Hyperstoichiometric compositions

The phase equilibrium of U4O9 has also been examined by many authors (1-6, 24-28), but the agreement between their data is rather poor. In Figs. 1-3, the phase equilibrium in this region is reproduced mainly from the results reported by Roberts and Walter (4), Ishii et al (26), and Matsui and Naito (29). The existence of a 114,09+ phase has been claimed from electric conductivity measurements under various oxygen partial pressures (29), but the range of this hyperstoichiometric composition has not yet been determined. 

Unlike the interstitial monocarbides, MC,, where C is never >1, the interstitial mononitrides, MN,, can have a composition where x >1. In substoichiometric compositions (x < 1), the sublattice of nitrogen is predominantly deficient while at hyperstoichiometric compositions (x > 1), the metal lattice is predominantly deficient. The lattice parameter is at a maximum at stoichiometry. Even at stoichiometry, a substantial fiaction of both nitrogen and metal sites are usually vacant. 

The fluorite-structure hyperstoichiometric range spans for high temperatures to compositions near to O/M = 2.25, which is the composition of the next oxide U4O9. 

Both the tensimetric and diffusion data clearly indicate two regimes for hyperstoichiometric Pr70ia. The change in slope for both sets of data (see Figs. 5 and 7) at about 214 mm oxygen pressure occurs at approximately the same temperature, thus indicating the close relationship between the defect concentration and the diffusion coefficient. Before this relationship can be fully determined, the pressure dependence of the composition must be decided. 

Diffusion coefficient and surface exchange coefficient measurements have been reported for the K2MF4 type oxide materials by a number of authors [4-8, 13-19] and have been complemented by electrochemical permeation measurements [20-27] all of which demonstrate the fast oxide ion conduction of hyperstoichiometric K2NiF4 type oxides. Early reports also demonstrate the relatively poor oxide ion mobility in those materials found to be hypostoichiometric [28,29]. Initial reports of the fast oxide ion conduction in La2Ni04+s [4, 6-8] have generated a number of further studies [13-19] regarding the optimization of composition and determination of the effects of anisotropy on the conduction properties of these materials. Each of these features will be discussed in more detail below. 

In the hyperstoichiometric UC, + phase, it is expected that the diffusion of carbon would occur through interstitial atoms, as substantiated by the increase of carbon diffusion coefficient with the increase of carbon content, but in the hypostoichiometric UC, j, phase, further careful examination of the composition dependence of the carbon diffusion coefficient must be undertaken before a reliable conclusion could be drawn. 

The results of the above-named authors (342) obtained in the composition range between UOi 934 and UO2.250 e reproduced in Fig. 27. It is seen from the figure that both the diffusion coefficient and the activation energy of fission gas differ between hyperstoichiometric, stoichiometric, and hypostoichiometric UO2, and that in the nonstoichiometric regions, D becomes almost independent of composition ... 

The composition of the sesquioxide on the hyperstoichiometric side depends very much on the particular oxide. The oxygen excess is small for any but those of cerium, praseodymium and terbium, where for the C-type it can be considerable. There are C-type phases of these latter oxides with composition near RO,.7o (called (T phasfe). For most of the rare earths only the sesquioxide is known. 

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