Theoretical investigation of stoichiometric lithium monoboride

Abstract

Structural energies around two recently reported diffraction results of nominally stoichiometric LiB samples have been investigated using full potential electronic structure methods. It is found that one of the reported structures is unstable to large relaxations of the hexagonal lattice constants. Calculations of a pseudodiamond structure and other B-chain derived structures indicate that this ambient pressure, stoichiometric lithium-monoboride system is indeed characterized by strongly double-bonded B chains. However, the structure is subject to very low-energy motions of Li atoms along the c axis, and also to relative displacements of B chains along the c direction. The B chains themselves are robust, with the boron bond-stretching mode at 138 meV. Although the deformation potential for this phonon mode is substantial, nevertheless the electron-phonon coupling is rather small and superconductivity can be expected at best only at low temperature. The lowest-energy geometry within this class of structures is identified, but due to only tiny differences in total energy, samples are likely to contain several types of structural imperfection as suggested by diffraction data.