Composition limits and vaporization behaviour of rare earth nitrides

Phase relationships in the praseodymium, holmium and erbium nitrogen systems have been investigated by chemical, X-ray and metallographic techniques. Particular attention was paid to the problem of oxygen contamination and an analytical method was devised for the determination of oxygen in rare earth oxynitrides. A substoichiometric existence range of less than 0·05 atomic units in MX_1·00 (where X is the total non-metal content) was found for praseodymium nitride, whereas the holmium and erbium systems showed a broader range (ca. 0·10 a.u.) at higher temperatures. The effects of substoichiometry on the nitride lattice parameters are small and can easily be hidden by changes due to sample contamination. The vaporization of lanthanum, cerium, praseodymium, gadolinium and terbium nitrides was found to be incongruent at 1600°C, whereas samarium, holmium, erbium and ytterbium nitrides vaporized congruently.