Raman scattering from a partly interrupted network glass former

Abstract

The authors have studied the Raman spectra of a network glass former that derives from GeS2 by interrupting the sulphur bridges by two halogens, concentrating on GeSBr2 where on average two of the four bridges per GeS4/2 tetrahedron are interrupted. The temperature range was 520-230 K (glass transition temperature Tg=250 K). At a resolution of 2 cm-1, a rich structure develops instead of the very broad lines of GeS2 glass. They concentrate on the lines that derive from the tetrahedral A1 (breathing) mode. In order to analyse them, the authors use a program for eigenfrequencies and eigenvectors neglecting symmetries (NORCOR of D Christen). Electro-optical parameters are estimated from the bond polarisability model. They consider quasi-molecular subunits of one, two or three tetrahedra with S or Br substituents. Then the fine structure of the Raman spectra can be understood as due to intermediate-range order, viz, coupling of neighbouring tetrahedra, probability of occurrence of various possible subunits and their geometrical configuration; about 100 subunits are included. A very satisfactory simulation of the Raman spectrum for the said modes is achieved. Edge-connected bitetrahedra and chair-shaped rings are essentially absent. Force constants show only minor changes.