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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].

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270102022680/ta1397sup1.cif
Contains datablocks global, I

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270102022680/ta1397Isup2.hkl
Contains datablock I

Comment top

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.

Experimental top

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 Li14.63Al3.26Si6. 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.

Refinement top

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

Computing details top

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 top
[Figure 1] 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) top
Crystal data top
Al3.39Li14.61Si6.0Dx = 1.501 Mg m3
Mr = 361.32Mo Kα radiation, λ = 0.71073 Å
Hexagonal, P63/mCell parameters from 451 reflections
a = 7.549 (1) Åθ = 0–31°
c = 8.097 (1) ŵ = 0.67 mm1
V = 399.61 (9) Å3T = 293 K
Z = 1Plate, black
F(000) = 171.900.32 × 0.20 × 0.12 mm
Data collection top
Oxford Xcalibur CCD area-detector
diffractometer
Rint = 0.049
Graphite monochromatorθmax = 31.2°, θmin = 4.0°
ω scansh = 107
3693 measured reflectionsk = 1010
451 independent reflectionsl = 1111
274 reflections with I > 2σ(I)
Refinement top
Refinement on F24 parameters
Least-squares matrix: fullPrimary atom site location: structure-invariant direct methods
R[F2 > 2σ(F2)] = 0.036 Chebychev polynomial with 3 parameters (Carruthers & Watkin, 1979), 0.0509, 7.70, -1.98
wR(F2) = 0.049(Δ/σ)max = 0.001
S = 1.10Δρmax = 0.62 e Å3
274 reflectionsΔρmin = 1.01 e Å3
Crystal data top
Al3.39Li14.61Si6.0Z = 1
Mr = 361.32Mo Kα radiation
Hexagonal, P63/mµ = 0.67 mm1
a = 7.549 (1) ÅT = 293 K
c = 8.097 (1) Å0.32 × 0.20 × 0.12 mm
V = 399.61 (9) Å3
Data collection top
Oxford Xcalibur CCD area-detector
diffractometer
274 reflections with I > 2σ(I)
3693 measured reflectionsRint = 0.049
451 independent reflections
Refinement top
R[F2 > 2σ(F2)] = 0.03624 parameters
wR(F2) = 0.049Δρmax = 0.62 e Å3
S = 1.10Δρmin = 1.01 e Å3
274 reflections
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/UeqOcc. (<1)
Si0.00879 (7)0.66238 (6)0.25000.0099
Al10.33330.33330.25000.0211
Li10.0064 (3)0.6635 (3)0.0840 (5)0.0211
Al20.00001.00000.25000.02190.545 (9)
Li20.00001.00000.25000.02190.455 (9)
Al30.33330.66670.25000.02650.148 (13)
Li30.33330.66670.25000.02650.852 (13)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Si0.0078 (3)0.0086 (3)0.0109 (3)0.00239 (18)0.00000.0000
Al10.0168 (4)0.0168 (4)0.0297 (7)0.00838 (18)0.00000.0000
Li10.0228 (15)0.0263 (15)0.0162 (17)0.014 (1)0.0001 (8)0.0004 (8)
Al20.0165 (8)0.0165 (8)0.0327 (12)0.0083 (4)0.00000.0000
Li20.0165 (8)0.0165 (8)0.0327 (12)0.0083 (4)0.00000.0000
Al30.018 (2)0.018 (2)0.043 (3)0.0092 (11)0.00000.0000
Li30.018 (2)0.018 (2)0.043 (3)0.0092 (11)0.00000.0000
Geometric parameters (Å, º) top
Si—Al12.4672 (4)Al1—Li1i2.885 (4)
Si—Al/Li22.5162 (4)Li1—Li1iv2.688 (11)
Si—Al/Li32.5667 (4)Li1—Li1v2.861 (6)
Si—Li12.704 (5)Li1—Li1i2.862 (8)
Si—Li1i2.855 (4)Li1—Li2vi2.853 (4)
Si—Li1ii2.865 (4)Li1—Li3i2.821 (4)
Si—Li1iii2.841 (4)
Al1—Si—Al2121.993 (17)Si—Al3—Six120.000
Al1—Si—Al3119.941 (15)Li1i—Li1—Li1iii101.2 (3)
Al2—Si—Al3118.066 (15)Li1i—Li1—Li1v97.3 (3)
Si—Al1—Sivii120.000Li1iii—Li1—Li1v99.3 (2)
Si—Al2—Siviii120.000Li1i—Li1—Li1iv118.4 (4)
Si—Al2—Siix120.000
Symmetry codes: (i) x, y+1, z; (ii) xy+1, x+1, z+1/2; (iii) y1, x+y, z; (iv) x, y, z1/2; (v) xy+1, x+1, z; (vi) x, y+2, z; (vii) x+y1, x, z+1/2; (viii) x+y1, x+1, z+1/2; (ix) y+1, xy+2, z; (x) x+y, x+1, z+1/2.

Experimental details

Crystal data
Chemical formulaAl3.39Li14.61Si6.0
Mr361.32
Crystal system, space groupHexagonal, P63/m
Temperature (K)293
a, c (Å)7.549 (1), 8.097 (1)
V3)399.61 (9)
Z1
Radiation typeMo Kα
µ (mm1)0.67
Crystal size (mm)0.32 × 0.20 × 0.12
Data collection
DiffractometerOxford Xcalibur CCD area-detector
diffractometer
Absorption correction
No. of measured, independent and
observed [I > 2σ(I)] reflections
3693, 451, 274
Rint0.049
(sin θ/λ)max1)0.729
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.036, 0.049, 1.10
No. of reflections274
No. of parameters24
No. of restraints?
Δρmax, Δρmin (e Å3)0.62, 1.01

Computer programs: CrysAlis CCD (Oxford Diffraction, 2002), CrysAlis RED (Oxford Diffraction, 2002), SHELXS86 (Sheldrick, 1985), CRYSTALS (Watkin et al., 2001), DIAMOND (Brandenburg, 2001).

Selected geometric parameters (Å, º) top
Si—Al12.4672 (4)Al1—Li1i2.885 (4)
Si—Al/Li22.5162 (4)Li1—Li1iv2.688 (11)
Si—Al/Li32.5667 (4)Li1—Li1v2.861 (6)
Si—Li12.704 (5)Li1—Li1i2.862 (8)
Si—Li1i2.855 (4)Li1—Li2vi2.853 (4)
Si—Li1ii2.865 (4)Li1—Li3i2.821 (4)
Si—Li1iii2.841 (4)
Al1—Si—Al2121.993 (17)Li1i—Li1—Li1v97.3 (3)
Al1—Si—Al3119.941 (15)Li1iii—Li1—Li1v99.3 (2)
Al2—Si—Al3118.066 (15)Li1i—Li1—Li1iv118.4 (4)
Li1i—Li1—Li1iii101.2 (3)
Symmetry codes: (i) x, y+1, z; (ii) xy+1, x+1, z+1/2; (iii) y1, x+y, z; (iv) x, y, z1/2; (v) xy+1, x+1, z; (vi) x, y+2, z.
 

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