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

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, Q(1)(angstrom(-1)), and the full-width at half-maximum (FWHM), Delta Q(1)(angstrom(-1)), of the FSDP peak. These parameters determine a correlation length R=2 pi/Q(1)(angstrom(-1)) and a coherence length L = 2 pi/Delta Q(1)(angstrom(-1)) that result from symmetry-determined contributions to the fundamental electronic structures Of SiO(2) and GeSe(2). 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 GeSe(2). The local bonding of threefold coordinated B-atoms in B(2)O(3) is planar rather than tetragonal and a similar O 2p pi-B 3d pi overlap is symmetry forbidden. Instead symmetry allowed O 2p pi-B 3p pi 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. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim