METASTABLE NICKEL SULFIDES WITH COMPOSITION CLOSE TO NI7S6-STABILITY AND STRUCTURAL-PROPERTIES

Four well-defined metastable phases with composition close to Ni7S6 ar e described. These phases are structurally closely related to the high -temperature phase Ni7+/-xS6, which is formed eutectoidally from Ni9S8 and Ni3S2 at similar to 675 K. The relative stability and a prelimina ry structural description of the metastable phases have been obtained from X-ray diffraction, transmission electron microscopy, calorimetry, and thermal analysis. Conditions for preparation of the different met astable phases are provided. On rapid quenching of Ni7+/-xS6 in ice water + NaCl, an as-quenched state is obtained. This state is rather u nstable and transforms on storing at room temperature to the metastabl e phase denoted alpha(iv) and subsequently to alpha(iii). The actual p hase sequence observed during heating of these modifications depends s trongly on the thermal history of the sample and especially on the hea ting rate used in the experiments. Either of the two quasi-reversible phase sequences alpha(iv) <->alpha(ii)<->alpha(i)<->Ni7+/-xS6 or alpha (iii)<->alpha(i)<->Ni7+/-xS6 is observed, where alpha(i), alpha(ii), a lpha(iii) and alpha(iv) denote the four well-defined metastable phases . The as-quenched samples appear rather similar to alpha(i). Furthermo re, an exothermal phase transformation is observed for alpha(iii). It has not, however, been possible to relate this transformation to struc tural changes. (C) 1996 Academic Press, Inc.