Фазовая диаграмма системы H-La

К оглавлению: Другие диаграммы (Others phase diargams)

H-La (Hydrogen-Lanthanum) D. Khatamian and F.D. Manchester The assessed La-H phase diagram is based on [66Kor], [66Pet], [80Dan], [82Tog], [84Bis], and [84Kla]. A diagram of pressure-composition isotherms has also been provided because the equilibrium pressure of the H surrounding the metal is always a significant thermodynamic variable. In the assessed diagram, solid lines are drawn through the averages of the data points and represent estimated phase boundaries; dashed lines indicate phase boundaries that are not definitely established. In the diagram of pressure-compositionisotherms, dash-dot lines have been obtained by mapping the phase boundaries of the assessed diagram into it. [66Pet] indicated that pure La transforms from a to b phase at 324 C, and the addition of H lowers this transformation temperature to 297 C at 1.1 at.% H. Because the accepted a = b transformation temperature for pure La is 310 C, the eutectoid temperature has been adjusted and lowered to 283 C. [66Pet] gave 860 C for the b = g transformation temperature of pure La and showed that the addition of H lowers this temperature to 773 C at 23.5 at.% H. Again, because the accepted b = g transformation temperature for pure La is 865 C, this eutectoid temperature has been adjusted by 5 C and raised to 778 C. [66Pet] and [84Bis] both have shown that the addition of H raises the melting point of La; however, their results do not agree with regard to (gLa) = d (the phase with H composition higher than LaH2) transformation. According to [66Pet] , the coexistence of the (gLa) and d phases terminates at a critical point of 960 C, and above this temperature complete solubility of H in La occurs. [ 84Bis] and [85Bis] believe that for structural reasons, the existence of the critical point, which would allow the formation of a homogeneity range with end-compositions (gLa) (bcc) and LaH2 (fcc), is impossible. They support this point of view by their own measurements (the diagram of pressure-composition isotherms) and propose a phase diagram with no critical point. Also, they suggest that the slopes of the phase boundaries indicate the existence of a peritectic line at about 1200 C; at this temperature, they predict H pressure of about 4 x 105 Pa. [84Kla] indicated the presence of a cubic-to-tetragonal distortion of ~1% below an onset temperature of about -25 C for the approximate concentration range of 72 to 74 at.% H. This onset temperature closely coincides with the peak of the heat capacity measurement observed for LaD2.76 and LaD2.91 by [82Ito]. Samples used by [63Sch] to obtain diffusion data had 0.05 to 0.3% paramagnetic impurities. According to [83Phu], who doped their samples with known amounts of La to study the effects of paramagnetic impurities on the nuclear magnetic resonance results, surprisingly small amounts of paramagnetic impurities have appreciable effects on the hydrogen diffusion parameters derived therefrom. In view of these findings and the levels of paramagnetic impurities reported by [63Sch], the diffusion results of [63Sch] might not be as certain an should be used with some caution. 55Hol: C.E. Holley, Jr., R.N. Mulford, F.H. Ellinger, W.C. Koehler, and W.H. Zachariasen, J. Phys. Chem., 59, 1026-1028 (1955). 59Goo: E.J. Goon, J. Phys. Chem., 63, 2018-2021 (1959). 63Sch: D.S. Schreiber and R.M. Cotts, Phys. Rev., 131, 1118-1132 (1963). 66Kor: W.L. Korst and J.C. Warf, Inorg. Chem., 5, 1719-1726 (1966). 66Pet: D.T. Peterson and J.A. Straatman, J. Phys. Chem., 70, 2980-2984 (1966). 77Bie: Z. Bieganski and M. Drulis, Phys. Status Solidi (a), 44, 91 (1977). 80Dan: P. Dantzer and O.P. Kleppa, J. Solid State Chem., 35, 34-42 (1980). 82Ito: T. Ito, B.J. Beaudry, and K.A. Gschneidner, Jr., J. Less-Common Met., 88, 425-430 (1982). 82Tog: K. Toguchi, M. Tada, and Y.C. Huang, J. Less-Common Met., 88, 469-478 ( 1982). 83Phu: T.-T. Phua, B.J. Beaudry, D.T. Peterson, D.R. Togeson, R.G. Barnes, M. Belhoul, G.A. Styles, and E.F.W. Seymour, Phys. Rev., 28, 6224-6250 (1983). 84Bis: R. Bischof, E. Kaldis, and M. Tellefsen, J. Cryst. Growth, 70, 491-496 ( 1984). 84Kla: P. Klavins, R.N. Shelton, R.G. Barnes, and B.J. Beaudry, Phys. Rev. B, 29, 5349-5353 (1984). Published in Bull. Alloy Phase Diagrams, 11(1), Feb 1990. Complete evaluation contains 6 figures, 4 tables, and 78 references. Special Points of the La-H System