Vapor pressures and thermodynamic properties of strontium silicides

The high temperature vaporization processes of strontium silicides were studied by means of the Knudsen Effusion Mass Spectrometry and Knudsen Effusion Weight Loss techniques in the temperature range 665–1300 K, with reference to the recent reinvestigation of the Sr–Si phase diagram [Palenzona A, Pani M. J Alloys Compd 2004;373:214.]. The only species detected in the vapor phase equilibrated with two-phase solid mixtures was monoatomic gaseous strontium. The vapor pressure of Sr(g) was measured as a function of the temperature, and the enthalpy changes associated with the decomposition processes were thereafter derived. The enthalpies of formation of the strontium silicide phases reported in Ref. [Palenzona A, Pani M. J Alloys Compd 2004;373:214.] (namely Sr2Si, Sr5Si3, SrSi and b-SrSi2) were finally obtained as, respectively (values in kJ/mol atoms, T=298 K): -39.7+/-3.2; -43.8+/-3.6; -51.7+/-4.1; -40.3+/-3.7. These results are definitely at variance with the scattered, strongly exothermic calorimetric data reported for some phases in the old literature, while they are in satisfactory agreement with recent density functional theory calculations.