LOCAL ARRANGEMENT IN GE-SE-I GLASSES

X-ray diffraction and Raman scattering measurements on amorphous (GeSe 3.5)(100-x)I-x have been carried out over a wide composition range. Th e position of the well-resolved first peak of the pair distribution fu nction gradually increases with increasing I content due to the substi tution of the Ge-I bond for the Ge-Se bond. The coordination number fo r the Ge atoms obtained by a least-squares fit between the observed an d theoretical intensity functions is nearly four at any composition, i ndicating that the glasses contain some types of tetrahedral unit with the Ge atoms located at the central position, as well as Se cluster m olecules. The nature of the Ge-I bonding in the tetrahedral unit is id eally covalent, similar to that for Ge-Se, which enables the formation of GeSe(4-n/2)I-n (n = 1-3) mixed-anion tetrahedral units. A series o f Raman bands for 160 less than or equal to v less than or equal to 20 1 cm(-1), which can bes assigned to the A(1) vibrational mode of these units, indicates their presence.