A new tetra­gonal structure type for Li₂B₂C

The ternary dilithium diboron carbide, Li₂B₂C (tetra­gonal, space group Pm2, tP10), crystallizes as a new structure type and consists of structural fragments which are typical for structures of elemental lithium and boron or binary borocarbide B₁₃C₂. The symmetries of the occupied sites are .m. and 2mm. for the B and C atoms, and m2 and 2mm. for the Li atoms. The coordination polyhedra around the Li atoms are cubocta­hedra and 15-vertex distorted pseudo-Frank–Kasper polyhedra. The environment of the B atom is a ten-vertex polyhedron. The nearest neighbours of the C atom are two B atoms, and this group is surrounded by a deformed cubocta­hedron with one centred lateral facet. Electronic structure calculations using the TB–LMTO–ASA method reveal strong B...C and B...B inter­actions.

S1. Introduction

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 Li_xBC 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 Li_1.25BC (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), Li_1.43–1.68B_{38.82–38.76}C₆ (space group R3m, a = 5.6031–5.6154 Å, c = 12.366–12.256 Å; Vojteer, 2008), Li_0.4–0.56BC (space group P6₃/mmc, a = 2.520–2.4809 Å, c = 7.333–7.4568 Å), and Li_0.83–1BC (space group P6₃/mmc, a = 7.0533–7.04798 Å, c = 46.025–46.092 Å; Fogg et al., 2006).

S2. Experimental

S2.1. Synthesis and crystallization

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).

S2.2. Refinement

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.

S3. Results and discussion

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 (E_F) 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).

Related literature

For related literature, see: Alcántara et al. (2002); Andersen (1975); Andersen & Jepsen (1984); Andersen et al. (1985, 1986); Andersson et al. (1968); Barth & Hedin (1972); Bauer & Nowotny (1971); Choi et al. (2004); Chumak et al. (2010, 2013); Dmytriv et al. (2005, 2007, 2010, 2011); Duijn et al. (2000); Farrugia (2012); Fogg et al. (2006); Kirfel et al. (1979); Lacroix-Orio, Tillard & Belin (2008); Langer et al. (2012); Mair et al. (1999); Pöttgen et al. (2010); Pavlyuk & Bodak (1995); Pavlyuk et al. (1989, 2005, 2007, 2008, 2011, 2012, 2014); Pavlyuk, Kevorkov, Bodak & Pecharskii (1995); Rosner et al. (2002); Scrosati & Garche (2010); Souptel et al. (2003); Sreeraj et al. (2006); Vojteer (2008); Vojteer & Hillebrecht (2006); Wörle et al. (1995); Zalkin & Templeton (1950); Zhang (2011).

Computing details

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).

Figures

	Fig. 1. A clinographic projection of the Li2B2C unit-cell contents and the coordination polyhedra of the atoms.
	Fig. 2. The relationship between the Li, B, B13C2 and Li2B2C structures.
	Fig. 3. (a) The borocarbon and (b) the lithium atomic networks in the Li2B2C structure.
	Fig. 4. The boron or borocarbon atomic networks in some related compounds.
	Fig. 5. (a) ELF mapping and (b) isosurfaces of ELF around the atoms in the Li2B2C structure.
	Fig. 6. (a) Total and partial DOS and (b) –COHP curves for Li2B2C from TB–LMTO–ASA calculations.

dilithium diboron carbide

Crystal data

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) Å^3	T = 293 K
Z = 2	Plate, metallic dark grey
F(000) = 44	0.07 × 0.05 × 0.01 mm

Data collection

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

Refinement on F^2	0 restraints
Least-squares matrix: full	Primary atom site location: structure-invariant direct methods
R[F^2 > 2σ(F^2)] = 0.017	Secondary atom site location: difference Fourier map
wR(F^2) = 0.057	w = 1/[σ^2(Fo^2) + (0.0091P)^2 + 0.0124P] where P = (Fo^2 + 2Fc^2)/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

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.

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

	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)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
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

Geometric parameters (Å, º)

C1—B2	1.5614 (17)	B2—B2^iii	1.6671 (13)
C1—B2^i	1.5614 (17)	B2—B2^i	2.0068 (2)
B2—B2^ii	1.6671 (13)	B2—B2^iv	2.1322 (2)
B2—C1—B2^i	79.98 (10)	B2^iii—B2—B2^i	129.75 (4)
C1—B2—B2^ii	137.55 (8)	C1—B2—B2^iv	129.99 (5)
C1—B2—B2^iii	137.55 (8)	B2^ii—B2—B2^iv	50.25 (4)
B2^ii—B2—B2^iii	79.51 (7)	B2^iii—B2—B2^iv	50.25 (4)
C1—B2—B2^i	50.01 (5)	B2^i—B2—B2^iv	180.0
B2^ii—B2—B2^i	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.