Structural analysis of amorphous Ge-Se alloys prepared by mechanical milling

The amorphizing process and short-range structure on amorphous GeSe2, Ge3Se4 and Ge4Se5 prepared by the mechanical milling technique (MM) have been investigated through X-ray diffraction and Raman scattering measurements. GeSe2 becomes amorphous in a shorter milling time. However, a much longer milling time is needed to amorphize Ge-enriched alloys such as Ge4Se5. Amorphous GeSe2 and Ge3Se4 prepared by the MM have a similar local order to the corresponding ones prepared by quenching the liquid (LQ), that is, the covalent 4(Ge)-2(Se) folded structure. This was confirmed by the EXAFS measurement at Ge and Se K-edges for amorphous GeSe2 from the MM and LQ. Amorphous Ge4Se5, which cannot be prepared by the LQ, has also the 4-2 folded structure, although its composition is nearer to GeSe, the crystalline structure of which is a distorted NaCl-type with the partial ionicity. The Raman spectrum in amorphous Ge4Se5 supported its covalent feature. Bond-lengths of Ge-Se and Ge-Ge involved in the first coordination shell in these alloys were estimated by the least squares fit to the observed diffraction function to be 2.36 Angstrom and 2.48 Angstrom, respectively, which are both composition-independent.