0.0219	0.545 (9)
Li2	0.0000	1.0000	0.2500	0.0219	0.455 (9)
Al3	0.3333	0.6667	0.2500	0.0265	0.148 (13)
Li3	0.3333	0.6667	0.2500	0.0265	0.852 (13)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Si	0.0078 (3)	0.0086 (3)	0.0109 (3)	0.00239 (18)	0.0000	0.0000
Al1	0.0168 (4)	0.0168 (4)	0.0297 (7)	0.00838 (18)	0.0000	0.0000
Li1	0.0228 (15)	0.0263 (15)	0.0162 (17)	0.014 (1)	−0.0001 (8)	0.0004 (8)
Al2	0.0165 (8)	0.0165 (8)	0.0327 (12)	0.0083 (4)	0.0000	0.0000
Li2	0.0165 (8)	0.0165 (8)	0.0327 (12)	0.0083 (4)	0.0000	0.0000
Al3	0.018 (2)	0.018 (2)	0.043 (3)	0.0092 (11)	0.0000	0.0000
Li3	0.018 (2)	0.018 (2)	0.043 (3)	0.0092 (11)	0.0000	0.0000

Geometric parameters (Å, º)

Si—Al1	2.4672 (4)	Al1—Li1^i	2.885 (4)
Si—Al/Li2	2.5162 (4)	Li1—Li1^iv	2.688 (11)
Si—Al/Li3	2.5667 (4)	Li1—Li1^v	2.861 (6)
Si—Li1	2.704 (5)	Li1—Li1^i	2.862 (8)
Si—Li1^i	2.855 (4)	Li1—Li2^vi	2.853 (4)
Si—Li1^ii	2.865 (4)	Li1—Li3^i	2.821 (4)
Si—Li1^iii	2.841 (4)
Al1—Si—Al2	121.993 (17)	Si—Al3—Si^x	120.000
Al1—Si—Al3	119.941 (15)	Li1^i—Li1—Li1^iii	101.2 (3)
Al2—Si—Al3	118.066 (15)	Li1^i—Li1—Li1^v	97.3 (3)
Si—Al1—Si^vii	120.000	Li1^iii—Li1—Li1^v	99.3 (2)
Si—Al2—Si^viii	120.000	Li1^i—Li1—Li1^iv	118.4 (4)
Si—Al2—Si^ix	120.000

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