Фазовая диаграмма системы Si-Sr

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Si-Sr (Silicon-Strontium) V.P. Itkin and C.B. Alcock The assessed Si-Sr phase diagram is based primarily on review of the work of [ 65Obi]. The Si primary crystals extracted from the Si-rich alloys contained about 0.2 at.% Sr, and [65Obi] suggested that this value represents solubility at 1000 C. However, this suggestion might be in error, because small portions of the eutectic can be trapped with the primary crystals. The work of [65Obi] should not be regarded as a highly accurate investigation. The number of compositions studied was small, the raw materials and alloys contained a substantial amount of impurities, the accuracy of the thermal analysis was probably not better than 10 C, the experimental points were connected without consideration of curvature at phase boundaries, and some segments of the liquidus clearly have the wrong shape. Nevertheless, [65Obi] is the only available complete study of the Si-Sr system, and the liquidus in the assessed diagram was drawn mostly through or near the experimental points. Corrections to the diagram of [65Obi] include the positions of the (Si) + Si2Sr eutectic (5 at.% to the left) and melting point of Si2Sr (50 C down) and the addition of SiSr2. SiSr2 is possibly formed by a peritectic reaction at about 1010 C. The corresponding phase boundaries are shown approximately by the dashed lines in the assessed diagram. The reaction due to the a = b transformation in Sr at 547 C is also shown by a dashed line. It is assumed here that solubilities of Sr in (Si) and Si in (aSr) and (bSr) are very small. A number of the intermetallic phases, including aSiSr, Si2Sr(II), Si3Sr2, Si7Sr4, and Si3Sr5 have been reported. However, none of these phases were included in the assessed diagram, because the conditions of their formation and their stabilities at different temperatures have not been sufficiently specified. [83Eve] investigated the behavior of Si2Sr equilibrated at 1 to 4 GPa and 600 to 1200 C. The compound transforms into a metastable modification, Si2Sr(II), with the aThSi2-type tetragonal structure. The transition temperature decreases linearly from 1200 C at 1 GPa to 600 C at 4 GPa [79Eve]. The metastable modification can be quenched and kept at room temperature indefinitely. On cooling to 3.1 с 0.3 K, it becomes a superconductor [80Eve]. On heating, the metastable modification transforms irreversibly into the stable cubic modification. The transition starts noticeably above 450 C, and it comes to completion very rapidly at 600 to 650 C [78Eve]. 65Obi: I. Obinata, Y. Takeuchi, K. Kurihara, and M. Watanabe, Metall, 19, 21- 35 (1965) in German. 67Jan: K.H. Janzon, H. Sch„fer, and A. Weiss, Z. Naturforsch. B, 22, 100-101 ( 1967) in German. 67Kri: P.I. Kripyakevich and E.I. Gladyshevskii, Sov. Phys. Crystallogr., 11, 693-695 (1967). 67Nag: G. Nagorsen, G. Rockt„schel, H. Sch„fer, and A. Weiss, Z. Naturforsch. B, 22, 101-102 (1967) in German. 67Rie: W. Rieger and E. Parthe, Acta Crystallogr., 22, 919-922 (1967). 74Eis: B. Eisenmann, H. Sch„fer, and K. Turban, Z. Naturforsch. B, 29, 464-468 (1974). 76Wid: A. Widera, B. Eisenmann, and H. Sch„fer, Z. Naturforsch. B, 31, 520-521 (1976) in German. 78Eve: J. Evers, J. Solid State Chem., 24, 199-207 (1978). 79Eve: J. Evers, J. Phys. Chem. Solids, 40, 951-954 (1979). 80Eve: J. Evers, G. ™ehlinger, and H.R. Ott, J. Less-Common Met., 69, 389-391 ( 1980). 83Eve: J. Evers, G. ™ehlinger, and A. Weiss, Z. Naturforsch. B, 38, 899-900 ( 1983) in German. Published in Bull. Alloy Phase Diagrams, 10(6), Dec 1989. Complete evaluation contains 1 figure, 4 tables, and 21 references. Special Points of the Si-Sr System