The effect of MgO/TiO₂ on structural and crystallization behavior of near invert phosphate-based glasses

Varying formulations in the glass system of 40P₂ O₅ ─(24 - x)MgO─(16 + x)CaO─(20 - y)Na₂ O─yTiO₂ (where 0 ≤ x ≤ 22 and y = 0 or 1) were prepared via melt-quenching. The structure of the glasses was confirmed by X-ray diffraction (XRD), Fourier transform infrared (FTIR), micro Raman and solid-state nuclear magnetic resonance (NMR) spectroscopies. The thermal properties and the activation energy of crystallization (E_c ) were measured using thermal analysis and the Kissinger equation, respectively. The glass forming ability of the formulations investigated was seen to decrease with reducing MgO content down to 8 mol% and the glass stability region also decreased from 106 to 90°C with decreasing MgO content. The activation energy of crystallization (E_c ) values also decreased from 248 (for 24 mol% MgO glass) to 229 kJ/mol (for the 8 mol% MgO content) with the replacement of MgO by CaO for glasses with no TiO₂ . The formulations containing less than 8 mol% MgO without TiO₂ showed a strong tendency toward crystallization. However, the addition of 1 mol% TiO₂ in place of Na₂ O for these glasses with less than 8 mol% MgO content, inhibited their crystallization tendency. Glasses containing 8 mol% MgO with 1 mol% TiO₂ revealed a 12°C higher glass transition temperature, a 14°C increase in glass stability against crystallization and a 38 kJ/mol increase in E_c in comparison to their non TiO₂ containing counterpart. NMR spectroscopy revealed that all of the formulations contained almost equal percentages of Q¹ and Q² species. However, FTIR and Raman spectroscopies showed that the local structure of the glasses had been altered with addition of 1 mol% TiO₂ , which acted as a network modifier, impeding crystallization by increasing the cross-linking between phosphate chains and consequently leading to increased E_c as well as their glass forming ability.