The Th-N-0 Phase Diagram

An appreciable N solubility in Th02 at 2550 and 2660X has been demonstrated metallogra-phically with the atom fraction values of x = 0.002 and 0.03, respectively [2]. Incorporation of N and of Th atoms in the ThOa lattice must be accompanied by creation of 0 ion vacancies in the nonmetal sublattice. Substoichiometry of pure ThOa is insignificant, ThO 993, above 2800 K [8]. 

The ternary compound Th2N20 is essentially a line compound at room temperature but exists over a range of 0/N ratios that fall in a very narrow section of the Th-N-0 phase diagram at elevated temperatures, 1600 to 2000°C. The range of compositions can be described by the general formula Th2N2(4/3 x/3)Ox, where V2 x 1 [5]. This formula represents compositions stable with the subsequently described crystal structure built up of Th, N , and 0 ions in which 1-x of the O ions are substituted for by % (1-x) ions on the O sublattice. No appreciable substitution of 0 ions for the N ions on the N sublattice seems to occur. 

Typical isotherms showing the transpiration of different 3-phase regions with temperature are illustrated in Fig. 17 to 20, drawn by the authors of this article on the basis of Ref. [2]. 

Th2N20 is prepared by equilibration of cold-pressed tablets of 3ThN + Th02 [1 to 3] or 3Th + Th02 [3 to 6] in 1 atm N2 at temperatures of 1000 to 2000°C. Although the low formation temperatures of 1000 [5], 1200 [3], and 1300°C [6] have been reported, higher temperatures can be expected to improve the crystallinity and homogeneity in a shorter period of time. 

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