Physical and optical properties of NaF–TeO${}_2$ glasses and glass–ceramics

Density, electrical conductivity and optical properties of xNaF⋅(100–x)TeO${}_2$ (0 ≤ x ≤ 75 mol%) glasses and glass–ceramics have been investigated. Density and molar volume decrease with increasing NaF content. The change of free volume is a primary factor in changing molar volume as well as the conductivity. Na${}^{+}$ ions are assumed as the main charge carriers in these glasses and glass–ceramics. The conductivity was found to have a limited change because of decreasing in free volume and the association between Na and F. Energy gap and Urbach energy decrease with increasing NaF content up to NaF ≤ 40 mol%, while refractive index increases. After that, these quantities remain nearly constant for further additions of NaF. These changes are due to the structural changes that occur with modifying the network by NaF. For NaF ≤ 10 mol%, the entire NaF content enters the structure to convert TeO${}_4$ units into TeO${}_{3/2}$F and Na${}^{+}$[TeO${}_{3+1}$]${}^{-}$ units; whereas, it starts to form its own matrix beside modifying the tellurite network, for NaF > 10 mol%. Molar refractivity and metallization criterion decrease with increasing NaF content. This shows an insulating behavior.