Glass and anti-glass phase co-existence and structural transitions in bismuth tellurite and bismuth niobium tellurite systems

Glass and anti-glass samples of Bi2O3-TeO2 and Bi2O3-Nb2O5-TeO2 were prepared by ice-water quenching and normal-quenching respectively. The bismuth tellurite system has poor glass forming ability (GFA) and forms a glassy phase at low Bi2O3 concentration of 2 to 5-mol%. On increasing Bi2O3 concentration to 10 mol%, a mixture of glass and anti-glass phases are formed by rapid quenching of the melt. A further increase in Bi2O3 concentration to 20 mol%, produces a sample consisting of entirely Bi2Te4O11 anti-glass upon melt-quenching. An anti-glass is a solid, which has long range order of cations (Te4+, Bi3+, Nb5+ etc.) but these are statistically distributed at their sites while the anion sites are partially vacant. Consequently the X-ray diffraction (XRD) patterns of bismuth tellurite and bismuth niobium tellurite anti-glass samples show sharp XRD peaks but the Raman spectra show broad phonon bands due to disturbed short-range order of the anions. The addition of Nb2O5 to the Bi2O3-TeO2 system significantly enhances the GFA. Samples from the system: xBi2O3-xNb2O5-(100-2x)TeO2 grow micro-inclusions or spherulites of size of several micron within the glass matrix on slow melt-cooling. Heat treatment of 7.5Bi2O3-7.5Nb2O5-85TeO2 sample show structural transitions from glass → anti-glass → crystalline phases.