A MOLECULAR-DYNAMICS AND NEUTRON-DIFFRACTION STUDY OF THE STRUCTURE OF NA2O-B2O3 MELTS

The structures of sodium berate melts with 10 and 30 mol%Na2O at 1073 K have been studied by the use of molecular dynamics simulation and pu lsed neutron total scattering measurements. With an increase in Na2O c ontent, the first peak of the experimental radial distribution functio n corresponding to nearest-neighboring B-O correlation spread asymmetr ically to the higher-r side, and the coordination number of O atoms ar ound a reference B atom, n(B-O), increased from 3.0 to 3.5 reflecting a structural change from BO3 triangle units to BO4 tetrahedral units. The observed n(B-O) agreed well with those obtained from the molecular dynamics calculations, which showed clear evidence of the appearance of BO4 units, in that the distribution of not less than O-B-O bond ang les spread from 120 degrees to the smaller-angle side for 30 mol%Na2O- B2O3. It can be concluded from these results that some BO3 units prese nt are converted to BO4 units on the addition of up to 30 mol%Na2O to the B2O3 melt in the same way as in Na2O-B2O3 glasses.