Effect of tantalum on phase formation in the Fe-S system

Phase formation in the Fe1 - x -Ta x -S system with 0 < x < 0.5 has been studied at temperatures up to 1273 K using X-ray diffraction. When the constituent elements are heated to 823 K, troilite (Tr), pyrrhotites (PoI, PoII, and Pom), and pyrite (FeS2) are formed. Tantalum at these temperatures only insignificantly dissolves in iron sulfides. In the range 823--1273 K, tantalum reacts with pyrrhotites PoII and Pom and pyrite to yield PoI, FeTa3S6, and a-Fe. The solubility limit of tantalum in PoI is near Fe0.98Ta0.02S. The initiation temperature of the reaction between troilite and tantalum producing FeTa3S6 and a-Fe is 873 K. The unit cell parameters of tantalum change at 500 K, presumably due to the dissolution of iron and possibly sulfur (iron sulfide).