Thermochemistry of ytterbium silicides S. Brutti a, *, G. Balducci a , A. Ciccioli a , G. Gigli a , P. Manfrinetti b , A. Palenzona b a Dipartimento di Chimica, Universita ` di Roma La Sapienza, Piazzale A. Moro 5, I-00185 Rome, Italy b INFM, Dipartimento di Chimica e Chimica Industriale, Universita ` di Genova,Via Dodecaneso 31, I-16146 Genoa, Italy Abstract 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 for- mation of all the six known Si–Yb intermediate phases. The following values  f H  298 are recommended: Si 3 Yb 5 =48.3  3.6, Si 4 Yb 5 =53.2  4.6, SiYb=51.1  5.1, Si 4 Yb 3 =48.0  3.1, Si 5 Yb 3 =41.3  2.6, Si 1.74 Yb=37.4  0.9, all in kJ/mol atoms. # 2003 Elsevier Ltd. All rights reserved. Keywords: A. Silicides, various; Rare-earth intermetallics; B. Thermodynamic and thermochemical properties 1. Introduction Transition metals and Rare-Earth (RE) metal silicides have attracted interest for their outstanding physical and chemical properties, that make some of them can- didates for technological applications e.g. as high tem- perature structural materials and contact materials in microelectronics. This interest is even higher for the Si– Yb intermediate phases where Yb mixed valence fluc- tuations can be observed between bulk silicides and their surfaces [1–4]. The modelling of solid-state silicon- metal interaction, interface formation, etc. would be aided by the knowledge of phase transformations and thermodynamic properties of the intermediate phases. The enthalpies of formation of several binary transi- tion and rare-earth metal silicides have been measured calorimetrically (see refs. [5,6]) but in general both the phase diagram and the thermodynamic information for RE-Si binary systems are still far from satisfactory. Regarding the Si–Yb system in particular, until recently several doubts on the number and identity of the inter- mediate phases still remained. Moreover experimental or computed phase diagram and thermodynamic prop- erties were completely missing. Very recently some works were devoted to the investigation of the phase diagram and the temperatures of the invariant equilibria in the whole composition range [7,8]. Two new phases (Si 4 Yb 5 and Si 4 Yb 3 ) have been identified and their magnetic and thermal low-temperature properties have been investigated [9]. As reported in Ref. [8] the exis- tence of a further compound SiYb 2 could not be defi- nitely proven. The present paper reports for the first time the hitherto missing thermochemical data for the formation of the hitherto known phases in the Si-Yb system, namely Si 3 Yb 5 , Si 4 Yb 5 , SiYb, Si 4 Yb 3 , Si 5 Yb 3 and Si 1.74 Yb. The thermodynamic properties of these phases were investigated by means of an equilibrium method based on vapour pressure measurements. Both Knudsen Effusion–Mass Spectrometry (KE–MS) and Knudsen Effusion–Weight Loss (KE–WL) techniques were used in order to measure the equilibrium vapour pressure of the effusing species in equilibrium over the various high temperature and low temperature biphasic regions in a wide temperature range. Only monatomic Yb is formed in the gas phase during the various decomposition reactions. From the temperature dependence of the measured p Yb the reaction enthalpies at room tempera- ture for the various processes were derived. From this set of experimental data we derived, for the first time, 0966-9795/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0966-9795(03)00152-3 Intermetallics 11 (2003) 1153–1159 www.elsevier.com/locate/intermet * Corresponding author. Fax: +39-064-991-3951. E-mail address: sergio.brutti@uniroma1.it (S. Brutti).