Фазовая диаграмма системы H-Na

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H-Na (Hydrogen-Sodium) A. San-Martin and F.D. Manchester The assessed phase diagram for the Na-H system at atmospheric pressure (0.1 MPa) consists of the following phases: bNa, the pure bcc solid; NaH, the hydride with fcc structure; and the liquid, L. The L/L + NaH boundary in the assessed diagram, representing values up to atmospheric pressure, was calculated with the assessed analytical approximation for the terminal solubility of H in liquid Na. The atomic ratio X (X = H/Na) amounts to less than 9 x 10-7 at 110 C, the lowest temperature measured so far [74Vis], and to ~ 4 x 10-3 at 428 C. The H solubility in liquid Na increases at H2 pressures higher than 0.1 MPa. The experimental pressure-composition isotherms reported by [82Klo] show that at a pressure of 66.4 MPa and a temperature of 900 C, the maximum observed solubility of H in liquid Na amounts to X = 0.186. The melting point temperature of NaH is assessed to be 638.8 C (at P = 11.25 MPa), derived by the procedure of [82Klo] and the equilibrium data of [55Ban], [76Sku], and [82Kle]. Solubility measurements of H in Na are difficult to obtain because of the chemical reactivity and high vapor pressure of Na. Thus, the H equilibrium pressure may be affected not only by the gases dissolved in the Na, but also by the products of the reaction of molten Na with the material of the cell used to contain it. [80Bas] predicted that alkali hydrides may transform from fcc NaCl-type structure to CsCl-type structure under pressure. However, based on comparative calculations of the specific volumes of the alkali hydrides in both structures, [80Bas] concluded that the NaCl <259> CsCl transitions seem to be possible only for CsH, RbH, and KH, because the specific volumes in the CsCl structure of those hydrides are 10 to 14% less than those in the NaCl- type structure. Results of high-pressure X-ray studies at room temperature [85Hoc1, 85Hoc2, 85Pon] are in line with the results of comparative calculations of the specific volumes of the alkali hydrides in both structures [85Pon]. The NaCl <259> CsCl transitions were observed only for KH, RbH, and CsH at pressures of around 4, 3, and 1 GPa, respectively, whereas in NaH, no transition was observed up to pressures of 28 GPa [85Hoc2]. Similar trends in NaCl <259> CsCl transitions were observed in the alkali halides, with a sodium halide requiring the highest transformation pressure recorded so far [76Chh]. Similarities between the alkali halides and alkali hydrides provided the motivation for the investigations of [80Bas]. Recently, the high-pressure behavior of NaH was studied theoretically by [86Sri]. 55Ban: M.D. Banus, J.J. McSharry, and E.A. Sullivan, J. Am. Chem. Soc., 77, 2007-2010 (1955). 56Els: R.E. Elson, H.C. Hornig, W.L. Jolly, J.W. Kury, W.J. Ramsey, and A. Zalkin, Univ. California Rad. Lab. Rep., UCRL 4519 Rev. (June 1956); cited in [ 60Lib]. 58Gun: S.R. Gunn and L.G. Green, J. Am. Chem. Soc., 80, 4782-4785 (1958). 60Lib: G.G. Libowitz, J. Nucl. Mater., 2, 1-22 (1960). 62Kuz: V.G. Kuznetsov and M.M. Shkrabkina, Zh. Strukt. Khim., 3, 553-558 (1962) in Russian; TR: J. Struct. Chem., 3, 532-537 (1962). 74Vis: D.R. Vissers, J.T. Holmes, L.G. Bartholme, and P.A. Nelson, Nucl. Technol., 21, 235-244 (1974). 76Chh: I.C. Chhabildas and A.L. Ruoff, J. Appl. Phys., 47, 4182-4187 (1976). 76Sku: O.A. Skuratov, O.N. Pavlov, V.I. Danikin, and I.V. Volkov, Zh. Neorg. Khim., 21, 2910-2913 (1976) in Russian; TR: Russ. J. Inorg. Chem., 21, 1605- 1608 (1976). 80Las: I.O. Bashkin, T.N. Dymova, and E.G. Pcyatovskii, Phys. Status Solidi (b) , 100, 87-92 (1980). 82Klo: W. Klostermeier and E.U. Franck, Ber. Bunsenges. Phys. Chem., 86, 606- 612 (1982) in German. 85Hoc1: H.D. Hochheimer, K. Strossuer, W. Honle, B. Baranowski, and F. Filipek, J. Less-Common Met., 107, L13-L14 (1985). 85Hoc2: H.D. Hochheimer, K. Strossuer, W. Honle, B. Baranowski, and F. Filipek, Z. Phys. Chem. (Neue Folge), 143, 139-144 (1985). 85Pon: E.G. Ponyatovskii and I.O. Bashkin, Z. Phys. Chem. (Neue Folge), 146, 137-157 (1985). 86Sri: G.M.S. Srivastava, G.G. Agrawai, and J. Shanker, Phys. Status Solidi (b) , 135, 523-531 (1986). Published in Bull. Alloy Phase Diagrams, 11(3), Jun 1990. Complete evaluation contains 4 figures, 7 tables, and 54 references. 1