Li2B12C2 and LiB13C2: Colorless Boron-Rich Boride Carbides of Lithium

Thermodynamic stability of Li-B-C compounds from first principles

Prediction of high-T_c superconductivity in hole-doped Li_xBC two decades ago has brought about an extensive effort to synthesize new materials with honeycomb B-C layers, but the thermodynamic stability of Li-B-C compounds remains largely unexplored. In this study, we use density functional theory to characterize well-established and recently reported Li-B-C phases. Our calculation of the Li chemical potential in Li_xBC helps estimate the (T,P) conditions required for delithiation of the LiBC parent material, while examination of B-C phases helps rationalize the observation of metastable BC₃ polymorphs with honeycomb and diamond-like morphologies. At the same time, we demonstrate that recently reported BC₃, LiBC₃, and Li₂B₂C phases with new crystal structures are both dynamically and thermodynamically unstable. With a combination of evolutionary optimization and rational design, we identify considerably more natural and favorable Li₂B₂C configurations that, nevertheless, remain above the thermodynamic stability threshold.

The binary boron lithium clusters B12Lin with n = 1-14: in search for hydrogen storage materials

Molecular structures and properties of the binary clusters containing twelve boron atoms mixed with n lithium atoms, B₁₂Li_n with n = 1-14, were investigated using density functional theory with the TPSSh functional and the 6-311+G(d) basis set. Energetic parameters including relative energies, average binding energies and second-order energies of the entire series were predicted using the coupled-cluster theory (U)CCSD(T) in conjunction with the cc-pVTZ basis set. Several lowest-lying isomers were determined for each size B₁₂Li_n whose energies differ from each other by <3 kcal mol^-1, except for n = 1, 2 and 4 (≤5 kcal mol^-1), and particularly n = 8 (∼13 kcal mol^-1). Electronic structure and chemical bonding in some specific sizes such as B₁₂Li₄, B₁₂Li₈ and B₁₂Li₁₄ were analyzed in detail. We established the electron shells of some magic clusters such as the B₁₂Li₄ cone for which we proposed a mixed cone-disk electron shell model. Thanks to both the phenomenological shell and Clemenger-Nilsson models, B₁₂Li₈ which contains a specific set of shells of 44 valence electrons is a high stability species. The arrangement of Li atoms around a fullerene B₁₂ framework shows that the mixed B₁₂Li₈ emerges as the most suitable of this cluster series to adsorb molecular hydrogen. Up to 32 H₂ molecules can strongly be attached to the B₁₂Li₈ cluster which is thus predicted to be a realistic candidate for hydrogen storage material with gravimetric density reaching up to a theoritical limit of 26 wt%. Attachment of the fifth H₂ molecule to each Li atom of B₁₂Li₈ results in weaker average bonds but can give rise to a total of 40 H₂ molecules, corresponding to 30 wt% of hydrogen.

Phase equilibria and crystal structure relationships in the ternary Li–B–C system

Phase equilibria in the Li–B–C ternary system have been investigated by X-ray powder diffraction, scanning electron microscopy, electron probe microanalysis and differential scanning calorimetry.

LiBC3: a new borocarbide based on graphene and heterographene networks

Li-B-C alloys have attracted much interest because of their potential use in lithium-ion batteries and superconducting materials. The formation of the new compound LiBC₃ [lithium boron tricarbide; own structure type, space group P-6m2, a = 2.5408 (3) Å and c = 7.5989 (9) Å] has been revealed and belongs to the graphite-like structure family. The crystal structure of LiBC₃ presents hexagonal graphene carbon networks, lithium layers and heterographene B/C networks, alternating sequentially along the c axis. According to electronic structure calculations using the tight-binding linear muffin-tin orbital-atomic spheres approximations (TB-LMTO-ASA) method, strong covalent B-C and C-C interactions are established. The coordination polyhedra for the B and C atoms are trigonal prisms and for the Li atoms are hexagonal prisms.

A new tetragonal structure type for Li2B2C

The ternary dilithium diboron carbide, Li2B2C (tetragonal, space group P-4m2, 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 -4m2 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.

Mechanical and electronic properties of B12-based ternary crystals of orthorhombic phase

Using first-principles calculations, the structural, mechanical and electronic properties of the experimentally synthesized B(12)-based ternary crystals (AlMgB(14), AlNaB(14), AlLiB(14), Mg(2)B(14), MgSi(2)B(12), MgC(2)B(12), Li(2)Si(2)B(12) and Li(2)C(2)B(12)) have been investigated. The theoretical equilibrium lattice constants of these crystals agree with the experimental values. The Vickers hardness (H(v)) estimated from the theoretical Young's moduli ranges from 20 to 30 GPa, and the MgC(2)B(12) compound (H(v) = 31.4 GPa) is harder than α-boron. Based on the electron density of states and Mulliken population analysis, the origination of hardness and interaction between the interstitial atoms and the B(12) framework were discussed. Scaled bond order of the B-B bonds was used to interpret the hardness of these B(12)-based ternary compounds. The crystal hardness is primarily determined by the B(12) icosahedral skeleton, whereas the contributions of metal atoms manifest as the electron transfer from metal to B atoms. We also calculated the ideal tensile strength of AlMgB(14) and MgC(2)B(12) and found that the <001> and <010> directions are their cleavage directions under tensile strains, respectively.

Synthesis, crystal structure, and properties of two modifications of MgB(12)C(2)

Single crystals of two modifications of the new magnesium boride carbide MgB(12)C(2) were synthesized from the elements in a metallic melt by using tantalum ampoules. Crystals were characterized by single-crystal X-ray diffraction and electron microprobe analysis (energy-dispersive (EDX) and wavelength-dispersive (WDX) X-ray spectroscopy). Orthorhombic MgB(12)C(2) is formed in a Cu/Mg melt at 1873 K. The crystal structure of o-MgB(12)C(2) (Imma, Z=4, a=5.6133(10), b=9.828(2), c=7.9329(15) A, 574 reflections, 42 variables, R(1)(F)=0.0208, wR(2)(I)=0.0540) consists of a hexagonal primitive array of B(12) icosahedra with Mg atoms and C(2) units in trigonal-prismatic voids. Each icosahedron has six exohedral B--B and six B--C bonds. Carbon is tetrahedrally coordinated by three boron atoms and one carbon atom with a remarkably long C--C distance of 1.727 A. Monoclinic MgB(12)C(2) is formed in an Al/Mg melt at 1573 K. The structure of m-MgB(12)C(2) (C2/c, Z=4, a=7.2736(11), b=8.7768(13), c=7.2817(11) A, beta=105.33(3) degrees , 1585 reflections, 71 variables, R(1)(F)=0.0228, wR(2)(I)=0.0610) may be described as a distorted cubic close arrangement of B(12) icosahedra. Tetrahedral voids are filled by C atoms and octahedral voids are occupied by Mg atoms. The icosahedra are interconnected by four exohedral B--B bonds to linear chains and by eight interstitial C atoms to form a three-dimensional covalent network. Both compounds fulfill the electron-counting rules of Wade and Longuet-Higgins.