Thermochemistry of ytterbium silicides

The results of the investigation of the high temperature decomposition reactions in vacuum under equilibrium conditions of ytterbium silicides in the whole composition range are reported. By means of the Knudsen Effusion–Mass Spectrometry (KE–MS) and the Knudsen Effusion–Weight Loss (KE–WL) techniques, the Yb(g) vapour pressures in equilibrium over the various high temperature and low temperature biphasic regions were measured in the temperature range 781–1395 K and the reaction enthalpies for the respective decompositions were derived. From this set of experimental data we derived for the first time the heats of formation of all the six known Si–Yb intermediate phases. The following values ΔfH°298 are recommended: Si3Yb5=−48.3±3.6, Si4Yb5=−53.2±4.6, SiYb=−51.1±5.1, Si4Yb3=−48.0±3.1, Si5Yb3=−41.3±2.6, Si1.74Yb=−37.4±0.9, all in kJ/mol atoms.