Structure Changes during Amorphization of Ge-Se Alloys by Mechanical Milling

Abstract

Amorphous GeSe₂, Ge₃Se₄ and Ge₄Se₅ have been prepared by mechanical milling (MM), and their structural changes have been investigated by X-ray diffraction, Raman scattering and EXAFS measurements. Amorphous Ge₄Se₅ was obtained at the milling time of 50 h in the present experiment, which has never been obtained by the rapid quenching from the melt. The short-range order in GeSe₂ remains almost unchanged during the milling. Since the configuration of tetrahedral units in the crystalline phase becomes rapidly less ordered with increasing the milling time, the amorphization is complete at a short milling time. On the other hand, Ge₃Se₄ and Ge₄Se₅ samples, both of which are eutectic mixtures of intermediate compounds of GeSe₂ and GeSe, need longer milling times for the amorphization, as they are accompanied by the change in local structures. The short-range order in these Ge–Se alloys is described by covalent 4(Ge)–2(Se) folded coordination structure. The structural feature of crystalline GeSe phase with a partial ionicity, i.e. 3(Ge)–3(Se) folded coordination, disappears not only in amorphous Ge₃Se₄ but also in amorphous Ge₄Se₅, where the concentration ratio of Ge/Se is close to that of the stoichiometric composition of GeSe. The bond-lengths of Ge–Se and Ge–Ge in these amorphous alloys are determined to be 0.236 and 0.248 nm, respectively, which are consistent with the corresponding covalent distances.