Фазовая диаграмма системы Ca-N

К оглавлению: Другие диаграммы (Others phase diargams)

Ca-N (Calcium-Nitrogen) V.P. Itkin and C.B. Alcock The equilibrium phases of the Ca-N system are (1) the liquid, L; (2) the terminal solid solution, (Ca), which exists at lower temperatures as fcc (aCa) and at higher temperatures as bcc (bCa); (3) tricalcium dinitride, Ca3N2, with four forms, one of which, aCa3N2, is stable; and (4) the gas, G. Several other compounds have been reported in the system-Ca(N3)2, Ca2N, Ca3N4, and Ca11N8. Some of them may be equilibrium phases of the condensed system. A portion of the phase diagram was studied by [30Ant]. Three alloys with 1.39, 5.93, and 12.61 at.% N, prepared from sublimed Ca and Ca3N2, were measured. The eutectic point was estimated to lie between 3 and 4 wt.% nitride (1.6 to 2. 1 at.% N) at 780 C. It should be pointed out that the accuracy of measurements in [30Ant] was not high. Extension of the phase diagram given in [ 30Ant] was not attempted because the locations of the phase boundaries in the system are mostly unknown. Pertinent data are lacking even for the types of invariant equilibria. [Hansen] reproduced the diagram of [30Ant] and added Ca3N2 to it. No information about the solubility of N in (Ca) is available, and this is probably low. [59Smi] reported that pure Ca contained less than 0.009 wt.% of N. (Ca) saturated with N coexists with aCa3N2 or with the liquid. The gas phase consists of Ca and N2. Three points in the boundary of the gas phase are known. These are the temperatures of evaporation of Ca (1440 C), N2 (-210 C), and Ca3N2 (1600 C). bCa3N2, gCa3N2, and Ca3N2(II) are metastable modifications of Ca3N2. The b modification was prepared by a reaction of Ca with N at low temperatures (300 to 650 C) in the presence of a trace of alkali metal [33Fra, 34Har]. On heating above 500 C, this modification transforms irreversibly into aCa3N2. Prolonged heating of the a form in vacuum or in nitrogen at temperatures above 300 C produces no b form [34Har]. The stability of bCa3N2 was studied by [66Bra] by holding samples at various temperatures in sealed gold capsules for a period up to 80 h followed by quenching in liquid nitrogen. After annealing at temperatures above 320 C, bCa3N2 disappeared. Below 320 C, the rate of the b <259> a transformation was too low to be measured. [66Bra] attempted to obtain stable bCa3N2 at high temperatures and pressures. Investigation of cubic and hexagonal material under pressures up to 9 GPa and temperatures up to 800 C produced no evidence of a phase transition. The gCa3N2 modification is obtained by heating aCa3N2 or by reaction of N with Ca at temperatures above 1050 C [33Fra]. The sublimation of aCa3N2 was considered a necessary step in preparation of gCa3N2. [68Lau] found that gCa3N2 has an orthorhombic structure. This modification transforms into the a modification after heating 6 h at 900 C. The latter product was heated at temperatures above 1050 C in vacuum or a N atmosphere, and no a <259> g transformation was observed [68Lau]. The high-pressure modification of tricalcium dinitride, denoted as Ca3N2(II), was obtained by heating a cubic phase at 1800 C and 4.6 GPa for 8.5 min and quenching rapidly [66Bra]. [68Kri] investigated anhydrous calcium azide Ca(N3)2 by means of X-rays. The crystals had an orthorhombic structure. This compound is thermodynamically unstable. 30Ant: A. von Antropoff and E. Falk, Z. Anorg. Allg. Chem., 187, 405-416 (1930) in German. 33Fra: H.H. Franck, M.A. Bredig, and G. Hoffmann, Naturwissenschaften, 21, 330- 331 (1933) in German. 34Har: H. Hartmann and H.J. Frohlich, Z. Anorg. Allg. Chem., 218, 190-192 ( 1934) in German. 59Smi: J.F. Smith and B.T. Bernstein, J. Electrochem. Soc., 106, 448-451 (1959) . 66Bra: R.S. Bradley, D.C. Munro, and M. Whitefield, J. Inorg. Nucl. Chem., 28, 1903-1912 (1966). 66Oka: Y. Okamoto and J.C. Goswami, Inorg. Chem., 5, 1281-1283 (1966). 68Kri: H. Krischner, Monatsh. Chem., 99, 2134-2141 (1968) in German. 68Lau: Y. Laurent, Rev. Chem. Minerale, 5, 1019-1050 (1968) in French. 69Lau: Y. Laurent and J. Lang, Acta Crystallogr. B, 25, 199-203 (1969) in French. Submitted to the APD Program. Complete evaluation contains 1 figure, 3 tables, and 23 references. 1