Structural studies of Bi₂O₃-Nb₂O₅-TeO₂ glasses

Bi₂O₃-Nb₂O₅-TeO₂ glasses show unusual annealing behavior with appearance of spherulites within the matrix glass structure for the Bi_0.5Nb_0.5Te₃O₈ composition. The textures resemble those found previously among polyamorphic Al₂O₃-Y₂O₃ glasses containing metastably co-existing high- and low-density phases produced during quenching. However the spherulites produced within the Bi₂O₃-Nb₂O₅-TeO₂ glass are crystalline and can be identified as an “anti-glass” phase related to β-Bi₂Te₄O₁₁. We used high energy synchrotron X-ray diffraction data to study structures of binary and ternary glasses quenched from liquids within the Bi₂O₃-Nb₂O₅-TeO₂ system. These reveal a glassy network based on interconnected TeO₄ and TeO₃ units that is related to TeO₂ crystalline materials but with larger Te…Te separations due to the presence of TeO₃ groups and non-bridging oxygens linked to modifier (Bi^3 +, Nb^5 +) cations. Analysis of the viscosity-temperature relations indicates that the glass-forming liquids are “fragile” and there is no evidence for a LLPT occurring in the supercooled liquid. The glasses obtained by quenching likely correspond to a high-density amorphous (HDA) state. Subsequent annealing above T_g shows mainly evidence for direct crystallization of the “anti-glass” tellurite phase. However, some evidence may exist for simultaneous formation of nanoscale amorphous spherulites that could correspond to the LDA polyamorph. The quenching and annealing behavior of Bi₂O₃-Nb₂O₅-TeO₂ supercooled liquids and glasses is compared with similar materials in the Al₂O₃-Y₂O₃ system.