The influence of Ho2O3 content on structural, spectroscopic, thermal, and dielectric properties of CaBTe glasses

A series of holmium-doped borotellurite glasses with the chemical formula 54B₂O₃–10TeO₂–26CaO–(10-x)CaF₂-xHo₂O₃ (CaBTe), where x = concentration of 0.00, 0.10, 0.25, 0.50, 0.75, and 1.00 mol% was prepared using the conventional melt-quenching method. Techniques such as X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, and impedance spectroscopy were employed to investigate the structural and dielectric properties of the synthesized glasses. It was observed that both the density and molar volume values increased with the rising dopant concentration. Ultraviolet–Visible spectroscopy analysis revealed characteristic bands of Ho³⁺ ions, showing that the absorption coefficient increased with Ho₂O₃ concentration. The refractive index, molar electronic polarizability, optical band gap, and Urbach energy were also calculated and discussed. The results indicated that the refractive index increased with the rise in Ho₂O₃ concentration, ranging from 1.60 (CaBTe) to 1.67 (CaBTe:Ho_1.00). Conversely, the optical band gap energy decreased from 3.27 eV (CaBTe) to 2.78 eV (CaBTe:Ho_1.00) for direct allowed transitions and from 3.90 eV (CaBTe) to 3.81 eV (CaBTe:Ho_1.00) for indirect allowed transitions of Ho³⁺ ions. Additionally, the electronic polarizability exhibited an exponential increase with Ho₂O₃ content, rising from 3.50 (matrix) to 3.86 (CaBTe:Ho_1.00). The Urbach energy values suggested that the incorporation of Ho₂O₃ species introduces greater disorder into the glass network, increasing from 0.30 eV (CaBTe) to 0.45 eV (CaBTe:Ho_1.00). Excitation and emission spectra identified excitation bands at 451 nm, corresponding to the ⁵I₈ → ⁵G₆ transition, which produced red emission at 623 nm (⁵F₅ → ⁵I₈). Thermoanalytical analysis demonstrated that Ho₂O₃ increases the crystallization temperature of the glasses from 742 °C (0.00% Ho₂O₃) to 760 °C (1.00% Ho₂O₃). These findings highlight the novelty of the work: the CaBTe:Ho glasses exhibit unique spectroscopic properties, making them promising candidates for low-cost, efficient red-emitting optical devices. This study provides new insights into the structural, optical, dielectric, and thermal properties of holmium-doped borotellurite glasses, emphasizing their potential for practical applications in photonic.