MD study of sodium borate glasses containing Al₂O₃

The short range structure of 0.05Al₂O₃.xNa ₂O.(0.95-x)B₂O₃ glasses (0.10<x<0.45) has been investigated using molecular dynamics calculations employing a fully ionic model. Regardless of x, the coordination number of oxygen atoms around the Al atom was approximately four. It was found that these Al atoms are present as network formers of AlO₄ units in the glasses in the same manner as are BO₄ units, and that they cause a decrease of the coordination number of O around Na. It is suggested that the exclusively formed AlO ₄ units may suppress the conversion of the BO₃ unit to the BO₄ unit caused by an increase of Na₂O. The decrease of the fraction of four coordinated boron atoms, N₄, due to the addition of Al₂O₃ is supported by the results obtained from preliminary neutron diffraction measurements.