STRUCTURAL CORRELATIONS IN GEXSE1-X GLASSES - A NEUTRON-DIFFRACTION STUDY

Neutron diffraction measurement is carried out on GexSe1-x glasses, wh ere 0.1 less than or equal to x less than or equal to 0.4, in a Q inte rval of 0.55-13.8 Angstrom(-1). The first sharp diffraction peak (FSDP ) in the structure factor, S(Q), shows a systematic increase in the in tensity and shifts to a lower Q with increasing Ge concentration. The coherence length of FSDP increases with x and becomes maximum for 0.33 less than or equal to x less than or equal to 0.4. The Monte-Carlo me thod, due to Soper, is used to generate S(Q) and also the pair correla tion function, g(r). The generated S(Q) is in agreement with the exper imental 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-1/2)(4) tetrahedra, in agreement with earlier re ports. Se-rich glasses contain Se-chains which are cross-linked with G e(Se-1/2)(4) tetrahedra. Ge-2(Se-1/2)(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. (C) 1998 Elsevier Science B.V. Al l rights reserved.