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 SiO and GeSe 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 2pπB 3dπ overlap is symmetry forbidden. Instead symmetry allowed O 2pπB 3pπ bonding interactions play the determining role in promoting second-neighbor BO atom coupling through intervening O-atoms, thereby extending the correlation length scale into the MRO regime.
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