First-principles study of structural, mechanical, lattice dynamical and thermal properties of nodal-line semimetals ZrXY (X=Si, Ge; Y=S, Se)

The nodal-line semimetals are new and very promising materials for technological applications. To understand their structural, mechanical, lattice dynamical and thermal properties in detail, we have investigated theoretical study of ZrXY (X = Si, Ge; Y = S, Se) using Density Functional Theory for the first time. Obtained lattice parameters are in excellent agreement with previous experimental data. These nodal-line semimetals obey the mechanical stability conditions for tetragonal structure. We obtain Bulk modulus, Shear modulus, Poisson's ratio, Pugh ratio, sound velocities and thermal conductivity using elastic constant. All the materials behave in brittle manner. Poisson's ratio data and Bader charge analysis results indicate that the ionic bonding characters are dominant. Next, the lattice dynamical properties are calculated. Phonon density of states shows that nodal-line semimetals ZrXY are also dynamically stable in the tetragonal structure. Raman and IR active phonon modes are determined. Highest optical mode at gamma point corresponds to A(2u) (IR active) and E-g (Raman active) modes for ZrXSe and ZrXS, respectively. Based on phonon density of states, thermal properties such as Helmholtz free energy, entropy, heat capacity at constant volume and Debye temperature are also presented and discussed.