MOLAR ATOMIZATION ENTHALPIES AND MOLAR ENTHALPIES OF FORMATION OF GESI, GESI2, GE2SI, AND GE2SI2 BY KNUDSEN-EFFUSION MASS-SPECTROMETRY

The partial pressures of Ge(g), Si(g), Si-2(g), Ge-2(g), GeSi(g), GeSi 2(g), Ge2Si(g), and Ge2Si2(g) were measured over a (germanium + silico n + silicon nitride + silver) condensed-phase mixture in the temperatu re range 1583 K to 1869 K by Knudsen-effusion mass spectrometry. The t hermal functions for the GexSiy gaseous species were calculated, and t he atomization equilibria: GexSiy(g) = xGe(g) + ySi(g), were evaluated . The values of Delta(atom)H(m)(o)(T --> 0)/(kJ . mol(-1)), where Delt a(atom)H(m)(o) denotes the standard molar atomization enthalpy, are (2 92.7 +/- 8.6) for GeSi(g), (700.4 +/- 16.8) for GeSi2(g), (665.3 +/- 1 6.7) for Ge2Si(g), and (1109.9 +/- 24.6) for Ge2Si2(g). These values, in combination with the standard molar enthalpies of formation Delta(r )H(m)(o) of Ge(g) and Si(g), yielded the values Delta(r)H(m)(o)(T --> 0)/(kJ . mol(-1)):(530.3 +/- 9.8) for GeSi(g), (573.9 +/- 18.9) for Ge Si2(g), (529.4 +/- 17.7) for Ge2Si(g), and (536.1 +/- 26.3) for Ge2Si2 (g). (C) 1995 Academic Press Limited