Bismuth modified physical, structural and optical properties of mid-IR transparent zinc boro-tellurite glasses

Tellurium oxide based quaternary glasses in the system TeO2⋅B2O3⋅Bi2O3⋅ZnO have been prepared by melt quenching technique. Amorphous nature of the samples has been ascertained by X-ray Diffractogram. The values of density and molar volume increase with increase in Bi2O3 content. Theoretical calculations of crystalline volume (Vc) have also been made. The glass transition temperature (Tg) has been determined using differential scanning calorimetry (DSC) and its value is observed to decrease with increase in Bi2O3 content. IR and Raman spectra of the present glass system indicate that ZnO acts as network modifier and exists in ZnO4 units. TeO2 exists as TeO3, TeO4, and TeO6 structural units and the number of TeO4 groups decreases with increase in bismuth content except for the glass sample with x = 5, which shows maximum number of TeO4 structural units among all other studied glass samples and transformation of some of TeO4 structural units into TeO3 structural units is observed with increase in bismuth content. Bismuth plays the role of network modifier with BiO6 octahedral structural units for glass samples with x = 5, 10, and 15, whereas, exists in network forming BiO3 pyramidal structural units for glass sample with x = 20. B2O3 exists in the form of BO3 trigonal and BO4 tetrahedral structural units. The transmittance of the present glasses is observed to be very high (up to 95%) in the spectral range (λ = 2.5-6.5 μm) which is of great technological importance. The percentage value of transmittance increases with increase in Bi concentration. The Hydrogenic excitonic model is applicable to the presently studied glasses and the optical band gap energies (Eg) calculated from this model and Tauc's plots for direct transitions are almost same. Eg decreases with increase in bismuth content, however, the molar refractivity (Rm) show the reverse trend. Metallization criterion for these materials shows that these glasses may be potential candidates for non linear applications.