Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S2053229614025510/fn3184sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S2053229614025510/fn3184Isup2.hkl |
CCDC reference: 1035565
Lithium intermetallides containing nonmetallic and p elements, such as B, Al, C, Si, Ge, Sn, Pb, Sb and others (Pavlyuk & Bodak, 1995; Pöttgen et al., 2010; Scrosati & Garche, 2010; Zhang, 2011; Langer et al., 2012), and metallic elements such as copper (Choi et al., 2004; Pavlyuk et al., 2008, 2011; Dmytriv et al., 2010), silver (Dmytriv et al., 2005; Pavlyuk et al., 2005, 2007; Sreeraj et al., 2006; Lacroix-Orio et al., 2008; Chumak et al., 2013), gold (Sreeraj et al., 2006; Dmytriv et al., 2011), palladium (Pavlyuk et al., 1989, 1995) and zinc (Alcántara et al., 2002; Dmytriv et al., 2007; Chumak et al., 2010; Pavlyuk et al., 2012, 2014) have been studied intensively as model systems for electrode materials in lithium-ion batteries. Another aspect of interest for Li–B–C compounds as possible structure materials is their ultralight weight.
The first ternary Li–B–C compound LiBC (space group P63/mmc, a = 2.7523 Å, c = 7.058 Å) represents a totally intercalated heterographite and was described by Wörle et al. (1995). This layered lithium borocarbide LiBC is isovalent with and structurally similar to the superconductor MgB2 (Rosner et al., 2002). Polycrystalline samples of LixBC samples were synthesized by a flux method for a wide range of flux compositions (x = 0.5–2.4), and a single phase was observed for the starting flux composition of Li1.25BC (Souptel et al., 2003). However, compared with graphite, the LiBC heterographite shows poor performance for both electrochemical Li insertion and extraction (Langer et al., 2012). In recent years, several new Li–B–C compounds have been reported: LiB13C2 (space group Imma, a = 5.6677 Å, b = 10.820 Å, c = 8.039 Å) and Li2B12C2 (space group Amm2, a = 4.706 Å, b = 9.010 Å, c = 5.652 Å (Vojteer & Hillebrecht, 2006), Li1.43–1.68B38.82–38.76C6 (space group R3m, a = 5.6031–5.6154 Å, c = 12.366–12.256 Å; Vojteer, 2008), Li0.4–0.56BC (space group P63/mmc, a = 2.520–2.4809 Å, c = 7.333–7.4568 Å), and Li0.83–1BC (space group P63/mmc, a = 7.0533–7.04798 Å, c = 46.025–46.092 Å; Fogg et al., 2006).
The Li–B–C samples were prepared from the following reactants: lithium (rod, cut into small pieces ~1 mm3, 99.9 at.%), boron (powder, 99.99 at.%) and carbon (graphite powder, 99.99 at.%). Appropriate amounts of all components were mixed according to the intended stoichiometry of the product and pressed in to a tablet at a pressure of 6 bar (1 bar = 100000 Pa). The tablet was closed inside a tantalum crucible in a glove-box under an argon atmosphere. The crucible was sealed by arc melting under a dry argon atmosphere. The reaction between the elements was initiated in an induction furnace at 1473 K. After 15 min, the sample was cooled rapidly to room temperature by removing the crucible from the furnace into ambient conditions. The reaction product was powdered in an agate mortar, filled into a capillary of 0.3 mm diameter and sealed for X-ray diffraction (XRD) on a Stoe STADI P (Mo Kα1 radiation) diffractometer in Debye–Scherrer mode (2θ from 5 to 45° in steps of 0.02°, linear position-sensitive detector with 6° aperture).
A laminar-like single crystal of the title compound, metallic dark grey in colour, was isolated from the Li60B30C10 alloy by mechanical fragmentation. This single crystal was protected from the air during X-ray data collection in a sealed thin-walled glass capillary (Hilgenberg, No. 10).
Crystal data, data collection and structure refinement details are summarized in Table 1. The analysis of systematic extinctions yielded the noncentrosymmetric space group P4m2, and was confirmed by the following structure refinement. Also, a statistical test of the distribution of the E values using the program E-STATS from the WinGX system (Farrugia, 2012) suggested that the structure is noncentrosymmetric. The structure was solved after the analytical absorption correction in space group P4, and after the extra symmetry or pseudosymmetry testing was transformed to space group P4m2. In the first stage of the refinement, the positions of all atoms were obtained correctly by direct methods. Initial refinement of the atomic parameters showed that the 1c, 1d and 2g positions were occupied by Li atoms. One 2g and one 4k positions were occupied by C and B atoms, respectively. In the final refinement cycles, all atoms were successfully refined with anisotropic displacement parameters.
The crystal of title compound contains no atom heavier than C and the data were collected with Mo radiation, so the absolute structure could not be determined.
The existence of a new tetragonal structure of Li2B2C was revealed by X-ray powder diffraction patterns, differing from powder patterns of all known Li–B–C phases. Therefore, a full structure analysis was performed using single-crystal X-ray diffraction. The obtained single-crystal data show that the title compound crystallizes with a new tetragonal structure type (space group P4m2). The unit-cell contents and the coordination polyhedra of the atoms are shown in Fig. 1. The number of neighbouring atoms correlates well with the sizes of the central atoms. The coordination polyhedra around Li3 and Li4 on the 1d and 1c sites, respectively, are cuboctahedra, [Li3Li4C4Li4] and [Li4Li4C4Li4]. Atoms Li5 on a 2g site are at the centres of 15-vertex distorted pseudo-Frank–Kasper polyhedra, [Li5B6C5Li4]. The coordination polyhedra around the B atoms are ten-vertex polyhedra, [BB4C3Li3]. The nearest neighbours of the C atoms are two B atoms, and this [–B–C–B-] group is surrounded by a 13-vertex cuboctahedron with one centred lateral facet.
A detailed crystal chemical analysis shows (Fig. 2) that the title structure consists of intergrown B13C2 and Li (W-type) structural fragments alternating along the c axis. The [–B4C2–] structural fragment of B13C2 borocarbide (Kirfel et al., 1979) is similar to the elemental boron structural fragment [–B6–] and it can be generated by the substitution of B atoms by C atoms.
Another way of describing this structure type is to analyse the network perpendicular to the longest unit-cell axis. The B and C atoms form a corrugated network, which accommodates the majority of the isolated square B4 groups, each of which is connected by means of C to the same four groups of atoms, forming 12-membered rings (Fig. 3a). This corrugated network has the symbol 12241. The Li atoms also form a corrugated network, as shown in Fig. 3(b).
Networks of B4 or substituted B2C2 squares are usually connected to eight-membered rings in borides and borocarbides. Such networks are compared in Fig. 4 for the known binary borides LiB3 (Mair et al., 1999), CrB4 (Andersson et al., 1968) and ThB4 (Zalkin & Templeton, 1950), and the ternary borocarbides CeB2C2 (van Duijn et al., 2000) and YB2C2 (Bauer & Nowotny, 1971), with the different connectivity scheme of the title compound Li2B2C.
The electronic structure of the title compound was calculated using the tight-binding linear muffin-tin orbital method in the atomic spheres approximation (TB-LMTO-ASA; Andersen, 1975; Andersen & Jepsen, 1984; Andersen et al., 1985, 1986), using the experimental crystallographic data presented here. The exchange and correlation were interpreted in the local density approximation (von Barth & Hedin, 1972).
The chemical bonding in the Li2B2C intermetallic compound is visualized by means of electron localization function (ELF) mapping, which is shown in Fig. 5(a). The isosurfaces of the ELF around the atoms for the title compound are shown in Fig. 5(b), and the total and partial densities of states (DOS) are shown in Fig. 6(b). The Fermi level (EF) lies in a continuous DOS region, indicating a metallic character for the title compound. The DOS is low over the entire energy range near the Fermi level and confirms its relative stability. The chemical bonding [integrated crystal orbital Hamilton populations (iCOHP) curve] exhibits strong B—C (d = 1.5654 Å and -iCOHP = 7.773 eV) interactions between -7 and -4 eV (Fig. 6b). A slightly weaker interaction is observed between B—B atoms, with d = 1.6667 Å and -iCOHP = 5.286 eV. The interaction between Li—B atoms has d = 2.102 Å and -iCOHP = 0.355 eV, so is much weaker. The weakest interaction is that between Li—Li atoms (d = 2.920 Å and -iCOHP = 0.073 eV).
Data collection: CrysAlis CCD (Oxford Diffraction, 2008); cell refinement: CrysAlis CCD (Oxford Diffraction, 2008); data reduction: CrysAlis RED (Oxford Diffraction, 2008); program(s) used to solve structure: SHELXS2014 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2006); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2008).
Li2B2C | Dx = 1.296 Mg m−3 |
Mr = 47.51 | Mo Kα radiation, λ = 0.71073 Å |
Tetragonal, P4m2 | Cell parameters from 182 reflections |
a = 4.1389 (4) Å | θ = 2.9–27.3° |
c = 7.1055 (11) Å | µ = 0.05 mm−1 |
V = 121.72 (3) Å3 | T = 293 K |
Z = 2 | Plate, metallic dark grey |
F(000) = 44 | 0.07 × 0.05 × 0.01 mm |
Oxford Xcalibur3 CCD area-detector diffractometer | 182 independent reflections |
Radiation source: fine-focus sealed tube | 178 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.026 |
Detector resolution: 0 pixels mm-1 | θmax = 27.3°, θmin = 2.9° |
ω scans | h = −5→5 |
Absorption correction: analytical (CrysAlis RED; Oxford Diffraction, 2008) | k = −5→5 |
Tmin = 0.891, Tmax = 0.998 | l = −9→9 |
1356 measured reflections |
Refinement on F2 | 0 restraints |
Least-squares matrix: full | Primary atom site location: structure-invariant direct methods |
R[F2 > 2σ(F2)] = 0.017 | Secondary atom site location: difference Fourier map |
wR(F2) = 0.057 | w = 1/[σ2(Fo2) + (0.0091P)2 + 0.0124P] where P = (Fo2 + 2Fc2)/3 |
S = 1.44 | (Δ/σ)max < 0.001 |
182 reflections | Δρmax = 0.22 e Å−3 |
19 parameters | Δρmin = −0.19 e Å−3 |
Li2B2C | Z = 2 |
Mr = 47.51 | Mo Kα radiation |
Tetragonal, P4m2 | µ = 0.05 mm−1 |
a = 4.1389 (4) Å | T = 293 K |
c = 7.1055 (11) Å | 0.07 × 0.05 × 0.01 mm |
V = 121.72 (3) Å3 |
Oxford Xcalibur3 CCD area-detector diffractometer | 182 independent reflections |
Absorption correction: analytical (CrysAlis RED; Oxford Diffraction, 2008) | 178 reflections with I > 2σ(I) |
Tmin = 0.891, Tmax = 0.998 | Rint = 0.026 |
1356 measured reflections |
R[F2 > 2σ(F2)] = 0.017 | 19 parameters |
wR(F2) = 0.057 | 0 restraints |
S = 1.44 | Δρmax = 0.22 e Å−3 |
182 reflections | Δρmin = −0.19 e Å−3 |
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. |
x | y | z | Uiso*/Ueq | ||
C1 | 0.0000 | 0.5000 | 0.2184 (2) | 0.0060 (4) | |
B2 | 0.2424 | 0.5000 | 0.0501 (2) | 0.0059 (3) | |
Li3 | 0.0000 | 0.0000 | 0.5000 | 0.0045 (9) | |
Li4 | 0.5000 | 0.5000 | 0.5000 | 0.0039 (8) | |
Li5 | 0.5000 | 0.0000 | 0.2113 (4) | 0.0069 (6) |
U11 | U22 | U33 | U12 | U13 | U23 | |
C1 | 0.0049 (11) | 0.0054 (11) | 0.0076 (8) | 0.000 | 0.000 | 0.000 |
B2 | 0.0053 (7) | 0.0054 (6) | 0.0069 (6) | 0.000 | −0.0003 (5) | 0.000 |
Li3 | 0.0038 (11) | 0.0038 (11) | 0.0059 (19) | 0.000 | 0.000 | 0.000 |
Li4 | 0.0034 (11) | 0.0034 (11) | 0.0048 (19) | 0.000 | 0.000 | 0.000 |
Li5 | 0.0072 (19) | 0.0064 (19) | 0.0071 (14) | 0.000 | 0.000 | 0.000 |
C1—B2 | 1.5614 (17) | B2—B2iii | 1.6671 (13) |
C1—B2i | 1.5614 (17) | B2—B2i | 2.0068 (2) |
B2—B2ii | 1.6671 (13) | B2—B2iv | 2.1322 (2) |
B2—C1—B2i | 79.98 (10) | B2iii—B2—B2i | 129.75 (4) |
C1—B2—B2ii | 137.55 (8) | C1—B2—B2iv | 129.99 (5) |
C1—B2—B2iii | 137.55 (8) | B2ii—B2—B2iv | 50.25 (4) |
B2ii—B2—B2iii | 79.51 (7) | B2iii—B2—B2iv | 50.25 (4) |
C1—B2—B2i | 50.01 (5) | B2i—B2—B2iv | 180.0 |
B2ii—B2—B2i | 129.75 (4) |
Symmetry codes: (i) −x, −y+1, z; (ii) y, −x+1, −z; (iii) −y+1, x, −z; (iv) −x+1, −y+1, z. |
Experimental details
Crystal data | |
Chemical formula | Li2B2C |
Mr | 47.51 |
Crystal system, space group | Tetragonal, P4m2 |
Temperature (K) | 293 |
a, c (Å) | 4.1389 (4), 7.1055 (11) |
V (Å3) | 121.72 (3) |
Z | 2 |
Radiation type | Mo Kα |
µ (mm−1) | 0.05 |
Crystal size (mm) | 0.07 × 0.05 × 0.01 |
Data collection | |
Diffractometer | Oxford Xcalibur3 CCD area-detector diffractometer |
Absorption correction | Analytical (CrysAlis RED; Oxford Diffraction, 2008) |
Tmin, Tmax | 0.891, 0.998 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 1356, 182, 178 |
Rint | 0.026 |
(sin θ/λ)max (Å−1) | 0.645 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.017, 0.057, 1.44 |
No. of reflections | 182 |
No. of parameters | 19 |
Δρmax, Δρmin (e Å−3) | 0.22, −0.19 |
Computer programs: CrysAlis CCD (Oxford Diffraction, 2008), CrysAlis RED (Oxford Diffraction, 2008), SHELXS2014 (Sheldrick, 2008), SHELXL2014 (Sheldrick, 2008), DIAMOND (Brandenburg, 2006).