Structural correlations in Ge_xSe_1-x glasses - a neutron diffraction study

Abstract

Neutron diffraction measurement is carried out on Ge_xSe_1-x glasses, where 0.1≤x≤0.4, in a Q interval of 0.55-13.8 Å^-1. The first sharp diffraction peak (FSDP) in the structure factor, S(Q), shows a systematic increase in the intensity and shifts to a lower Q with increasing Ge concentration. The coherence length of FSDP increases with x and becomes maximum for 0.33≤x≤0.4. The Monte-Carlo method, due to Soper, is used to generate S(Q) and also the pair correlation function, g(r). The generated S(Q) is in agreement with the experimental data for all x. Analysis of the first four peaks in the total correlation function, T(r), shows that the short range order in GeSe₂ glass is due to Ge(Se_½)₄ tetrahedra, in agreement with earlier reports. Se-rich glasses contain Se-chains which are cross-linked with Ge(Se_½)₄ tetrahedra. Ge₂(Se_½)₆ molecular units are the basic structural units in Ge-rich, x=0.4, glass. For x=0.2, 0.33 and 0.4 there is evidence for some of the tetrahedra being in an edge-shared configuration. The number of edge-shared tetrahedra in these glasses increase with increasing Ge content.