In the fused salt structure of sodium borate, Na2O and B2O3 phases co-exist. The basicity of the fused salt is defined by the Na2O activity which corresponds to the activity of O 2-since Na2O can be decomposed into Na + and O 2-at high temperatures following reaction (1). The formed O 2-can then ionize part of the B2O3 network into the BO 2-ions, following equation ( ), when Na + is generated .