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.
KeywordsCarbide Welding Enthalpy Argon Tungsten
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