Transition metal-chalcogen systems, VII.: The iron-selenium phase diagram

Abstract

The binary iron-selenium system was investigated by thermoanalytical and isopiestic methods and by X-ray analysis. Combining all results a temperature-concentration diagram was constructed from 20 to 66 at% Se and between 623 and 1,373 K. Two compounds and several NiAs-related structures could be identified. Tetragonal β-FeSe with a narrow stability range between 49.0 and 49.4 at% Se at 653 K decomposes by a peritectoid reaction at 730 K. The NiAs-related structures Fe1-xSe have a wide range of homogeneity from 51.5 to 58.5 at% Se at 823 K. The iron-rich hexagonal δ-phase extends, from 51.5 to 54.3 at% Se, and transforms to a high temperature modification δ′ of unknown structure with a maximum transformation temperature of 1,338 K at 52.8 at% Se. The congruent melting point of the δ′-phase is located at 1,351 K and 52.0 at% Se. At ≈ 54 at% Se the δ-phase undergoes a λ-transformation to the monoclinic γ′-phase which is stable between 54.3 and 54.6 at% Se. Between 54.6 and 56.4 at% Se two very similar monoclinic phases coexist: the γ′-and the γ-phase with similara- andb-axes but a simplec′- and a doublec-axis, resp. Between 56.4 and 58.5 at% Se the γ-phase is stable. Orthorhombic e-FeSe2 (marcasite structure) with a very narrow range of homogeneity at 66.6, at% Se decomposes peritectically at 858 K. On the iron-rich side there are three invariant equilibria at high temperatures: a monotectic at 1,234 K and 46.5 at% Se, a eutectic at 1,215 K and 5.5 at% Se, and a eutectoid at 1,149 K. On the selenium-rich side there is a eutectoid at 1,122 K, a monotectic at 1,068 K and 71.5 at% Se, and a peritectic at 1,001 K.