CeN: Phase Relationships and Enthalpies of Solution
Single crystals and polycrystalline samples of CeN with nitrogen contents 47.0<x<50.5 at %N, do not show inclusions of cerium metal either in metallographic or in X-ray investigations. A T-x plot of the growth or nitridation temperature (2100 C>T>900 C) and the nitrogen concentration show a temperature dependent nitrogen-rich phase boundary. Variation of the nitrogen partial pressure during nitridation of cerium turnings from 1140 to 57 Torr, showed only a slight change of the nitrogen content. Attempts to locate the cerium-rich phase boundary are now concentrated on annealing CeN+Ce mixtures in sealed W-crucibles.
The lattice constants of CeN vary by 0.14% in the range of 47.0–50.5 at%N, with the value for stoichiometric CeN being 5.018 Å, as compared with 0.23% for the corresponding change of δ-NbN. Most instructive is a histogram of the lattice constants vs. nitrogen concentration of 70 samples. Comparison with the lattice constant, a, of samples which have been doped with oxygen, shows clearly that a>5.023 A belong to oxygen contaminated samples. Carefully measured lattice constants are therefore a sensitive criterion in differentiating between contamination and nonstoichiometry. This was supported by chemical analyses both of nitrogen (Kjeldahl; accuracy ±0.1%) and cerium (complexometrically; accuracy ± 0.25%) in aliquot parts of the samples used for measurements of the lattice constants and enthalpies of solution.
Enthalpies of solution ΔHS in 4n HCl have been measured at 25 C as a function of nonstoichiometry for 50 undoped and 20 oxygen doped (and alloyed) CeN samples. The proposed value for pure, stoichiometric CeN is ΔHS=122 kcal/mol. The dependence of ΔHS on nonstoichiometry indicates an instability of the mixed valent CeN at 49.2%N. The most striking effect however, is the strong decrease of the heat of solution i.e. increase of the stability of samples doped with not more than 1% oxygen. This is explained as the result of destabilisation of the delicate balance of electronic states in mixed valent CeN.
KeywordsLattice Constant Homogeneity Range Large Single Crystal Nitridation Temperature Nitrogen Partial Pressure
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