Congruent vaporization properties as a tool for critical assessment of thermodynamic data: The case of gaseous molecules in the La–O and Y–O systems

In the first part of this contribution, definitions of congruent or azeotropic vaporizations are given as a function of experimental conditions. Then, we show how to calculate these vaporization states by using thermodynamic software. Experimental aspects of congruent vaporizations are analysed in relation to chemical interactions between the sample and the container that change the original chemical system of the sample into a system of higher order for the analysis of congruent behaviours. In experimental determinations, three points must be checked: the partial vapour pressure ratios must be constant for all temperatures; the chemical nature of the sample must be unchanged and controlled after the experiments; and thermodynamic data derived from the experiments must lead to the calculation of the same congruent vaporization conditions. The concept of congruent vaporization can be used to assess thermodynamic data. For example, Y2O3 is observed experimentally to vaporize at high temperature according to the main reaction Y2O3(s)→2YO(g)+O(g). From thermodynamic databases, the calculated main reaction is Y2O3(g)→YO(g)+YO2(g), and we conclude that the tabulated thermodynamic data for the species YO2 are erroneous. The same conclusion is reached for the La2O3 compound and the LaO2 gaseous molecule.