BAND STRUCTURES OF CSCL-STRUCTURED BAS AND CASE AT HIGH-PRESSURE - IMPLICATIONS FOR METALLIZATION PRESSURES OF THE ALKALINE-EARTH CHALCOGENIDES

The indirect energy gaps of CsCl-structured BaS and CaSe are measured at high pressures using absorption spectroscopy to pressures of 41 and 53 Cpa, respectively. Band-structure calculations using density-funct ional theory and quasiparticle corrections are also conducted on BaS a nd CaSe as a function of pressure. The measured indirect gaps of BaS a nd CaSe shift linearly with pressure with a slope, (dE(gap)/dP), of - 1.8(2) x 10(-2) and - 1.5(3)x 10(-2) eV/GPa, respectively. These value s are in reasonable agreement with the theoretical results of - 2.2 x 10(-2) eV/GPa for BaS and - 2.3 x 10(-2) eV/GPa for CaSe. Our calculat ions indicate that the energy gap in both compounds is indirect with t he valence-band maxima at M and the conduction-band minima at Gamma. B ased;on a simple linear extrapolation of our data, we estimate the met allization pressures of CsCl-structured BaS and CaSe to be above 113 a nd 125 GPa, respectively. Our estimates of metallization pressures are compared with those of other alkaline earth monochalcogenides to dete rmine the degree to which changes in the electronic structures of thes e compounds under pressure can be described using systematic trends ba sed on ionic radii. [S0163-1829(98)03839-9].