Phase stability and structural properties of Ni 7 ± δS 6 and Ni 9S 8 Heat capacity and thermodynamic properties of Ni 7S 6 at temperatures from 5 K to 970 K and of Ni 9S 8 from 5 K to 673 K

Abstract

Two stable solid phases with composition in the range from (45 to 48) moles per cent of S exist for (nickel + sulfur): Ni 9S 8 which disproportionates to Ni 1 - δS and Ni 7 ± δS 6 above T = (709 ± 5) K, and Ni 7 ± δS 6. The latter non-stoichiometric phase forms eutectoidally from Ni 9S 8 and Ni 3S 2 at T = (675 ± 3) K. It is stable in a rather narrow temperature interval and disproportionates to Ni 1 - δS and Ni 3 ± δS 2 at T = (850 ± 2) K. The phases are characterized structurally, and a revised phase diagram is presented. The heat capacity of a sample with composition Ni 7S 6 was determined over the temperature range T = 5 K to 970 K by adiabatic calorimetry. Effects from metastable modifications of the high-temperature phase were observed during thermal analysis, X-ray diffraction, and calorimetry on incompletely equilibrated samples. Thermodynamic-function values for Ni 7S 6 and Ni 9S 8 are presented.