Local atomic arrangement in amorphous As–Se–Br System

Raman spectroscopic, EXAFS and time of flight (TOF)-neutron diffraction measurements have been carried out on amorphous As40−xSe60Brx with x=0∼20. The Raman spectra have indicated that Br atoms incorporated instead of As in amorphous As2Se3 are bonded not to As but to Se, and extra Se atoms produced in the system associate together in the formation of Sen cluster units. The present result differs from the bonding structure in amorphous As40−xSe60Ix, in which I atoms are bonded to As to form molecular AsI3 units. The bond-length of Se–Br estimated from the Br K-edge EXAFS datum for amorphous As25Se60Br15 was 2.36 Å, which is in excellent agreement with that, 2.359 Å, reported previously in liquid Se–Br alloys. Bond-lengths of the remaining two atomic pairs contained in the first coordination shell obtained by the present EXAFS and/or neutron diffraction data were 2.41 Å for As–Se and 2.30 Å for Se–Se, respectively. These values have denoted that all three bonds are covalent. The result of the least squares fitting analysis to observed neutron structure factors in amorphous As40−xSe60Brx has verified that these bond-lengthsʹ values are reasonable within the accuracy of the fit. In addition, coordination numbers of As, Se and Br in the system were determined to be three, two and unity, respectively, irrespective of alloy composition, in the fitting analysis.