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
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.
The highest residual density and the deepest hole in the final difference Fourier map were located near Si and Al1 sites, respectively.
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).
Lithium aluminium silicide (15/3/6)
top
Crystal data top
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) Å3 | T = 293 K |
Z = 1 | Plate, black |
F(000) = 171.90 | 0.32 × 0.20 × 0.12 mm |
Data collection top
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 top
Refinement on F | 24 parameters |
Least-squares matrix: full | Primary 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 = 1.10 | Δρmax = 0.62 e Å−3 |
274 reflections | Δρmin = −1.01 e Å−3 |
Crystal data top
Al3.39Li14.61Si6.0 | Z = 1 |
Mr = 361.32 | Mo Kα radiation |
Hexagonal, P63/m | µ = 0.67 mm−1 |
a = 7.549 (1) Å | T = 293 K |
c = 8.097 (1) Å | 0.32 × 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 reflections | Rint = 0.049 |
451 independent reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.036 | 24 parameters |
wR(F2) = 0.049 | Δρmax = 0.62 e Å−3 |
S = 1.10 | Δρmin = −1.01 e Å−3 |
274 reflections | |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | 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 (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
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) top
Si—Al1 | 2.4672 (4) | Al1—Li1i | 2.885 (4) |
Si—Al/Li2 | 2.5162 (4) | Li1—Li1iv | 2.688 (11) |
Si—Al/Li3 | 2.5667 (4) | Li1—Li1v | 2.861 (6) |
Si—Li1 | 2.704 (5) | Li1—Li1i | 2.862 (8) |
Si—Li1i | 2.855 (4) | Li1—Li2vi | 2.853 (4) |
Si—Li1ii | 2.865 (4) | Li1—Li3i | 2.821 (4) |
Si—Li1iii | 2.841 (4) | | |
| | | |
Al1—Si—Al2 | 121.993 (17) | Si—Al3—Six | 120.000 |
Al1—Si—Al3 | 119.941 (15) | Li1i—Li1—Li1iii | 101.2 (3) |
Al2—Si—Al3 | 118.066 (15) | Li1i—Li1—Li1v | 97.3 (3) |
Si—Al1—Sivii | 120.000 | Li1iii—Li1—Li1v | 99.3 (2) |
Si—Al2—Siviii | 120.000 | Li1i—Li1—Li1iv | 118.4 (4) |
Si—Al2—Siix | 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. |
Experimental details
Crystal data |
Chemical formula | Al3.39Li14.61Si6.0 |
Mr | 361.32 |
Crystal system, space group | Hexagonal, P63/m |
Temperature (K) | 293 |
a, c (Å) | 7.549 (1), 8.097 (1) |
V (Å3) | 399.61 (9) |
Z | 1 |
Radiation type | Mo Kα |
µ (mm−1) | 0.67 |
Crystal size (mm) | 0.32 × 0.20 × 0.12 |
|
Data collection |
Diffractometer | Oxford Xcalibur CCD area-detector diffractometer |
Absorption correction | – |
No. of measured, independent and observed [I > 2σ(I)] reflections | 3693, 451, 274 |
Rint | 0.049 |
(sin θ/λ)max (Å−1) | 0.729 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.036, 0.049, 1.10 |
No. of reflections | 274 |
No. of parameters | 24 |
No. of restraints | ? |
Δρmax, Δρmin (e Å−3) | 0.62, −1.01 |
Selected geometric parameters (Å, º) topSi—Al1 | 2.4672 (4) | Al1—Li1i | 2.885 (4) |
Si—Al/Li2 | 2.5162 (4) | Li1—Li1iv | 2.688 (11) |
Si—Al/Li3 | 2.5667 (4) | Li1—Li1v | 2.861 (6) |
Si—Li1 | 2.704 (5) | Li1—Li1i | 2.862 (8) |
Si—Li1i | 2.855 (4) | Li1—Li2vi | 2.853 (4) |
Si—Li1ii | 2.865 (4) | Li1—Li3i | 2.821 (4) |
Si—Li1iii | 2.841 (4) | | |
| | | |
Al1—Si—Al2 | 121.993 (17) | Li1i—Li1—Li1v | 97.3 (3) |
Al1—Si—Al3 | 119.941 (15) | Li1iii—Li1—Li1v | 99.3 (2) |
Al2—Si—Al3 | 118.066 (15) | Li1i—Li1—Li1iv | 118.4 (4) |
Li1i—Li1—Li1iii | 101.2 (3) | | |
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. |
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.