Symmetry determined medium range order (MRO) contributions to the first sharp diffraction peak (FSDP) in non-crystalline oxide and chalcogenide glasses

Abstract

The first sharp diffraction peak (FSDP) in the structure factor, S(Q), obtained from X-ray and neutron diffraction data, is a characteristic feature of oxide and chalcogenide glasses. This feature reflects intermediate, or medium range order (MRO), and is expressed in terms of two experimentally determined parameters, the position, Q1(Å−1), and the full-width at half-maximum (FWHM), ΔQ1(Å−1), of the FSDP peak. These parameters determine a correlation length R = 2π/Q1(Å−1) and a coherence length L = 2π/ΔQ1(Å−1) that result from symmetry-determined contributions to the fundamental electronic structures of SiO2 and GeSe2. Narrow distributions of third neighbor SiO and GeSe pair correlation distances are forced by strongly correlated symmetries of Si and Ge d-states connected through intervening O and Se atoms, as well as Se inter-atom lone-pair repulsions in GeSe2. The local bonding of threefold coordinated B-atoms in B2O3 is planar rather than tetragonal and a similar O 2pπB 3dπ overlap is symmetry forbidden. Instead symmetry allowed O 2pπB 3pπ bonding interactions play the determining role in promoting second-neighbor BO atom coupling through intervening O-atoms, thereby extending the correlation length scale into the MRO regime.