A neutron diffraction study of six M2O.M'2O.5B2O3 mixed-modifier di-pentaborate glasses.

Twin-axis neutron diffraction experiments have been performed to study structural aspects of the mixed-modifier effect for six ternary di-pentaborate glasses of composition M₂O.M'₂O.5B₂O₃ (M/M'=Li/Na, Li/Rb, Na/K, Na/Rb, Na/Cs, and Na/Ag). In reciprocal space, the method of additivity is employed to deduce information concerning the thickness/ size of the network cages containing the network-modifying cations, and the zero-Q limit of the diffraction pattern, I(0), to investigate the extent of the nanoheterogeneity in the glasses. The fraction of 4-fold co-ordinated boron atoms, x₄, is obtained from the area under the first (B-O) peak in the real-space total correlation function, T(r) and, with the exception of vitreous Na₂O.Cs₂O.5B₂O₃ and Na₂O.Ag₂O.5B₂O₃, no evidence is found to indicate the presence of a significant fraction of non-bridging oxygen atoms. An interpretation of the neutron diffraction data, in terms of a thermodynamically-based structural model for the network-modifying cation conductivity and mixed modifier effect in borate glasses, is presented in an accompanying paper (pp. 98-107). [ABSTRACT FROM AUTHOR]