A Knudsen effusion mass spectrometric study of the molecule Si3N

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Abstract
Condensed phase mixtures (Ge+Si+Si3N4) and (Si+Si3N4) were evaporated from boron nitride Knudsen cells and the partial pressures of Si3N(g),Si2NSi2(g), and Si(g) measured with a mass spectrometer at temperatures between 1770 and 2000 K. The thermal functions for Si3N(g) were calculated from theoretical molecular constants, and the enthalpy change for the dissociation reaction: Si3N(g)=Si2N(g)+Si(g) determined. With the revised atomization enthalpy of Si2N(g), ΔatomHmo=1011±12 kJ mol−1 at T=0 K and 1020±12 kJ mol−1 at T=298.15 K, the atomization enthalpy of Si3N(g) was derived as 1298±19 kJ mol−1 at T=0 and 1312±19 kJ mol−1 at T=298.15 K. These values in combination with the enthalpies of formation of Si(g) and N(g) yielded the enthalpies of formation ΔfHmo at T=298.15 K: 352±15 kJ mol−1 for Si2N(g), and 511±22 kJ mol−1 for Si3N(g). Experimental and theoretical bond dissociation energies have been compared and discussed, indicating a very strong bonding of nitrogen to Si3.