Niobium doped BGO glasses: Physical, Thermal and Optical Properties

Abstract

IR-transparent niobium substituted heavy metal oxide glass system in formula composition (100-x) BGO75-xNb 2 O 5 ; 5 ≤ x ≤ 25 was fabricated by quenching and press molding technique. Glassy state was confirmed by XRD. Density ρ exp varied 6.381 g/cc-7.028 g/cc ±0.06%. Modifying behavior of Nb 2 O 5 was corroborated by rate of increase in theoretical volume V th , measured volume V exp and oxygen molar volume V MO -2 . Nb 5+ had larger cation radius and greater polarizing strength as compared to Ge 4+ ions. It replaced Ge 4+ sites introducing more NBOs in the network. Transformation temperatures T g , T x and T p1 were 456 -469 o C ±2 o C, 516 -537 o C ±2 o C and 589 -624 o C ±2 o C respectively. In the range from room temperature to 400 o C the coefficient of linear thermal expansion α was 5.431±0.001*10 -6 K -1 to 7.333±0.001*10 -6 K -1 . ΔT = T x -T g and ΔT P1 = T P1 -T g varied collinearly with increase in niobium concentration and revealed thermal stability against devitrification. The direct bandgap E g values lay in 3.24 -2.63 eV ±0.01 eV range and decreased due to impurity states of Nb 5+ within the forbidden band. Mobility edges obeyed Urbach law verifying amorphousness of the compositions. The IR-transparency window extended from 0.38 -6.1 μm in MIR which is wider than binary BGO glass as well as BGO single crystal.