Exploring the impact of CuCl2 modification on structural variations and dielectric constant in bismuth borate tellurite glasses

Copper chloride doped boro-tellurite-based glass series with compositions 60TeO₂-(25-x) Bi₂O₃–15B₂O₃-xCuCl₂ (x = 0, 5, 10, 15, and 20) were synthesized using the traditional melt-quench procedure. The X-ray diffraction patterns of the examined glasses reveal their amorphous nature. The effect of CuCl₂ addition on the glass network/structural units was studied using vibrational (FTIR/Raman) spectroscopy, and CuCl₂ inclusion in the glass matrix resulted in a transformation from compact TeO₄ to TeO₃ structural units This shows that copper chloride acts as a modifier in the examined glass series, resulting in the creation of non-bridging oxygen (NBOs), meaning that the loosening of the glass network is further supported by a decrease in the glass transition temperature as CuCl₂ content increases. UV–Vis DRS spectra of examined glasses reveal an absorption band corresponding to the typical transition of Cu²⁺ ions from ²B_1g → ²B_2g, situated in deformed octahedral sites in absorption spectra. As Cu⁺ concentration rises, the indirect optical band gap values drop from 2.94 eV to 1.75 eV. The metallization criterion values lie between 0.383 and 0.300 suggesting that these glasses are potential candidates for nonlinear optical applications. With an increase in Cu⁺ concentration, the values of molar refractive index (R_m) and molar electronic polarizability ${\alpha }_m$ increases from 22.412 to 26.205 and 8.894 to 10.398 respectively, which correlate with increasing dielectric constant (ε′) values as the Cu⁺ amount increases. Further dielectric studies reveal non-Debye-type relaxation behaviour.