Three-dimensional structure of multicomponent (Na₂O) _0.35[(P₂O₅)_1-x(B₂O ₃)_x]_0.65 glasses by high-energy x-ray diffraction and constrained reverse Monte Carlo simulations

Experimental structure functions for (Na₂O) _0.35[(P₂O₅)_1-x(B₂O ₃)_x]_0.65 glasses, where x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0, have been measured by high-energy x-ray diffraction up to wavevectors of 28 Å^-1 to obtain atomic pair distribution functions with high real space resolution. The experimental diffraction data have been used to guide constrained reverse Monte Carlo simulations of the three-dimensional structure of the glasses. The resulting models show that the glasses exhibit a very complex atomic-scale structure that evolves from an assembly of chains of corner shared P(O)₄ tetrahedra for x= 0 to a network of B(O)₄ tetrahedra and planar B(O)₃ units for x= 1. In the glasses of intermediate composition (i.e. 0 <x < 1), P, B and oxygen atoms sit on the vertices of P(O)₄, B(O)₄ and B(O)₃ units mixed in various proportions. Sodium atoms are found to fill up the cavities in between the P/B-oxygen units in a more or less random manner. The new data can provide a firm structural basis for an explanation of the mixed glass former effect where a nonlinear behavior of Na ion conductivity is observed in the (Na₂O)_0.35[(P₂O ₅)_1-x(B₂O₃)_x] _0.65 glass system. (Some figures in this article are in colour only in the electronic version)