Topological phonons and low lattice thermal conductivity of Li2CaX (X = Sn and Pb) type Heusler compounds

The topological phonons, electronic structure, and transport properties of Li2CaX (X = Sn and Pb) have been investigated using first-principles. Topological phononic states are facilitated by Dirac-like band crossings observed in optical phonon branches, which are ensured by calculated surface states in the (001) plane. Li2CaPb has very low lattice thermal conductivity(κl), as evidenced by shorter phonon lifetime, acoustic phonon mode dispersion, and low Debye temperature. The value of κl is low at high temperatures due to Dirac-like crossings in optical phonon branches exhibiting non-trivial topological properties. The inclusion of hybrid functional transits Li2CaPb into an indirect bandgap semiconductor. Li2CaPb has a figure of merit of 0.15 for holes and 0.2 for electrons at 500 K. Topological phonons combined with promising transport features in Li2CaX (X = Sn and Pb) may be desirable for future device applications.