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

Abstract

Amorphous GeSe 2 , Ge 3 Se 4 and Ge 4 Se 5 have been prepared by mechanical milling (MM), and their structural changes have been investigated by X-ray diffraction, Raman scattering and EXAFS measurements. Amorphous Ge 4 Se 5 was obtained at the milling time of 50h in the present experiment, which has never been obtained by the rapid quenching from the melt. The short-range order in GeSe 2 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, Ge 3 Se 4 and Ge 4 Se 5 samples, both of which are eutectic mixtures of intermediate compounds of GeSe 2 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 Ge 3 Se 4 but also in amorphous Ge 4 Se 5 , 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 he 0.236 and 0.248 nm, respectively, which are consistent with the corresponding covalent distances.