Neutron diffraction and reverse Monte Carlo modelling of v-B₂O₃ and 75B₂O₃-25Na₂O glasses

A neutron diffraction study has been performed on newly synthesized v-B₂O₃ and 75B₂O₃-25Na₂O glasses. The structure factor has been measured over a broad momentum transfer range, 0.4-30 Å^-1, which made fine r-space resolution available for real space analyses. The reverse Monte Carlo (RMC) method was successfully applied to calculate a possible 3-dimensional atomic configuration that is consistent with the measured structure factor. The partial atomic pair-correlation functions, nearest neighbor atomic distances, coordination number distributions, average coordination number values and three-particle bond angle distributions have been revealed. It was established that addition of 25 mol% Na₂O to v-B₂O₃ transforms 41% of the 3-fold oxygen coordinated boron atoms into 4-fold coordinated ones. The first neighbor B-O distance is at 1.37 Å in v-B₂O₃, while two well resolved distances are at 1.38 and 1.50 Å in the 75B₂O₃-25Na₂O glass. The three-particle 〈O-B-O〉 and 〈B-O-B〉 bond angle distributions show an extra peak at around 90° in the 75B₂O₃-25Na₂O glass, which may be attributed to the BO₄ units, in addition to the peak at 120° characteristic for BO₃ trigonal units. The ring size distribution of the v-B₂O₃ network indicate that only 12% of boron atoms form boroxol group (B₃O₆), while the fraction of the larger 7-11 boron membered rings is 52%, and a tail of the distribution extends up to 20-membered rings.