First-principles prediction of half-metallicity at the low index surfaces of rocksalt KS

The structural, electronic and magnetic properties of the low index (111), (110) and (001) surfaces of the rocksalt Potassium Sulfide (rs-KS) are investigated by using pseudo-potential calculations in the framework of spin density functional theory. The results show that the bulk half-metallicity of this compound is well preserved on its low index surfaces. The spin flip gap at the studied surfaces is close to the bulk value except for the potassium terminated (111) surface which exhibits about half of the bulk spin flip gap. The calculated surface energies in the framework of ab-initio atomistic thermodynamics indicate that the potassium terminated (111) surface of KS has the lowest energy over entire allowed range of the chemical potentials, while the surface energy of the (001) and (110) surfaces approaches the surface energy of the stable surface at the sulfur rich environment. In the framework of the Heisenberg model, it is argued that the surface effects have small effects on the interlayer exchange interaction in the stable K-terminated (111) surface, compared with the bulk KS.