The K-Sn (Potassium-Tin) System
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The K-Sn (Potassium-Tin) System J. Sangster* and C.W. Bale École Polytechnique de Montréal
Equilibrium Diagram The assessed K-Sn phase diagram is shown in Fig. 1. The special points of the diagram are listed in Table 1. Phase diagram data were obtained by thermal analysis [08Smi, 81Dri]. The earlier investigator [08Smi] reported considerable experimental difficulty, due primarily to the high volatility of K at the liquidus temperatures (boiling point of K = 774 °C). The data of the later investigators [81Dri] were only presented as points on a phase diagram, from which they were read. Although there are few experimental details given in the later work [81Dri], the assessed phase diagram has been drawn in accordance with these results [81Dri], assumed here to be more reliable. In particular, [81Dri] shows a rather flat liquidus in the range 20 to 40 at.% Sn, suggesting incipient liquid-liquid phase separation that is also observed in the Rb-Sn and Cs-Sn systems [81Dri]. Both reports [08Smi, 81Dri] indicate two eutectics in this system, the temperatures of which were indistinguishable from the melting points of the elements. There are no data on solid solubilities nor on the eutectic compositions; eutectic arrests were observed [08Smi] only as low as 5 and as high as 97 at.% Sn. In the absence of any specific information, it is reasonable to assume that there is zero solid solubility at both extremes and that the eutectic compositions are almost identical with the pure elements. The remaining features of the phase diagram are discussed with reference to intermetallic compounds. The possible existence of “K4Sn” was suggested [80Guk] on the basis of limited XRD data. If this compound were present in the phase diagram, it would probably melt incongruently somewhere in the interval 63 to 535 °C, but no peritectic arrests were observed [08Smi, 81Dri]. K2Sn was identified [08Smi, 81Dri] as the species corresponding to a peritectic at 535 °C. The peritectic composition is ap-
proximately 20 at.% Sn [08Smi, 81Dri]. Limited x-ray data [80Guk] as well as emf measurements [21Kre] corroborate the existence of this compound. KSn was prepared by direct reaction of the elements [64Hew, 81Dri]. It was characterized by its single-crystal XRD pattern [64Hew]. According to recent results [81Dri], KSn melts incongruently at 740 °C, peritectic composition about 49 at.% Sn. In the older work [08Smi] thermal arrests were observed at 670 °C, which were thought [08Smi] to correspond either to KSn peritectic melting or KSn solid transformation. This phenomenon was not reported in the later work [81Dri]. K2Sn3 was prepared from the elements [08Smi, 81Dri], although in the earlier work [08Smi] it was identified mainly as an alloy of 60 at.% Sn. Its congruent melting point is the highest temperature on the K-Sn liquidus, reported as 824 °C [08Smi], 830 °C [64Hew], or 860 °C [81Dri]. The compound is not mentioned, however, in the emf [21Kre] or x-ray [80Guk] investigations. The existence of KSn2 was inferred from the early thermal analysis resu
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