Lattice dynamics, electronic structure, and optical properties of LiBeSb: A hexagonal ABC-type hyperferroelectrics

The recently discovered hexagonal ABC-type hyperferroelectrics, in which the polarization persists in the presence of the depolarization filed, exhibit a variety of intriguing and potentially useful properties [Garrity et al., Phys. Rev. Lett. 112, 127601 (2014)]. For the existing prototype of LiBeSb, we present detailed first-principles calculations concerning the lattice dynamics, electronic structure, and optical properties. An unstable longitudinal optic mode in the high-symmetry structure and a large polarization of 0.5 C/m2 in the polar phase are reported, including the remarkable dependence of Born effective charges on structural distortion. Using the HSE06 hybrid functional, we predict that LiBeSb has a small band-gap of 1.5 eV and shows dominant asymmetric covalent bonding character. Importantly, we find that there are remarkable absorptions in the whole visible spectrum. These features, combined with the enhanced carrier mobility, make LiBeSb as well as the whole family of hexagonal ABC-type hyp...