Rare-earth doped oxyfluoride glass ceramics represent a new generation of tailorable optical materials with high potential for optical-related applications such as optical amplifiers, optical waveguides, and white LEDs. Their key features are related to the high transparency and remarkable luminescence properties, while keeping the thermal and chemical advantages of oxide glasses. Sol-gel chemistry offers a flexible synthesis approach with several advantages, such as lower processing temperature, the ability to control the purity and homogeneity of the final materials on a molecular level, and the large compositional flexibility. The review will be focused on optical properties of sol-gel derived nano-glass ceramics related to the RE-doped luminescent nanocrystals (fluorides, chlorides, oxychlorides, etc.) such as photoluminescence, up-conversion luminescence, thermoluminescence and how these properties are influenced by their specific processing, mostly focusing on the findings from our group and similar ones in the literature, along with a discussion of perspectives, potential challenges, and future development directions.

In this study, we report on the structural, magnetic, and optical properties of Tb³⁺-doped CeF₃ nanocrystals prepared via a polyol-assisted route, followed by calcination. X-ray diffraction analysis and electron microscopy investigations have shown the formation of a dominant Ce_0.75F₃ nanocrystalline phase (of about 99%), with a relatively uniform distribution of nanocrystals about 15 nm in size. Magnetization curves showed typical paramagnetic properties related to the presence of Ce³⁺ and Tb³⁺ ions. The magnetic susceptibility showed a weak inflexion at about 150 K, assigned to the cerium ions’ crystal field splitting. Under UV light excitation of the Ce³⁺ ions, we observed Tb³⁺ green luminescence with a quantum yield of about 20%.

An europium doped BaO–B₂O₃–BaCl₂ chloroborate glass-ceramic containing a BaCl₂ nanocrystalline phase was produced by melt-quenching followed by glass crystallization during annealing. Structural and morphological investigations using x-ray diffraction and scanning electron microscopy have shown fvBaCl₂ nanocrystals of about tens of nm size accompanied by a smaller amount of the BaB₂O₄ crystalline phase. Photoluminescence spectra have indicated the reduction of Eu³⁺ to Eu²⁺ during processing in air or a reducing atmosphere. The spectra analysis showed the presence of Eu³⁺ ions in the borate glass matrix, while the Eu²⁺ were incorporated in both the BaCl₂ nanocrystals and glass matrix. Thermoluminescence properties were due to the recombination of F(Cl) centers and Eu²⁺ related hole centers produced by irradiation within the BaCl₂ nanocrystals. The color impression of the samples and the photoluminescence quantum efficiency were influenced by the glass processing.

The remarkable properties of Eu²⁺-activated phosphors, related to the broad and intense luminescence of Eu²⁺ ions, showed a high potential for a wide range of optical-related applications. Oxy-fluoride glass-ceramic containing Europium (II)-doped CaF₂ nanocrystals embedded in silica matrix were produced in two steps: glass-ceramization in air at 800° with Eu³⁺-doped CaF₂ nanocrystals embedded followed by Eu³⁺ to Eu²⁺ reduction during annealing in reducing atmosphere. The broad, blue luminescence band at 425 nm and with the long, weak tail in the visible range is assigned to the d → f type transition of the Eu²⁺ located inside the CaF₂ nanocrystals in substitutional and perturbed sites, respectively; the photoluminescence quantum yield was about 0.76. The X-ray photoelectron spectroscopy and Electron paramagnetic spectroscopy confirmed the presence of Eu²⁺ inside the CaF₂ nanocrystals. Thermoluminescence curves recorded after X-ray irradiation of un-doped and Eu²⁺-doped glass-ceramics showed a single dominant glow peak at 85 °C related to the recombination between F centers and Eu²⁺ related hole within the CaF₂ nanocrystals. The applicability of the procedure can be tested to obtain an oxy-fluoride glass-ceramic doped with other divalent ions such as Sm²⁺, Yb²⁺, as nanophosphors for radiation detector or photonics-related applications.

This study aims to obtain uniform and homogeneous bismuth germanate oxides thin films by spin coating and using the sol-gel technique with different precursors, followed by low-temperature annealing at 560 °C. By using Bi(NO₃)₃ precursors, we have obtained transparent, yellowish thin films with a 200 nm thickness. The structural analysis of the initial sol-gel powder has shown the presence of two crystalline structures, the cubic Bi₄Ge₃O₁₂ (BGO) and monoclinic Bi₂GeO₅ crystallites, which evolves towards the BGO structure after annealing. The elemental analysis confirmed the composition of the desired compound Bi₄Ge₃O₁₂ with 60 wt % GeO₂ and 40 wt % Bi₂O₅. On the other hand, by changing the precursor to (Bi(CH₃COO)₂, the film thickness increased to 500 nm thicker due to the high viscosity of the sol, and a dominant monoclinic Bi₂GeO₅ crystalline structure appeared. The elemental analysis revealed a nonstoichiometric composition with 38 wt % GeO₂ and 62 wt % Bi₂O₃. Due to the low GeO₂ phase content that reacted with metastable Bi₂GeO₅, we obtained cubic Bi₄Ge₃O₁₂ as a secondary phase, with Bi₂GeO₅ as a dominant crystalline phase. The redshifts of both absorptions and emissions spectra peaks confirmed a different disorder structure as an interplay between the cubic Bi₄Ge₃O₁₂ (BGO) and monoclinic Bi₂GeO₅ phases.

Glassy nanocomposites containing Yb³⁺/Er³⁺-doped GdF₃ and LiGdF₄ nanocrystals have been prepared by controlled crystallization of the xerogel and the structural, up-conversion luminescence, and magnetic properties were analyzed and discussed. Structural and morphological analysis showed uniform distribution of both GdF₃ and LiGdF₄ nanocrystals (tens of nm size), embedded in silica glass matrix as the result of thermal decomposition of the trifluoracetates, revealed as a strong exothermic peak at about 300 °C; the Li-ions co-doping showed a strong influence on the GdF₃ and LiGdF₄ nanocrystalline fraction. The energy dispersive spectrometry mapping showed Gd, F and Yb, Er within the nanocrystals but not in the silica glass matrix. X-ray diffraction pattern analysis indicated the crystalline lattice distortion consistent with the Yb/Er incorporation in both fluoride nanocrystals. The “green” ((²H_11/2, ⁴S_3/2) →⁴I_15/2) and “red” (⁴F_9/2→⁴I_15/2) up-conversion luminescences at 525, 545, and 660 nm observed under 980 nm laser light pumping were assigned to the Er³⁺ ions deexcitation through a two-photon process. The magnetic properties of the nanocomposite are strongly temperature dependent. The magnetization hysteresis loops show a ferromagnetic behavior at low temperatures (5K) related to the rare-earth ions contribution and the saturation magnetization of 39 emu/g. At 300 K a paramagnetic behavior was observed that was ascribed to the non-interacting localized nature of the magnetic moment of the rare-earth ions. Hence, such novel, multifunctional magnetic and optical materials can allow the intertwining between magnetism and photonics and might offer new opportunities for new magneto-optical device development.