Фазовая диаграмма системы Cr-Te

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Cr-Te (Chromium-Tellurium) M. Venkatraman and J.P. Neumann The assessed Cr-Te phase diagram is taken from [83Ips], who investigated the system in the composition range 30 to 100 at.% Te. Five intermediate phases with a NiAs-type crystal structure occur in this system: hexagonal Cr1-xTe, monoclinic Cr3Te4, monoclinic Cr5Te8-I, hexa-gonal Cr2Te3, and hexagonal Cr3Te8-II. No data are available in the composition range 0 to 30 at.% Te. The eutectic shown at the Cr-rich end of the assessed diagram is only speculative. By analogy with the Cr-S and Cr-Se systems, the uppermost invariant temperature is the monotectic equilibrium temperature as suggested by [83Ips]. The invariant temperature at 1181 C was attributed to a eutectic between Cr and Cr1-xTe by [83Ips]. The region between 50 and 62 at.% Te is characterized by the occurrence of several intermediate phases. It has been found that stoichiometric CrTe does not exist. However, with less Cr, metal vacancies are formed and several superstructures derivable from the NiAs-type crystal structure appear. Some confusion exists in the literature regarding the formulas of these phases, because different authors have designated the same phase by different formulas. In the present evaluation, the formulas given by [83Ips] for the intermediate phases are adopted. The homogeneity of Cr1-xTe is 52.5 to 53.4 at.% Te [83Ips]. [83Ips] suggested that this phase decomposes below 800 C. This phase was synthesized by explosive shock method by [68Bat]. The existence of an intermediate phase with a monoclinic crystal structure in the composition region 53.3 to ~59 at.% Te between 700 to 1200 C has been observed by several investigators, but it was variously designated as Cr7Te8 and Cr5Te6. [83Ips] observed that the lattice parameters of this phase changed continuously as a function of composition between 53.5 and 59.2 at.% Te, and for this reason, they designated the entire composition region as a single phase, denoted by Cr3Te4 (HT). Cr3Te4 (HT) melts congruently at about 1283 C and 55 at.% Te [83Ips]. The original experimental data [83Ips] showed a gap between the liquidus and the solidus maximum of Cr3Te4 (HT). In the assessed diagram, it is shown so that the solidus and the liquidus meet at a congruent maximum. The separation between the solidus and liquidus may be due to impurity (ternary) effects, as suggested by [83Ips]. Cr3Te4 (HT) undergoes a solid state transformation at 635 C to Cr3Te4 (LT) [83Ips]. Cr3Te4 (HT) decomposes eutectoidally at 574 C [83Ips]. Some investigators [ 69Has, 75Leg] observed an anomaly in the magnetic susceptibility vs composition curves at 53.3 at.% Te (Cr7Te8) around 477 C [69Has] and between 400 to 1000 C [75Leg]. They attributed this to a crystallographic transformation of Cr7Te8. However, according to the phase diagram, this temperature probably corresponds to the eutectoidal decomposition of Cr3Te4 ( HT). Cr5Te8-I has a monoclinic crystal structure. It is separated from Cr3Te4 (LT) by a narrow two-phase field of ~59.2 to 59.6 at.% Te. The phase has a homogeneity of 59.6 to ~60.4 at.% Te. Further work is needed to define the exact location of the phase boundaries. Cr2Te3 is stable between 59.5 to 60.0 at.% Te [83Ips]. It decomposes peritectoidally to Cr3Te4 (LT) and Cr5Te8-I at 455 C. The hexagonal modification of Cr5Te8 has been reported to exist. At the stoichiometric composition (61.5 at.% Te), the monoclinic modification is stable at low temperatures [83Ips]. Quenching from about 800 C yields the hexagonal modification. At low temperatures, they observed that this phase occurs at slightly higher Te content (~62 at.% Te). CrTe3 is formed peritectically at 480 C by means of the reaction L + Cr5Te8- II = CrTe3, and it forms a eutectic with (Te) at 445 C. In the composition range between 64.1 to 74 at.% Te, [83Ips] observed a thermal arrest at 461 C in addition to those at 480 and 445 C. They suggested that this could be due to a peritectoid decomposition of a new phase, but because of slow kinetics, its existence has not been established. All the intermediate phases that occur in the Cr-Te system are ferromagnetic below 300 K. The Curie temperature remains almost constant at ~343 to 353 K in the composition range 50 to 58 at.% Te, but drops to about -193 to -183 K at 60 at.% Te [37Har, 70And]. The Curie temperature of Cr1-xTe was reported in the range ~340 to 360 K by numerous investigators. [56Tsu] reported a value of about ~308 K. The Curie temperature of Cr3Te4 was reported as 325 K [70And], 329 K [64Ber], and 326 K [72Oza]. The N‚el temperature has been reported as 80 K [64Ber] and 85 K [70And]. The discrepancy may be due to slight differences in composition. Below this temperature, the phase is antiferromagnetically ordered. Nearly the same values have been reported for the Curie temperature of samples at compositions 53.3 at.% Te (Cr7Te8) by [69Has] and [71Oza] and at 54.5 at.% Te ( Cr5Te6) by [70And]. According to the assessed phase diagram, no intermediate phases occur at these compositions at low temperatures. It is probable that their data correspond to the metastable Cr3Te4 (HT) phase or to the two-phase ( Cr) + Cr3Te4 (LT) mixture. According to [70And], even though Cr5Te6 and Cr3Te4 have the same crystal structure, their magnetic moments are differently aligned in each case; whereas the ferromagnetic component is pointing along the a axis in both phases, the antiferromagnetic component is pointing along the b axis in Cr5Te6 and close to the (b + c) axis in Cr3Te4. The Curie temperature of Cr2Te3 is 197 с 3 K [71Has]. The effect of pressure on the Curie temperatures of Cr1-xTe [73Sha] and Cr3Te4 [75Leg] has been studied as a function of pressure up to in the range 5 to 50 kbar. The Curie temperature decreased linearly with increasing pressure at the rate of ~5 to 6 C per kbar (up to 25 kbar) for Cr1-xTe and ~6 C per kbar (up to 5 kbar) for Cr3Te4. 27Oft: I. Oftedal, Z. Phys. Chem., 128, 135-153 (1927) in German. 37Har: H. Haraldsen and A. Neuber, Z. Anorg. Allg. Chem., 234, 353-371 (1937) in German. 56Tsu: I. Tsubokawa, J. Phys. Soc. Jpn., 11(6), 662-665 (1956). 57Lot: F.K. Lotgering and E.W. Gorter, J. Phys. Chem. Solids, 3, 238-249 (1957) . 60Hir: T. Hirone and S. Chiba, J. Phys. Soc. Jpn., 15(11), 1991-1994 (1960). 63And: A.F. Anderson, Acta Chem. Scand., 17, 1335-1342 (1963). 63Che: M. Chevreton, E.F. Bertaut, and F. Jellinek, Acta Crystallogr., 16, 431 (1963) in French. 64Ber: F. Bertaut, G. Roult, R. Aleonard, R. Pauthenet, M. Cheureton, and R. Jansen, J. Phys. (Paris), 25, 582-595 (1964) in French. 64Gro: F. Gronvold and E. Westrum, Z. Anorg. Allg. Chem., 328, 272-282 (1964). 67Ido: H. Ido, T. Kaneko, and K. Kamigaki, J. Phys. Soc. Jpn., 22(6), 1418- 1420 (1967). 68Bat: S.S. Batsanov and E.S. Zolotova, Dokl. Akad. Nauk SSSR, 180(1), 93-94 ( 1968) in Russian; TR: Dokl. Chem., 180, 383-384 (1968). 69Has: T. Hashimoto and M. Yamaguchi, J. Phys. Soc. Jpn., 27(5), 1121-1126 ( 1969). 69Nag: H. Nagasaki, I. Wakabayashi, and S. Minomura, J. Phys. Chem. Solids, 30, 2405-2408 (1969). 70And: A.F. Anderson, Acta Chem. Scand., 24, 3495-3509 (1970). 71Has: T. Hashimoto, K. Hoya, M. Yamaguchi, and I. Ichitsubo, J. Phys. Soc. Jpn., 31(3), 679-682 (1971). 71Oza: K. Ozawa, T. Yoshimi, and S. Yanagisawa, Phys. Status Solidi (b), 44, 681-686 (1971). 72Oza: K. Ozawa, T. Toshimi, M. Irie, and S. Yanagisawa, Phys. Status Solidi ( a), 11(2), 581-588 (1972). 73Gro: F. Gronvold, J. Chem. Thermodyn., 5, 545-551 (1973). 73Sha: V.A. Shanditsev, L.F. Vereshchagin, E.N. Yakovlev, N.P. Graz-hdankina, and T.I. Alaeva, Fiz. Tverd. Tela, 15(1), 212-215 (1973) in Russian; TR: Sov. Phys.-Solid State., 15(1), 146-148 (1973). 73Zav: E.A. Zavadskii and B.Ya. Sinelnikov, Fiz.-Tekh. Inst., (Donetsk), 18(3), 662-663 (1973) in Ukrainian. 74Zav: E.A. Zavadskii and B.Ya. Sinelnikov, Fiz. Tverd. Tela (Kharkov), 4, 18- 21 (1974) in Russian. 75Leg: J.M. Leger and J.P. Bastide, Phys. Status Solidi (a), 29, 107-113 (1975) . 83Kle: K.O. Klepp and H. Ipser, Angew. Chem. Suppl., 2004-2009 (1982) in German. 83Ips: H. Ipser, K.L. Komarek, and K.O. Klepp, J. Less-Common Met., 92(2), 265- 282 (1983). Submitted to the APD Program. Complete evaluation contains 2 figures, 5 tables, and 44 references. 1