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hase Diagram Evaluations: Section II

Fe-Ga-Sb (Iron-Gallium-Antimony) V. Raghavan

Recently, Deputier et al. [2002Dep] determined an isothermal section for this system at 600 °C, which depicts an extensive solubility of Ga in the Fe3Sb2 (␧) binary phase.

is stoichiometric and has the cubic ZnS (sphalerite) type structure. The mutual solubility between Ga and Sb is very limited.

Binary Systems

Ternary Isothermal Section

The Fe-Ga system [1993Oka1] is characterized by the presence of a closed ␥ loop and several ordered forms of the bcc Fe based solid solution (␣Fe). ␣⬘ has the CsCl type ordered structure. The structure of ␣⬙ is not known. ␣⵮ has the cubic BiF3 type structure. The intermediate phases of the system are: Fe3Ga, Fe6Ga5, Fe3Ga4, and FeGa3. The first two have high- and low- temperature modifications. For crystal structure data, see [1993Oka1] and [Pearson3]. The Fe-Sb phase diagram [1993Oka2] depicts two intermediate phases. The NiAs-type B81 phase Fe3Sb2 (␧) has a homogeneity range of 40-47 at.% Sb. The other intermediate phase FeSb2 is stoichiometric and has orthorhombic symmetry. [Pearson3] lists two orthorhombic types for this phase: FeS2-type (Pnnm) and FeSb2-type (Pnn2). The GaSb phase diagram [1988Nga] is characterized by the presence of the congruently-melting intermediate phase GaSb. It

With starting materials of purity of ⱖ99.99%, [2002Dep] heated about 30 alloy compositions to 1000 °C for 72 h in evacuated silica tubes. The cooled samples were ground to powder and annealed at 600 °C for a long time to ensure homogeneity and then quenched in ice-water mixture. The phase equilibria were studied by x-ray powder diffraction and by energy dispersive spectroscopy attached to a scanning electron microscope (SEM-EDS). The isothermal section at 600 °C constructed by [2002Dep] is redrawn in Fig. 1 to agree with the accepted binary data. The ordered form of bcc Fe (␣⵮) and the Sb-rich liquid, omitted by [2002Dep], are schematically indicated. The main feature of the section in Fig. 1 is the large solubility of Ga in the Fe3Sb2 (␧) phase, with Ga substituting for Sb. The Ga-rich end has the composition range of FetGa0.8Sb1.2 (2.15 ⱕ t ⱕ 2.80), that corresponds to an Fe

Fig. 1 Fe-Ga-Sb isothermal section at 600 °C [2002Dep]

Journal of Phase Equilibria and Diffusion Vol. 25 No. 1 2004

85

Section II: Phase Diagram Evaluations range of 52-58 at.%. This range does not cover the composition Fe t GaSb found at 800 °C by [1994Moz]. FetGa0.8Sb1.2 has hexagonal symmetry and a structure intermediate between B81 and B82 types. No extra lines are seen in the x-ray powder patterns, indicating that Ga and Sb atoms are randomly distributed in the non-metal sublattice. In NiAs (B81), there are two types of interstitial sites which are vacant. In Ni2In (B82), both the interstitial sites are occupied by Ni atoms. In FetGa2−xSbx (␧), the extra Fe atoms (as compared to the NiAs structure) are distributed randomly over the two interstitial sites. This phase can be considered either as a partially-filled NiAs type struct