HIGH-PRESSURE PROPERTIES OF THE ALKALINE-EARTH SULFIDES

The alkaline-earth sulphides MgS, CaS, SrS, and BaS naturally occur in the rocksalt (B1) structure. Under pressure they undergo a structural phase transformation to the cesiumchloride (B2) structure. Ab-initio local density nonlocal pseudopotential calculations of the total energ y of the first three compounds are performed in the B1, B2, and in the zincblende (B3) structure. It is found that the B1 structure is stabl e at normal conditions while the B3 structure is unstable under all co nditions. The B1 to B2 phase transformation occurs at, respectively, 2 30.7, 49.8, and 20.9 GPa. Recent experimental estimates of these trans ition pressures are 44 and 37.1 GPa for CaS. The calculated lattice co nstants, bulk moduli, and their pressure derivatives agree well with t he experimental values.