A Comparative Study of (Cu-III-Se2)x-(FeSe)1?x Alloys (III: Al, Ga, In) (0 ? x ? 1) by X-Ray Diffraction, Differential Thermal Analysis and Scanning Electron Microscopy

Abstract

Polycrystalline samples of (Cu-III-Se 2 )-(FeSe) 1-x alloys (III: Al, Ga and In) were prepared by the usual melting and annealing method in the entire composition range 1≤ x < 0. X-Ray Diffraction (XRD), Differential Thermal Analysis (DTA) and Scanning Electron Microscopy (SEM) techniques were used to characterize the products. From SEM technique, the stoichiometric relation for each sample was checked to be very close to the nominal values, with deviations less than 10%. XRD measurements showed that, for the three alloy families, there is a wide single-phase field, with a chalcopyrite-like structure, for 1 < x < 0.5. The lattice parameters vary linearly with composition and the variation can be predicted using Jaffe and Zunger's model for chalcopyrites, Pauling's set of covalent radii, and the covalent radius of 1.35 A for Fe. From DTA measurements, thermal transitions in the temperature range 300 K < T < 1400 K were obtained. The melting point decreases from that of extreme compounds (x = 0 and x x = 1) towards an intersection point (local minimum) at around x = 0.3. These results suggest that under adequate thermal treatments, the single-phase field could be increasing up to around x 0.3. In a closer analysis, a pronounced local minimum has been observed at x = 2/3 for Ga and In-based alloys, suggesting that the composition x 2/3 is a new class of compounds, the (Cu-III) 2 FeSe 5 .