Influence of MeF2 (Me = Mg, Ca, Sr, and Ba) on the electrical properties of glasses in the MeF2-Na2B4O7 system

Glasses in the MeF₂-Na₂B₄O₇ (Me = Mg, Ca, Sr, and Ba) system have been synthesized. It is shown that the glass formation is observed at a MeF₂ content of up to 40 mol %. The influence of the MeF₂ content on the electrical conductivity and the fluorine concentration in the glass bulk is examined. From the analysis of the concentration dependence of the electrical conductivity with due regard for the fluorine content, it is concluded that the glass structure is predominantly built up of the polar groupings Na⁺BO_4/2]^-, Na⁺F-BO_3/2], Me _1/2 ²⁺ BO_4/2]⁻, Me _1/2 ²⁺ F⁻BO_3/2], MeF_4/2], and MeF_6/3] and the BO_3/2 nonpolar structural-chemical units. The electricity transport is governed by the migration of sodium ions formed upon dissociation of the Na+BO_4/2]^-and Na+F^-BO_3/2] groupings. An increase in the MeF₂ content leads to a decrease in the total concentration of sodium ions, a decrease in the Na⁺BO_4/2]^- concentration, and an increase in the Na⁺F^-BO_3/2] concentration. Upon introduction of MeF₂ up to ∼20 mol %, the fluorine losses during the synthesis are caused by the dehydration of glass melt. An addition of 20–25 mol % MeF₂ brings about the saturation of the glass by the F^-BO_3/2]-type structural units, so that the fluorine concentration reaches a saturation in the structures of calcium-, strontium-, and barium-containing glasses and increases in magnesium-containing glasses, owing to the formation of the MgF+_6/3] groupings.