Local atomic order of Ge–Se–Br glasses

Abstract

Raman scattering, X-ray diffraction and EXAFS measurements on (GeSe 2) 1− x (GeBr 4) x glasses with x=0 to 0.265 and (GeSe 2) 1− x Br x 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 Se n cluster-units was observed in the Br rich concentration range in (GeSe 2) 1− x Br x glasses. On the other hand, there were not any indications of Se–Br bonding and GeBr 4 molecular units in these spectra. These results suggest that Br atoms incorporated into GeSe 2 glass are preferentially bonded to Ge to form mixed-anion type Ge(Se (4− n)/2 Br n ) 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.