Structural, electronic and magnetic properties of the (001), (110) and (111) surfaces of rocksalt sodium sulfide: A first-principles study

First principles study of the structural, electronic and magnetic properties of the (111), (110) and (001) surfaces of rocksalt sodium sulfide (rs-NaS) are reported. The results show that the bulk half-metallicity of this compound is well preserved on the surfaces considered here except for Na-terminated (111) surface. The spin-flip gap at the S-terminated (111), (001) and (110) surfaces are close to the bulk value. Using ab-initio atomistic thermodynamics, we calculate the surface energies as a function of chemical potential to find the most stable surface. We find that the Na-terminated (111) surface is the most stable one over the whole allowed range of chemical potential, while the surface energies of the (001) and (110) surfaces approach the most stable surface energy at the sulfur rich environment. We have also calculated the interlayer exchange interaction in bulk and Na-terminated (111) surface by classical Heisenberg model and we found that the surface effects do not change these kinds of interactions significantly.