Phase evolution of KLaF4 nanocrystals and their effects on the photoluminescence of Nd3+ doped transparent oxyfluoride glass-ceramics

Abstract

Transparent oxyfluoride glass-ceramics (GCs) were prepared starting from the glass composition 70SiO₂–7Al₂O₃–16K₂O–7LaF₃ (mol%) doped with four different Nd³⁺ concentrations (0.1–2 mol%). The glass-ceramics were prepared by heat treatment above the corresponding glass transition temperatures (T_g + 10–100 °C). Depending on the Nd³⁺ concentration, cubic (α-phase) and hexagonal (β-phase) KLaF₄ nanocrystals (NCs) precipitated from the starting glass. By X-ray diffraction (XRD) and high resolution transmission electron microscopy (HR-TEM), the crystallization of the cubic structure was observed for all compositions, while β-KLaF₄ nanocrystals were mostly detected for the glass-ceramic samples doped with 0.5 mol% NdF₃. This noteworthy result is confirmed by site-selective laser spectroscopy. The presence of α-KLaF₄ and β-KLaF₄ crystalline phases is unambiguously identified from the emission and excitation spectra and lifetime measurements of the ⁴F_3/2 state of Nd³⁺ ions. The spectroscopic results confirm those obtained with XRD and HR-TEM techniques: α-KLaF₄ NCs are found to be present for all NdF₃ concentrations whereas β-KLaF₄ NCs are predominant in the glass-ceramic samples doped with 0.5 mol%.