Фазовая диаграмма системы As-Eu

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As-Eu (Arsenic-Europium) K.A. Gschneidner, Jr. and F.W. Calderwood The assessed phase diagram for the Eu-As system is based on review of the literature [70Hul, 71Ono, 74Tay, 78Tay, 78Wan1, 78Wan2, 80Wan]. [78Tay] reported the rare earth-rich phase, Eu5As3, to occur in two hexagonal forms- one with an Mn5Si3-type structure [78Wan1] and the other with a Ca5Pb3-type structure. [78Wan1] clarified the relationships between these structures and reported the Mn5Si3-type to be the a or low-temperature form, whereas the Ca5Pb3-type is the b or high-temperature form of Eu5As3. [78Tay] reported that two phases exist between Eu5As3 and Eu3As2; these phases were designated as Eu3+xAs2 and Eu3+yAs2. They also found that Eu3As2 did not form during annealing below ~1000 C. [70Hul] previously reported that Eu3As2 has a bcc structure of the Ba3P2-type, which is related to the anti-Th3P4-type structure. They also reported that Eu3As2 melts near 1300 C. Eu4As3 has a cubic structure of the anti-Th3P4-type [80Wan]. [78Tay] found that samples with 40 to 50 at.% As, when annealed at ~600 C, contained Eu and Eu5As4. Precession photographs on single crystals of Eu5As4 indicated an orthorhombic cell similar to that of Sm5Ge4. [78Wan2] refined the structure of Eu5As4 and showed it to be a more symmetrical version of the Sm5Ge4 type with the space group Ccmb rather than Pnma. Eu11As10 was obtained with EuAs in samples containing 40 to 50 at.% As that had been annealed at ~900 C. It was not formed below this temperature, but was stable to at least ~1400 C. X-ray precession photographs of single crystals indicated that the structure is an orthorhombic distortion of the Ho11Ge10-type cell. [71Ono] reported that the equiatomic compound EuAs has a distorted structure of the hexagonal Na2O2-type. EuAs is the only equiatomic Re-As compound that does not crystallize in the cubic NaCl-type structure. They assigned a peritectic decomposition temperature of 990 C to EuAs. [71Ono] found Eu3As4 (57.1 at.% As) to be the next As-rich compound. This alloy was reported to form peritectically from the melt and EuAs at 950 C. It has a type of orthorhombic structure that had not been previously reported. [71Ono] also reported an Eu2As3 compound, a nominal stoichiometry that was assigned on the basis of crystallographic data. The phase lies at a more metal- rich composition of ~58.7 at.% As. This phase, which forms peritectically at 912 C, has a monoclinic structure. They also reported an Eu2As3+x compound with an unknown structure that forms peritectically at 842 C and decomposes to Eu2As3 and EuAs2 at 697 C by a eutectoid reaction. [71Ono] also found EuAs2, which forms peritectically at 912 C and has an orthorhombic structure with a primitive unit cell. EuAs3 forms peritectically at 800 C and has a monoclinic symmetry. No data are available for the a = b transformation temperatures for those compounds for which more than one form has been reported-Eu5As3, Eu4As3, and EuAs3. The compounds Eu3+xAs2 and Eu3+yAs2 reported by [74Tay] are shown by a single dashed line. Because As sublimes at 614 C, no details are shown for the high-As region beyond EuAs3. However, [74Tay] indicated a eutectic between EuAs3 and (As) at 790 C and at a composition greater than 95% As. No data have been found regarding the solid solubility of either element in the adjacent intermediate compound. [82Wer] attempted to transform EuAs, which has the Na2O2-type structure, to the NaCl-type structure by means of pressures up to 23 GPa but were unsuccessful. Magnetic susceptibility shows Eu to be divalent in this compound. In contrast, EuP forms an NaCl-type structure with trivalent Eu. 70Hul: F. Hulliger and O. Vogt, Solid State Commun., 8, 771 (1970). 71Ono: S. Ono, F.L. Hui, J.G. Despault, L.D. Calvert, and J.B. Tayler, J. Less- Common Met., 25, 287 (1971). 74Tay: J.B. Tayler, L.D. Calvert, J.G. Despault, E.J. Gabe, and J.J. Murray, J. Less-Common Met., 37, 217 (1974). 78Tay: J.B. Tayler, L.D. Calvert, T. Utsonomiya, Y. Wang, and J.G. Despault, J. Less-Common Met., 57, 39 (1978). 78Wan1: Y. Wang, L.D. Calvert, E.J. Gabe, and J.B. Taylor, Acta Crystallogr., A34, 2281 (1978). 78Wan2: Y. Wang, L.D. Calvert, E.J. Gabe, and J.B. Taylor, Acta Crystallogr., B34, 1962 (1978). 80Wan: Y. Wang, L.D. Calvert, and J.B. Taylor, Acta Crystallogr., B36, 221 ( 1980). 82Wer: A. Werner, H.D. Hochheimer, M. Wittmann, and H.G. von Schnering, J. Less-Common Met., 87, 327 (1982). Published in Bull. Alloy Phase Diagrams, 7(3), Jun 1986. Complete evaluation contains 1 figure, 1 table, and 20 references. 1