Fig. 1. Glass forming regions of La- and Nb-doped BT2. Open circles, half-filled circle and crosses represent glasses, partially crystallized glass and crystals, respectively. The inset figures show the digital camera photos of the glass spheres with different compositions.

Fig. 2. The composition dependence of density, ρ, and molar volume, VM, of (a) La-doped BT2 and (b) Nb-doped BT2 glasses. The lines through the symbols are the linear fittings of the data.

Fig. 3. (a) DTA curves for La-doped BT2 glasses. The inset figure shows the expanded view of DTA curve for x = 0 in the rectangle showing the glass transition temperature. (b) The glass-transition temperature Tg, the first crystallization temperature Tx1, and the glass thermal stability ΔT (ΔT =Tx1-Tg, inset figure) as a function of the La-doping level, x.

Fig. 4. (a) DTA curves for Nb-doped BT2 glasses. (b) The glass-transition temperature Tg, the first crystallization temperature Tx1, and the glass thermal stability ΔT (ΔT =Tx1-Tg, inset figure) as a function of the Nb-doping level, y.

Fig. 5. (a) Raman spectra of La-doped BT2 glasses. The inset figure shows the deconvolution of the Raman spectrum of BT2 in the range between 130 and 950 cm-1 using four Lorentz peaks. (b) Variation of the wavenumber of each peak in Region I, II and III as a function of La-doping level, x. (c) Fraction of the [TiOn] polyhedra calculated based on the peak area from spectrum deconvolution. The dash lines through the symbols in (b) and (c) shows the trend.

Fig. 6. (a) Raman spectra of Nb-doped BT2 glasses. (b) Variation of the wavenumber of each peak in Region I, II and III as a function of Nb-doping level, y. The solid lines through the symbols show the trend in the composition range 0 ≤ y ≤ 0.20, and the vertical dash line indicates the critical composition y = 0.20. (c) Fraction of the [TiOn] polyhedra calculated based on the peak area from spectrum deconvolution. The dash lines through the symbols in (b) and (c) show the trend.