Isothermal crystallization and effect of soak time on phase evolution, microstructure and ionic conductivity of Li2O–Al2O3–TiO2–P2O5 glass–ceramic

Glass–ceramics derived from Li ₂O–Al ₂O ₃–TiO ₂–P ₂O ₅ vitreous system were obtained by isothermal heat treatment at 700 °C, for different soak times ranging from 0 to 60 h. Phase analysis revealed mixture of amorphous and crystalline phases for short soak times (< 1 h). By 1 h of soak time, crystallization was complete and phases were ascribed to dominant Li _{1 + x}Al _xTi _{2 − x}(PO ₄) ₃ (LATP) along with secondary AlPO ₄ and TiO ₂ , previously reported. Microstructural evolution with soak time was imaged using high resolution scanning electron microscope (SEM). Crystalline volume fraction in glass–ceramics was estimated from image analysis and it followed Kolmogorov–Johnson–Mehl–Avrami (KJMA) equation. Conductivity of glass–ceramics was derived using impedance spectroscopy data for all soak times and correlated to the crystalline volume content. After attaining a critical volume of crystalline phase/s, conductivity increased significantly and was observed to follow the scaling equation σ _dc ~ (V − V _c) ^β, where σ _dc is dc conductivity, V is crystalline volume content, V _c is threshold value of crystalline volume content above which sudden increase in conductivity was observed and β is scaling exponent. V _c ≈ 30% was in accordance with percolation scaling law and is attributed to formation of 3D conducting channel network in the glass–ceramics microstructure.