Silicene-terminated surface of calcium and strontium disilicides: properties and comparison with bulk structures by computational methods

The bulk and surface structures of calcium and strontium disilicides are investigated by computational methods using density functional theory. The investigated structures are R6, R3 and P1-CaSi₂ and P1-SrSi₂. The investigated properties are the cleavage energy at the silicene sheet, buckling of the bulk and surface silicene layers, charge transfer from calcium to silicon, band structure of bulk and surface-terminated structures and adsorption energies on H atoms and H₂ molecules on the silicene-terminated surface of the R3 phase. The cleavage energy at the silicene surface is low in all cases. Structures P1-CaSi₂ and R3-CaSi₂ contain silicene sheets with different coordination to Ca, while R6-CaSi₂ contains both types of the sheets. It is shown that the properties of the two types of silicene-like sheets in R6-CaSi₂ are similar to those of the corresponding sheets in P1-CaSi₂ and R3-CaSi₂, and the thermodynamically stable R6 phase is a good candidate for experimental investigation of silicene-terminated surface in calcium disilicide.