Фазовая диаграмма системы Ga-Rb
К оглавлению: Другие диаграммы (Others phase diargams)
Ga-Rb (Gallium-Rubidium)
A.D. Pelton and S. Larose
Information on the Ga-Rb system is not well established, apparently because of
problems of oxidation and volatilization of Rb. The assessed phase diagram is
tentative, and is based on the experimental data of [70Thu], [73Yat], and [
77Yat]. Although Ga and Rb have very low melting points, the phase diagram is
dominated by compounds with much higher melting points, complex crystal
structures, and a large degree of ionic or covalent bonding.
The liquidus curves of the two studies in the range 0 to 25 at.% Rb differ by
up to 75 C. In view of their more careful experimental technique, the results
of [73Yat, 77Yat] are given preference in the assessed diagram.
The solubility of Ga in liquid Na at 500 C is about 3 at.%, whereas in liquid
K at 500 C Ga is soluble to only 0.01 at.%. It is expected that the
solubility in liquid Rb will be even less. Hence, in the assessed diagram, a
solubility of much less than 1% at 620 C is indicated. At the melting point
of Rb, the solubility of Ga must be immeasurably small, such that the
temperature of the eutectic reaction at 39.48 C is indistinguishable from the
melting point of pure Rb.
The solubilities of Na and K in liquid Ga at 400 C are 2.5 and 0.3 at.%,
respectively. The solubility of Rb is expected to be even less. At the melting
point of Ga, the solubility should be less than 1 ppm, so that the eutectic
temperature is identical to the melting point of pure Ga.
The peritectic melting temperature of Ga7Rb, 354 C, has been clearly
established by [73Yat]. First proposed as Ga5Rb by [73Yat], the compound has
been shown by [81Bel] and [85Van] to have the stoichiometry Ga7Rb.
Ga3Rb was identified by [81Lin]. Ga3K and Ga3Cs, with identical crystal
structures, exist in the Ga-K and Ga-Cs systems. [70Thu] showed what might be
an invariant reaction near 520 C, although [73Yat] observed no thermal
effects here. In the assessed diagram, Ga3Rb is tentatively shown as melting
peritectically at this temperature. It is also possible that Ga3Rb may
decompose at lower temperatures. [81Bel] reported the compound to be "very
unstable and oxidizable." [85Van] showed that both Ga3K and Ga3Cs have
measurable stoichiometry ranges. By analogy, Ga3Rb probably also exists over a
composition range.
There is much doubt as to the existence of Ga8Rb5 and even more doubt as to
its stoichiometry. Although [73Yat] reported identification of "Ga2Rb,"
similar studies by the same authors on "Ga2K" have been shown to exhibit
diffractograms very similar to those of Ga3K. Probably there was loss of
alkali by oxidation during sample preparation. In a later study, these same
authors [82Chu] concluded from their attempts at crystal growth that Ga3Rb is
the most Rb-rich compound in the system. [81Lin] reported that they isolated
only Ga7Rb and Ga3Rb.
The Rb-rich limit of the miscibility gap in all probability lies above 99 at.%
Rb. The Ga-rich limit is less well established, but appears to lie in the
range 20 to 35 at.% Rb.
The invariant reaction observed at 620 C by [73Yat] is tentatively associated
with the syntectic melting of Ga2Rb in the assessed diagram. However, if
Ga8Rb5 does not exist, then the invariant reaction at 620 C is most probably
a syntectic or monotectic reaction associated with the melting of Ga3Rb, and
the invariant reaction at 520 C does not exist.
In their diagram, [73Yat, 77Yat] show a consolute temperature of about 675 C
for the miscibility gap. That a gap extending to >99 at.% Rb at 620 C should
disappear at a temperature only 55 C higher is extremely unlikely.
70Thu: R. ThЃmmel and W. Klemm, Z. Anorg. Allg. Chem., 376, 44-63 (1970) in
German.
73Yat: S.P. Yatsenko, K.A. Chuntonov, and S.I. Alyamovskiy, Russ. Metall., 3,
196-198 (1973).
77Yat: S.P. Yatsenko, J. Chim. Phys., 74, 836-843 (1977).
81Bel: C. Belin, Acta Crystallogr., B37, 2060-2062 (1981).
81Lin: R.G. Ling and C. Belin, Z. Anorg. Allg. Chem., 480, 181-185 (1981).
82Chu: K.A. Chuntonov, L.Z. Melekhov, A.N. Kuznetsov, A.N. Orlov, G.G.
Ugodnikov, and S.P. Yatsenko, J. Less-Common Met., 83, 143-153 (1982).
85Van: J.H.N. Van Vucht, J. Less-Common Met., 108, 163-175 (1985).
Published in Bull. Alloy Phase Diagrams, 11(4), Aug 1990. Complete evaluation
contains 1 figure, 2 tables, and 7 references.
1
