High temperature thermodynamic investigation of the iridium-rich portion of the titanium-iridium system by Knudsen cell mass spectrometry and predicted enthalpies of formation for some transition metal-iridium intermetallic compounds

Abstract

The vapor pressures of titanium over solid titanium-iridium alloys ( Ti = 0.5, 0.4 and 0.25) were determined in the temperature range 1850–2100 K by means of the Knudsen cell mass spectrometric method. The activity of titanium (standard state- solid Ti) was determined as 3.32 × 10 −4 ( N Ti = 0.5), 1.67 × 10 −4 ( N Ti = 0.4) and 1.76 × 10 −5 ( N Ti = 0.25). The activity of iridium (standard state- solid Ir) for the composition N Ti = 0.25 was estimated to be 0.35 by analogy with that of Rh in the Ti-Rh system of the same composition. From the Gibbs-Duhem treatment of the titanium activities, the activity of iridium was derived as 0.187 ( N Ti = 0.4) and 0.135 ( N Ti = 0.5). The derived, highly-negative Gibbs energies of formation of Tilr (−19.9 ± 1.5 kcal/ g- at.) and TiIr 3 (−14.0 ± 1.0 kcal/ g- at.) attest to the predominant role played by electron-electron interactions in determining the stability, which is consistent with the Engel-Brewer concepts of alloying behavior in transition metals. In the light of the present experimental stability values for TiIr and TiIr 3, the literature estimates for the enthalpies of formation of other transition metal-iridium compounds have been revised and, when necessary, new values have been predicted from the knowledge of stability trends in M-Pt systems.