Effect of niobium doping on physical, thermal, and spectral parameters of infrared transmitting BGO70 glasses

Heavy metal oxide glasses doped with 2d transition metal niobium were casted through normal melt-quench technique in the formula composition (100?x) 3Bi₂O₃–7GeO₂ (BGO70)]?xNb₂O₅ where 5 ≤ x ≤ 25. Experimentally measured values of density d _exp were 6.737–7.149 g/cc ± 0.06 %. Corresponding molar volume V _m exp had values 29.677–31.550 cc ± 0.04 %, V _pyc varied 32.28–34.71 cc ± 0.03 % and oxygen molar volume $ V_{{{\text{mO}}^{2-} }} $ increased linearly from 17.761 to 20.467 cc ± 0.06 %. Thermal coefficient of linear expansion was between 5.316 ± 0.001 × 10^?6 and 8.033 ± 0.001 × 10^?6 K^?1. Glass transition temperature T _g, onset of crystallization temperature T _x, and the stability factor ΔT were noted from DTA curves. Direct allowed energy gap E _g was between 1.809–2.988 eV and Urbach energy had value 0.32–1.49 eV. Maximum transmission efficiency was 74 % for glass BGO70-Nb10. FTIR spectra revealed that lattice vibration modes were active in 400–1,300 cm^?1 range. A modifying behavior was assigned to Nb⁵⁺ ion in the system.