Фазовая диаграмма системы Se-V

К оглавлению: Другие диаграммы (Others phase diargams)

Se-V (Selenium-Vanadium) J.F. Smith No phase diagram for the complete Se-V system is available. A variety of sources have been used as the basis for construction of a tentative phase diagram. Published information indicates that the central portion of the system around 33.3 to 60 at.% V shares many features of the analogous regions in the S-V and Te-V systems. However, several details remain to be clarified. The Se-V system differs from the other two systems in both the V-poor and V-rich regions. In the V-poor regions, the Se-V system has a phase reported [84Fur] at Se9V2, an S4V phase has been reported for the S-V system [Elliott], but no Te-V phase below 33.3 at.% V has been reported. In the V-rich regions above 60 at.% V, phases with XV3 stoichiometry have been reported to occur in both the S-V [ Elliott] and Te-V [68Mon] systems, but not in the Se-V system. Se9V2 [84Fur] was formed by direct reaction between the elements between 280 and 400 C, but the phase decomposed into Se and Se2V1+x at and above 410 C. There is no indication of the existence of other phases between this phase and Se or between this phase and Se2V1+x. In analogy with the Te-V system, a broad range of homogeneity has been reported for an Se2V1+x-SeyV phase field (x minimum is slightly >0 and y minimum is slightly <1) at high temperatures [38Kle, 64Ros], with approximate compositions from 33.8 to 51.2 at.% V. The CdI2 crystal structure was observed to persist in this range for Se2V1+x from 33.8 to near 37.0 at.% V, where distortion to a monoclinic structure occurred. The monoclinic structure in turn gave way to the NiAs structure for SeyV near 47.2 at.% V. The NiAs structure persisted to approximately 51 at.% V. The CdI2 structure for the Se2V1+x phase is stable to and below room temperature at V-poor compositions, and vapor pressure measurements show that there is a two-phase region between this phase and the adjacent monoclinic phase. However, above 700 C the hexagonal and monoclinic phases interlayer, apparently coherently, in this two- phase region to form a pseudo-single phase [78Hay]. This complicates delineation of the boundaries of the two-phase field. A hexagonal superlattice of the basic NiAs structure has been reported [66Car, 68Car] in a narrow composition range between 47.4 and 47.6 at.% V. A report of two different monoclinic structures at a stoichiometry of Se8V7 (46.7 at.% V) has also been made [72Bru]. Neither the superlattice nor the monoclinic structures were observed in earlier work [64Ros], nor have they been confirmed by subsequent work [78Hay, 83Miy]. Further examination of alloys from 45 to 48 at.% V is needed. Below room temperature, the Se2V1+x phase undergoes transitions that are attributable to changes among commensurate and incommensurate charge-density- waves [76Bay, 78Van]. One transition occurs near 100 K and a second near 290 K, and electron diffraction [78Van] indicates that there is probably a third transition. This phase is a metallic conductor, but studies to 1.5 K [76Bay] and to 0.05 K [67Maa] showed no superconductivity. Also below room temperature, the SeyV phase with the NiAs structure exhibits Curie-Weiss type magnetic behavior with a N‚el point at 163 K [59Tsu]. 38Kle: W. Klemm, Z. Angew. Chem., 51, 756 (1938) in German. 59Tsu: I. Tsubokawa, J. Phys. Soc. Jpn., 14, 196-198 (1959). 64Bru: S. Brunie and M. Chevreton, Compt. Rend., 258, 5847-5850 (1964) in French. 64Ros: E. Rost and L. Gjertsen, Z. Anorg. Allg. Chem., 328, 299-308 (1964). 66Car: F.M.A. Carpay, J. Inorg. Nucl. Chem., 28, 2827-2831 (1966). 67Maa: M.H. van Maaren and G.M. Schaeffer, Phys. Lett. A, 24, 645-646 (1967). 68Car: F.M.A. Carpay, Philips Res. Rept., Supple. 10, 99 p (1968). 68Mon: E. Montigne, Z. Anorg. Allg. Chem., 362, 329-330 (1969). 72Bru: S. Brunie, M. Chevreton, and J.-M. Kauffmann, Mater. Res. Bull., 7, 253- 259 (1972). 76Bay: M. Bayard and M.J. Sienko, J. Solid State Chem., 19, 325-329 (1976). 78Hay: K. Hayashi and M. Nakahira, J. Solid State Chem., 24, 153-161 (1978). 78Van: J. van Landuyt, G.A. Wiegers, and S. Amelinckx, Phys. Status Solidi (a), 46, 479-492 (1978). 83Miy: K. Miyauchi, K. Hayashi, and M. Nakahira, Mater. Res. Bull., 18, 757- 764 (1983). 84Fur: S. Furuseth and B. Klewe, Acta Chem. Scand. A, 38, 467-471 (1984). Published in Phase Diagrams of Binary Vanadium Alloys, 1989. Complete evaluation contains 3 figures, 6 tables, and 23 references. Special Points of the Se-V System