Influence of rare earth additives and boron component on electrical conductivity of sodium rare earth borate glasses

Sodium rare earth borate glasses (Na₂O)_35.7(RE₂O₃)_7.2(B₂O₃)_57.1 (RE = Sm, Gd, Dy, Ho, Y, Er, and Yb), were prepared from a mixture of Na₂CO₃, RE₂O₃ and B₂O₃, and their properties as an Na⁺ ionic conductor were investigated. Density increased with increasing atomic weight of RE. Crystallization temperature and crystal melting temperature of the present borate system was lower than that of the previously reported silicate and germanate system. Results of the ¹¹B NMR measurement suggested that half of all boron atoms are coordinated by four oxide ions to give a BO₄] tetrahedral unit and the others are coordinated by three oxide ions to give a BO₃] planar triangular unit. The electrical conductivity slightly decreased with increasing the ionic radius of RE³⁺. (Na₂O)_35.7(Y₂O₃)_7.2(B₂O₃)_57.1 glass exhibited the electrical conductivity which is about one order of magnitude lower than those of the previously reported (Na₂O)_35.7(Y₂O₃)_7.2(SiO₂)_57.1 and (Na₂O)_35.7(Y₂O₃)_7.2(GeO₂)_57.1 glasses. It was assumed that this lower electrical conductivity is due to the lower content of Na⁺ ions as conduction species in the former glass, compared with the latter two glasses.