Add time:07/27/2019         Source:sciencedirect.com

The vaporization of gadolinium and holmium mononitrides was studied and the partial pressures of the vapour species were determined by Knudsen effusion mass spectrometry. No evidence of gaseous GdN or HoN was found. The nitrides vaporize congruently by dissociation into metal vapour and molecular nitrogen. The synthesis by the amalgam method of gadolinium and holmium nitrides is described and the difficulties in obtaining pure and stoichiometric samples are critically discussed. Single crystals of these compounds have been grown by sublimation in sealed molybdenum crucibles. The growth rates were found to be of the order of 10-7 (HoN) to 10-8 (GdN) cm/sec. To find out the reason for these low values an estimation of the average evaporation-condensation coefficient of these compounds has been made. This was accomplished by comparing the maximum flux calculated by the Langmuir-Knudsen equation from the measured vapour pressures and the crystal growth temperatures, with the flux corresponding to the experimentally measured growth rate. Values of the order of 10-3 were found for the average evaporation-condensation coefficient of both nitrides, indicating that the surface reaction is the rate determining process during crystal growth. From the mass spectrometric measurements second law enthalpy values of ΔH0298 = 174 ± 6 kcal and ΔH0298 = 157 ± 6 kcal have been found for the reactions GdN (s) = Gd (g) + 12N2 (g) and HoN (s) = Ho (g) + 12N2 (g) respectively. When these enthalpies are combined with the known literature values for the enthalpy of vaporization of the metals, the standard enthalpies of formation of GdN (s),ΔH0f, 298 = −79 ± 6 kcal/mole, and of HoN (s) ΔH0f, 298 = − 6 ± 6 kcal/mole are obtained.

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