Phase stability and band degeneracy of quasi-one-dimensional boron chain polymorphs embedded in LiB crystals

Polymorphs of boron provide various types of atomic structures that lead to the emergence of physical phenomena and become playgrounds to examine electronic states. There are lithium-boron compounds that embed quasi-one-dimensional (quasi-1D) boron incommensurately in the hexagonal lithium sublattices. Based on first-principles calculations, k groups, and topological invariants, we unveiled that the quasi-1D structures show double π bonding orbitals, unavoidable band crossings, and nodal lines around the Fermi level. The origin of this phenomenon is attributed to the interaction between B-B chains through sublattice Li atoms and to the nonsymmorphic crystal symmetry.