Effects of mixed glass former on ac conductivity, dieletric and optical properties of 70[xTeO2+(1-x)B2O3]+15Na2O+15K2O glasses / Shima Asyurah Shuhaimi

Mixed glass former of composition 70[xTeO2+(l-x)B2O3J+15Na2O+15K2O where x = 0 - 0.7 mol% have been prepared by melt quenching method to investigate their structural, AC conductivity, dielectric and optical properties. The variation of conductivity (er') with Te02 showed a non-linear behaviour, where decrease to a minimum value at x = 0.4 mol% before increasing for x > 0.4 mol%. The minimum is suggested to be due to low migration of Na+ and K+ ions caused by the mixed glass former effect (MGFE). Meanwhile, dielectric constant (e') showed a slight increase for x < 0.4 mol% followed by a large increase for x > 0.5 mol% Te02. This result is attributed to the formation of TeCb give out a larger effect compared to BO3 and is suggested to be related to MGFE. Structural analysis of the present glass system reveals N4 reached minima at x = 0.2 mol% and 0.4 mol% with addition of Te02 which attributed to the structural changes due to the conversion of BO4 to BO3 units. Glass transition temperature, Tg exhibited a non-linear increase for x < 0.5 mol% followed by a large increase at x > 0.5 mol%. The conduction mechanism at low frequency region was found to be Inverse - Overlapping Large Polaron Tunnelling (Inverse - OLPT) for x < 0.5 mol%, while the mechanism transformed to the OLPT model for x = 0.7 mol%. The electric modulus of the present glass system showed asymmetric peak of M\ that reflected a non-Debye type relaxation. The optical energy gap, E0pt for both transition exhibited a minimum at x = 0.4 mol%, whereas refractive index, n and Urbach energy, Eu showed a maxima at the same concentration, thereby indicating variation in polarizability due to changes in concentration of bridging and nonbridging oxygen. On the other hand, the large decrease in ao2.and A for x > 0.2 mol% because of increasing number of bridging oxygen attributed to BO4 that increase the covalency and decrease the electron donor power. The changes observed showed that the continuous addition of Te02 into the glass system influence the properties of this borotellurite glasses.