Фазовая диаграмма системы Ag-Se
К оглавлению: Другие диаграммы (Others phase diargams)
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
