Фазовая диаграмма системы Ag-Rh

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Ag-Rh (Silver-Rhodium) I. Karakaya and W.T. Thompson Because of extremely limited data, the assessed phase diagram for the Ag-Rh system is an approximation. The phase boundaries have been established mainly on the basis of a simple thermodynamic model that makes the most of the limited experimental data. The condensed equilibrium phases are believed to be the liquid with a miscibility gap, an fcc Ag-rich phase with extremely small Rh solubility [00Ros, 33Dri], and an fcc Rh-rich solid solution with relatively large Ag solubility. The maximum solubility of Ag in solid (Rh) is reported to be higher than 10 at.% [59Rud]. In the thermodynamic modeling of the solid and liquid phases, the value for Gibbs energies of transformation for Ag are the same as those used in previous evaluations [86Kar]. The corresponding data for Rh were obtained by using DCp values from [77Bar] together with the accepted melting point [Melt] and boiling point [75CRC]. Because a large miscibility gap is reported for the liquid phase, the effect of excess entropy on the excess Gibbs energy may be assumed to be small with respect to a large positive enthalpy of mixing for this phase. A subregular solution model for the liquid is therefore considered to be appropriate. A eutectic reaction was reported at about 960 C and less than 1 at.% Rh [ 59Rud]. This conclusion was based on a very small freezing point depression and constant freezing point measurements from a differential thermal analysis study for Ag-rich alloys [59Rud]. In a study of the Rh-rich liquidus, a monotectic reaction was reported at about 1900 C and near 20 at.% Ag [59Rud]. The accuracy of these observations is questionable, because the melting point of pure Rh differed from the presently accepted value by about 25 C [Melt]. Rh was reported to be insoluble in liquid Ag [00Ros]. Exceedingly small solubility of Rh in solid (Ag) was concluded by [33Dri]; this study also indicated the absence of any intermediate phases. Data of Ag-rich alloys confirmed Rh as nearly insoluble in (Ag) up to 800 C [59Rud]. 00Ros: H. Rossler, Chem. Ztg., 2, 733-735 (1900) in German. 33Dri: R.W. Drier and H.L. Walker, Philos. Mag., 16, 294-298 (1933). 59Rud: A.A. Rudnitskii and A.N. Khotinskaya, Russ. J. Inorg. Chem., 4, 1053- 1056 (1959). 77CRC: CRC Handbook of Chemistry and Physics, 56th ed., R.C. Weast, Ed., B31, CRC Press Inc., Cleveland, OH (1975). 77Bar: I. Barin, O. Knacke, and O. Kubaschewski, Thermochemical Properties of Inorganic Substances (Supplement), 590, Springer-Verlag, Berlin (1977). 86Kar: I. Karakaya and W.T. Thompson, Bull. Alloy Phase Diagrams, 7(3), 259- 263 (1986). Published in Bull. Alloy Phase Diagrams, 7(4), Aug 1986. Complete evaluation contains 3 figures, 2 tables, and 6 references. 1