Energetic evaluation of Ba- and Sr-Si clathrate formation at high pressures by first-principle pseudopotential calculations

Energetic evaluations of BaSi(2), Ba(8)Si(46), BaSi(6), SrSi(2), Sr(8)Si(46), and SrSi(6) at 0 to 15 GPa (or 20 GPa) have been performed using first-principle calculations based on the density-functional theory. From the calculations. Ba(8)Si(46) is expected to be present as an equilibrium phases at 10 GPa and 15 GPa, while BaSi(6) is considered to be formed at 15 GPa and 20 GPa. SrSi(6) is expected to be present at 10 GPa and 15 GPa. Sr(8)Si(46) would not be present at any pressure from 0 to 15 GPa, since SrSi(6) + SrSi(2) is more stable at pressures of 10 GPa and 15 GPa than Sr(8)Si(46), while SrSi(2) + Si is more stable at 0 GPa and 5 GPa. This is consistent with the observed absence of Sr(8)Si(46) in contrast to the presence of Ba(8)Si(46). In addition, the phase transformation of disilicides by pressurization could almost be properly predicted, except for the observed higher stability of the cubic phase of BaSi(2) compared to its trigonal phase at lower temperatures and that of the cubic phase of SrSi(2) compared to its tetragonal phase at ambient pressure. (C) 2009 Elsevier Ltd. All rights reserved.