Фазовая диаграмма системы Na-O

К оглавлению: Другие диаграммы (Others phase diargams)

Na-O (Sodium-Oxygen) H.A. Wriedt The equilibrium solid phases of the Na-O system are (1) the terminal cph solid solution, (aNa); (2) the terminal bcc solid solution, (bNa); (3) the fcc oxide Na2O; (4) the hexagonal peroxide Na2O2-I; (5) the noncubic, but otherwise structurally identified, peroxide Na2O2-II; (6) the orthorhombic superoxide NaO2(III); (7) the cubic superoxide NaO2(III); (8) the fcc superoxide NaO2(I); and (9) the bct ozonide NaO3. A third form of the peroxide, Na2O2-Q, with an unidentified structure, might be an equilibrium phase [61Tal], but this suggestion is uncomfirmed. A complete phase diagram for the condensed Na-O system has not been published. The assessed diagram is mainly schematic, because most phase boundaries and temperatures or forms of the three-phase equilibria have not been determined. For the condensed system at compositions up to 75 at.% O, there are twelve possible three-phase equilibria, few of which have been observed, and nine possible transitions. At 0.1 MPa hydrostatic pressure, aNa is stable up to -237C [56Bar], and from - 237 C to the melting point of 97.83 C, bNa is stable. The solid solutions ( aNa) and (bNa) saturate with respect to Na2O at very small oxygen concentrations, which have not been determined. The (bNa) solidus is also undetermined. Na2O exhibits only the fcc structure. On its Na-rich side, Na2O is in equilibrium with (aNa) below -237 C, with (bNa) from -237 to 98 C, with L1 from 98 to 1130 C, and with L2 from 1130 C to its congruent melting point at 1134 C. Except for the latter temperature, these values are approximate. The 1130 C reaction has been identified as monotectic, but, except as noted, the types of the other reactions among the three condensed phases have not been established. On its O-rich side, Na2O is in equilibrium with Na2O2-I below ~512C, with Na2O2-II from ~512 C to the probable eutectic temperature of ~570 C, and with L2 from ~570 to 1134 C. The range of Na2O composition is undetermined, but is probably narrow, at least at low temperatures. Although a third known polymorph of the peroxide, Na2O2-Q, may be stable, there is insufficient information to include it in the assessed diagram, which shows only Na2O2-I and Na2O2-II as stable peroxide polymorphs. At temperatures below 512 C, where it undergoes a polymorphic transformation to Na2O2-II at 0. 1 MPa hydrostatic pressure, Na2O2-I is in equilibrium with Na2O on its Na-rich, and on its O-rich side, it is in equilibrium with NaO2(III) below about -77 C, with NaO2(II) from about -77 C to about -50 C, with NaO2(I) from about -50 C to the temperature (assumed to be below 512 C) of the required but apparently unobserved Na2O2-I + L2 + NaO2(I) equilibrium, and with L2 above. No observations have been reported of the three-phase equilibria at about - 77 or -50 C. On its Na-rich side from ~512 to 570 C, Na2O2-II is in equilibrium with Na2O; otherwise, on both sides, it is in equilibrium with L2 below its 675 C melting point. Although Na2O2-II may exhibit an appreciable variability of composition, systematic studies of the phase boundaries and of its compositions as a function of temperature and oxygen fugacity have not been reported. Decomposition of Na2O2-I is supressed to at least 500 C by oxygen gas at 0.1 MPa pressure (fugacity) [47Bun]. Although the superoxide NaO2 exhibits only three crystal structures, a magnetic transition from NaO2(III) to NaO2(IV) without change of crystal symmetry occurs at low temperature. On their Na-rich sides, the NaO2 phases are in equilibrium with Na2O2-I or with L2. On their O-rich sides, the NaO2 phases are in equilibrium only with NaO3, except for NaO2-I, which also may coexist with L2. Appreciable variability in the composition of NaO2-I has been detected, but systematic studies of the phase boundaries have not been reported. The highest known oxide, the ozonide NaO3, has been reported to occur in two varieties exhibiting differences in chemical behavior, but structural differences or an equilibrium transformation have not been observed. The melting points and deviations from the stoichiometric composition are also unknown. Na-rich NaO3 would be in equilibrium, according to temperature, with each of the NaO2 modifications; on the O-rich side, the equilibria are unknown. Only three liquidus segments have been investigated. Numerous measurements of the L1 liquidus of Na2O have been made; the data of [73Nod] were used to construct the curve below 500 C in the assessed diagram. The O-rich liquidus of Na2O and the Na-rich liquidus of Na2O2-II were studied by [47Bun], but only the data for the latter curve are possibly near the correct values. 47Bun: E.G. Bunzel and E.J. Kohlmeyer, Z. Anorg. Chem., 254(1-2), 1-30 (1947) in German. 56Bar: C.S. Barrett, Acta Crystallogr., 9, 671-677 (1956). 57Fop: H. F”ppl, Z. Anorg. Chem., 291, 12-50 (1957) in German. 61Tal: R.L. Tallman and J.L. Margrave, J. Inorg. Nucl. Chem., 21, 40-44 (1961). 73Nod: J.D. Noden, J. Brit. Nucl. Energy Soc., 12(3), 329 (1973). Published in Bull. Alloy Phase Diagrams, 8(3), Jun 1987. Complete evaluation contains 1 figure, 2 tables, and 144 references. Special Points of the Na-O System