Thermodynamic properties of barium silicides from vapor pressure measurements and density functional calculations

The high temperature decomposition of the intermediate phases of the Ba-Si system (namely, Ba2Si, Ba5Si3, BaSi, Ba3Si4 and BaSi2) was investigated by means of Knudsen effusion-mass spectrometry and Knudsen effusion-weight loss techniques. All barium silicides decompose at high temperature by losing monatomic gaseous barium. The standard enthalpy changes of the decomposition reactions were derived from vapor pressure measurements by the second- and third-law methods of analysis. From these values the heats of formation of all the barium silicides were calculated. First principles electronic structure calculations for the same compounds were also performed by the density functional theory approach, in the generalized gradient approximation, using plane wave basis functions and Vanderbilt ultrasoft pseudopotentials. The experimental ground state lattices of the barium silicides have been studied: structures have been relaxed and total energy values derived. These were used in combination with calculated total energies of diamond-silicon and fcc-barium to derive the heats of formation of the barium silicides. First principles predictions and experimental heats of formation are in satisfactory agreement. The selected values of the heats of formation of the intermediate phases of the Ba-Si system are (kJ mol-at.−1): Ba2Si, −35.8 ± 7.0; Ba5Si3, −42.4 ± 5.2; BaSi, −50.1 ± 2.9; Ba3Si4, −50.7 ± 3.1; and BaSi2, −46.7 ± 3.5.