Thermochemistry of gaseous OSiI, OSiI2, SiI, and SiI2

Gaseous reaction equilibria in the Si-O-I system were investigated by mass spectrometry, leading to identification and thermochemical characterization of the species OSiI, OSil(2), Sil, and SiI2. Species were generated by the addition of gaseous I-2 to an effusion cell reactor containing a Si/SiO2 m ixture of composition "SiO". Equilibria involving the Si-O-I species were m easured in the range 1350 to 1540 K. From third law analysis, the enthalpie s of formation of OSLI, OSiI2, Sil, and SiI2 were derived as greater than o r equal to -21.7 +/- 5, greater than or equal to 74.5 +/- 5, 61.0 +/- 2, an d 21.6 +/- 3.3 kcal mol(-1), respectively. Although the values for SiI and SiI2 are in good agreement with selected values in a recent compilation, th e results for OSLI and OSiI2 are 25-30 kcal mol(-1) less stable than semiem pirical estimates developed for modeling of metal halide lamp chemistry. In some instances, other estimated thermochemical database values differ subs tantially from both theoretical and experimental results; reasons for these discrepancies are discussed.