Фазовая диаграмма системы Ag-Se

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Ag-Se (Silver-Selenium) I. Karakaya and W.T. Thompson The assessed Ag-Se phase diagram is essentially the same as that of [Hansen]; the approximate position of the two-phase liquid-vapor region has been added. The main features of the diagram are based on the thermal analysis and microscopic investigations of [08Fri] and [15Pel]. A small portion of L3/L3 + g phase boundary (Se-rich side) in the assessed diagram was calculated from thermodynamic properties of Se evaporation by assuming an ideal (Raoultian) behavior both in liquid and vapor phases. L2/L2 + g phase boundary for concentrations of Se near 40 at.% was approximately located using the data of [78Bla] for the liquid. The vapor phase was assumed to be composed of equilibrium concentrations of Se, Se2, Se3, Se4, Se5, Se6, Se7, and Se8, with Se2 being the most abundant over the temperature range covered. The condensed phases are (1) the liquid (L), possibly with two miscibility gaps; (2) fcc (Ag); (3) cph (Se); and (4) Ag2Se with two polymorphic modifications (a and b). The low-temperature crystal structure of Ag2Se is orthorhombic; the high-temperature structure is bcc. Coexisting solid phases are essentially mutually insoluble. The portion of the diagram between Ag2Se and Se is based on [15Pel], which shows a monotectic with a likely miscibility gap. The miscibility gap between 45 and 95 at.% Se and the monotectic temperature of 616 C has been recently confirmed by [84Hou]. The solubility of Se in solid (Ag) is very limited. [08Fri] reported that an alloy with 0.27 at.% Se was heterogeneous after cooling from the melt. There are no reported measurements of Ag solubility in (Se). The existence of the compound Ag2Se and its polymorphic a/b transformation have long been known [1890Bel]. The melting point of bAg2Se was reported as 897 C from thermal analysis [15Pel]. However, there is uncertainty in the temperature for the a/b polymorphic transformation. The a/b transformation of a thin film was reported to be as high as 166 C on heating and 107 C on cooling [69Sha]. The transformation temperature was reported to increase to 298 C at a pressure of 47 kbar [65Ban]. The transformation of aAg2Se to a complex tetragonal structure at 65 C has been proposed [55Boe]. This however, is not fully substantiated. In related research, [60Con] claims the existence of Ag16Se7 based upon precipitation from aqueous solutions. A metastable fcc phase with lattice parameter 0.565 nm was reported [76Con] for Ag2Se from electron diffraction studies of thin films above 220 C. Appearance of a tetragonal metastable intermediate phase was observed for this compound during crystallization at room temperature [72Nur]. All of the equilibrium studies indicate the presence of no phase other than Ag2Se in equilibrium with either (Ag) or (Se). 1890Bel: M. Bellati and S. Lussana, Z. Phys. Chem. (Leipzig), 5, 282 (1890) in German. 08Fri: K. Friedrich and A. Leroux, Metallurgie, 5, 355-358 (1908) in German. 15Pel: G. Pellini, Gazz. Chim. Ital., 45, 533-539 (1915) in Italian. 55Boe: A. Boettcher, G. Haase, and H. Treupel, Z. Angew. Phys., 7, 478-487 ( 1955) in German. 60Con: J.B. Conn and R.C. Taylor, J. Electrochem. Soc., 107, 977-982 (1960). 65Ban: M.D. Banus, Science, 147, 732-733 (1965). 69Sha: S.K. Sharma, J. Mater. Sci., 4, 189-194 (1969). 72Nur: I.R. Nuriev and R.B. Safizade, Izv. Akad. Nauk Azerb. SSSR, Ser. Fiz. Tehn., 2, 13-17 (1972). 76Con: L.V. Constantinescu, Rev. Roum. Phys. Tome., 21, 595-600 (1976) in French. 78Bla: R. Blachnik and G. Bolte, J. Less-Common Met., 57, 21-28 (1978) in German. 84Hou: D. Houphouet-Boigny, R. Eholie, R. Ollitrault-Fichet, and J. Flahaut, J. Less-Common Met., 98, 11-31 (1984) in French. Published in Bull. Alloy Phase Diagrams, 11(3), Jun 1990. Complete evaluation contains 3 figures, 4 tables, and 42 references. Special Points of the Ag-Se System