Abstract
The pressure-composition-temperature (P-C-T) relationships of the Sm 2Fe 17N system were measured volumetrically at temperatures between 623 and 673 K. Clear pressure plateaux could be reproducibly measured between [N] [Sm 2Fe 17] = 2.0 and 3.0, indicating the coexistence of Sm 2Fe 17N 2 and Sm 2Fe 17N 3 phases. As the N concentration [N] [Sm 2Fe 17] approaches from 0 to 2.0, the N chemical potentials and the lattice parameters increase sharply, indicating the induction and accumulation of internal stresses caused by the uptake of N. The inclinations of the measured plateaux suggest that the formation of Sm 2Fe 17N 3 from Sm 2Fe 17N 2 seems to proceed under highly constrained lattice conditions. The Lacher model was fitted to the measured P-C-T curves and a partial phase diagram of the Sm 2Fe 17N system was constructed. The critical temperature below which Sm 2Fe 17N 3 exists was estimated as 793 ± 30 K. The exposure of a stoichiometric Sm 2Fe 17 sample to N 2 gas at 673 K over a period of 1 month exhibited no appreciable disproportionation. The partial molar enthalpy and entropy calculated for the formation of Sm 2Fe 17N 3 were −46.9 ± 2.4 kJ (mol N 2) −1 and −25.4 ± 1.3 J K −1 (mol N 2) −1 respectively.
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