Local atomic order of Ge–Se–Br glasses

Raman scattering, X-ray diffraction and EXAFS measurements on (GeSe2)1−x(GeBr4)x glasses with x=0 to 0.265 and (GeSe2)1−xBrx glasses with x=0 to 0.562, have been carried out in order to elucidate structural units contained in Ge-chalcobromide glasses. Raman spectra in both glass types showed that the 200 and 218 cm−1 bands of Ge-tetrahedral units contained in the glass varied their feature with the incorporation of Br atoms. Another sharp 260 cm−1 band of Sen cluster-units was observed in the Br rich concentration range in (GeSe2)1−xBrx glasses. On the other hand, there were not any indications of Se–Br bonding and GeBr4 molecular units in these spectra. These results suggest that Br atoms incorporated into GeSe2 glass are preferentially bonded to Ge to form mixed-anion type Ge(Se(4−n)/2Brn) tetrahedra. The formation of purely covalent Ge–Br bonds in these glass types was indicated also by analyzing EXAFS oscillation functions at the Br K-edge. Bond-lengths of Ge–Se, Ge–Br and Se–Se pairs in the first co-ordination shell in these glass types were estimated to be 2.36, 2.27 and 2.32 Å, respectively, through the least-squares fit to observed X-ray diffraction functions, which are equal roughly to the corresponding covalent distances. In addition, the total co-ordination numbers of Ge, Se and Br appeared to be roughly four, two and unity, respectively.