Фазовая диаграмма системы Ga-K

К оглавлению: Другие диаграммы (Others phase diargams)


Ga-K (Gallium-Potassium) A.D. Pelton and S. Larose Although the elements Ga and K have very low melting points, the assessed phase diagram for the Ga-K system is dominated by stable compounds with much higher melting points, complex crystal structures, and a large degree of ionic or covalent bonding. The diagram is based mainly on [61Fes], with review of the data of [73Yat]. The existence of the compounds Ga13K3 and Ga3K is well established. Ga13K3 decomposes peritectically at 510 с 10 C. Ga3K melts, probably peritectically, near 595 C. There is a region of liquid immiscibility above about 620 C, the K-rich boundary of which lies above 99 at.% K. The solubilities of K in liquid Ga and of Ga in liquid K at their melting points are on the order of 1 ppm. Even at 400 C, the solubilities are much less than 1 at.%. Apart from these facts, the phase diagram is not well established, apparently because of problems of oxidation and volatilization of K. The Ga-rich limit of the miscibility gap is not well known. Reported liquidus temperatures may be in serious error. Thus, the assessed diagram is only tentative. Because of the very low mutual solubilities of K and Ga, the eutectics at 29.7 and 63.7 C are indistinguishable from the melting points of the elements. [73Yat] observed an invariant reaction occurring near 595 C. In the assessed diagram, this has been tentatively associated with the peritectic melting of Ga3K. Ga8K5 was identified as Ga8K5 by [61Fes] and [61Rin] and as Ga2K by [73Yat], although it is most likely that [73Yat] was really examining Ga3K. The error could have been due to oxidation or volatilization of K during sample preparation. Enthalpy of formation data [73Yat] from quantitative differential thermal analysis confirm the approximate stoichiometry Ga2K. However, the same problems of K loss could have occurred in these measurements as well. [82Chu] concluded that the compound richest in K in this system is Ga3K, and [82Bel] reported that Ga13K3 and Ga3K are the only compounds in the system. Thus, it is quite possible that Ga8K5 does not exist. However, there is a stable compound of very similar stoichiometry in the Ga-Na system, and there is also some evidence for similar compounds in the Ga-Rb and Ga-Cs systems. Hence, in the assessed diagram, a compound with approximate stoichiometry Ga8K5is indicated tentatively. An invariant reaction near 620 C associated with the syntectic melting of Ga8K5 is suggested in the assessed diagram. A plateau at 595 C at higher K contents observed by [73Yat] could result from nonequilibrium cooling. The position of the Ga-rich boundary of the miscibility gap cannot be fixed with certainty. However, enthalpy of mixing data for the liquid alloys suggest that it may occur near 38 at.% K. 61Fes: P. Feschotte, Ann. Chim. (Paris), 6, 1029-1070 (1961) in French. 61Rin: E. Rinck and P. Feschotte, Compt. Rend., 252, 3592-3594 (1961) in French. 73Yat: S.P. Yatsenko and K.A. Chuntonov, Russ. Metall., 6, 109-112 (1973). 80Bel: C. Belin, Acta Crystallogr., B36, 1339-1343 (1980). 82Bel: C. Belin and R.G. Ling, Compt. Rend., 294, 1083-1086 (1982). 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). 83Yat: S.P. Yatsenko, E.I. Gladyshevskii, K.A. Chuntonov, Ya.P. Yarmolyuk, and Yu.N. Grin', Sov. Phys. Crystallogr., 28, 479-480 (1983). 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 2 figures, 2 tables, and 13 references. 1