Surface inducing high-temperature superconductivity in layered metal carborides Li2BC3 and LiBC by metallizing σ electrons

Metallizing σ electrons provides a promising route to design high-temperature superconducting materials, such as MgB2 and high-pressure hydrides. Here, we focus on two MgB2-like layered carborides Li2BC3 and LiBC; their bulk does not have superconductivity because the B-C σ states are far away from the Fermi level (EF), however, based on first-principles calculations, we found that when their bulk systems are cleaved into surfaces with B-C termination, high Tc of ∼80 K could be observed in the exposed B-C layer on the surfaces. Detailed analysis reveals that surface symmetry reduction, due to lattice periodic breaking, not only introduces hole self-doping into surface B-C layers and shifts the σ-bonding states towards the EF - associated with emergent large electronic occupation, but also makes in-plane stretching modes on the surface layer experience significant softness. The enhanced σ states and softened phonon modes work to produce strong coupling, thus yielding high-Tc surface superconductivity, which distinctly differs from the superconducting features of the MgB2 film, which generates phonon stiffness accompanied by suppressed superconductivity. Our findings undoubtedly provide a novel platform to realize high-Tc surface superconductivity, and also clearly elucidate the microscopic mechanism of surface-enhanced superconductivity in favor of creating more high-Tc surface superconductors among MgB2-like layered materials.

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.

New cubic cluster phases in the Mg-Ni-Ga system

The crystal structure of new Mg₉Ni₆Ga₁₄ and Mg₃Ni₂Ga compounds were investigated by single-crystal diffraction. Both structures can be described as three-core-shell cluster compounds. In the Mg₆Ni₉Ga₁₄ structure, the [Ni₆Ga₆] icosahedron is encapsulated within the [Mg₂₀] dodecahedron, which is again encapsulated within a [Ni₁₈Ga₄₂] fullerene-like truncated icosahedron, thus the three core-shell cluster [Ni₆Ga₆@Mg₂₀@Ni₁₈Ga₄₂] results. In the Mg₃Ni₂Ga structure, the [Mg₆] octahedron is encapsulated within the [Ni₁₂Ga₆] flattened icosahedron in vertices of which there are 12 nickel atoms, and six lateral edges are centered by gallium atoms, which in turn is encapsulated within a [Mg₃₆] pseudo-rhombicuboctahedron with 12 additional atoms centering the lateral faces; thus for Mg₃Ni₂Ga the three-shell cluster is [Mg₆@Ni₁₂Ga₆@Mg₃₆].