L. V. Moiseeva

Glasses based on fluorides of metals of the I–IV groups: Synthesis, properties, and application

A review of the current studies on the synthesis, glass formation, structure, and optical properties of fluoride glasses in various glass-forming systems is presented. With a view to form and select active optical media for broad spectrum lasers, extensive studies are conducted to clarify the fluorescence mechanisms in fluoride glasses of various compositions doped with REE activators. Studies on obtainment of REE-activated fluoride transparent glass ceramics are discussed. Promising directions of practical application of such materials in optics and laser technology are shown.

Preparation and properties of heavy-metal halide glasses

We have studied the effect of In3+, Pb2+, Gd3+, and Cl (heavier ions) substitutions for Al3+, Ba2+, La3+, and F− on the crystallization stability and UV/IR optical properties of HBLAN fluorohafnatc glasses (HfF4-BaF2-LaF3-AlF3-NaF system). We obtained stable glasses containing InF3 and BaCl2 instead of AlF3 and BaF2, respectively, and offering increased IR transmission. The presence of CCl4 in the process atmosphere and the removal of oxygen-containing impurities via directional solidification are shown to have an advantageous effect on the optical quality of the glasses. The fluoride-chloride glasses are capable of accommodating about 1.5 times higher levels of rare-earth activators in comparison with their fluoride analogs.

Formation and crystallization of chlorine- and bromine-containing hafnium fluoride-based glasses

We have studied glass-forming regions for ZBLAN-type hafnium fluoride-based glasses with the compositions (63–53)HfF4 · 20BaF2 · 4LaF3 · (1–3)InF3 · (0–20)NaF in which fluorine anions are partially replaced by chlorine and bromine anions. The crystallization mechanism of the glasses has been investigated by differential thermal analysis and X-ray diffraction. The results demonstrate that characteristic features of the glasses are a significant decrease in glass transition temperature (tg) and the precipitation of fine-particle crystalline chloride and bromide phases at temperatures well below the crystallization temperature (tx) of their fluoride analogs. We have studied the effect of the Cl/F and Cl/Br ratios in the glass batch and the melt cooling rate after glass preparation on the crystallization behavior of the glasses and determined the heat treatment temperature and time for the formation of transparent glass-ceramic samples upon heat treatment in the temperature range where chloride and bromide phases precipitate. Doping of the glasses with 1 at % or smaller amounts of rare-earth metals is found to have no significant effect on the phase segregation process during heat treatment.

Synthesis and luminescence of fluorochloride glasses activated by Er3

Fluorochloride glasses activated by Er3+ and glass ceramics of 57HfF4 · 20BaCl2 · 3LaF3 · 3InF3 · 17NaF composition were synthesized. For the given glasses, Er3+ luminescence in the region of 1.55 μm was studied in comparison with fluoride glasses and glass ceramics of similar composition.

Optical properties of europium-activated hafnium fluoride-based glasses

We have studied the optical absorption, luminescence, and electron paramagnetic resonance of EuF3- and EuF2-activated fluorohafnate glasses. The glasses prepared with EuF2 contain both di- and trivalent europium. The fraction of divalent europium clusters in the glass host decreases with decreasing EuF2 concentration. Eu2+ luminescence in the fluorohafnate glasses is quenched, which is due the overlap of Eu2+ excited state levels with the conduction band of the glass, resulting in nonradiative relaxation through Hf3+ levels in the conduction band. The Eu3+ luminescence spectra contain lines corresponding to transitions from several levels of the 5D multiplet to levels of the 7F multiplet. The relationship between transitions from different 5D levels depends on europium concentration and temperature.

Glasses on the basis of heavy metal fluorides

In this review, the information on glass-forming fluoride systems is presented, and the main methods for synthesizing glasses on the basis of fluorides of the metals of Groups I–IV, their physicochemical properties, techniques for producing fibers and areas of application, and the techniques for purifying them from undesired impurities are discussed. Modern materials science studies in the area of fluoride glasses are aimed at searching for glasses activated with rare-earth elements (REEs) with a broad IR transmission range and high optical homogeneity with the purpose of creating efficient active optical media in a wide spectral range, as well as converters of IR radiation into the visible range for enhancing the efficiency of solar cells and exciting photocatalysts. The analysis of the influence of impurities on the optical transparency, crystallization, and phase purity of the glasses under study is presented. The methods for the deep purification of the initial substances for preparing fluoride glasses with the minimum content of impurities are considered. Original procedures for synthesizing fluorides of elements and glasses free from oxygen-containing impurities are created on the basis of physicochemical studies of the interactions of fluorinating agents with the components of fluoride glasses.

Synthesis and glass formation in the BaO-B2O3-BaCl2 system

We have studied synthesis, glass formation, and crystallization processes in the oxychloride barium borate system BaO-B2O3-BaCl2 and determined the highest chlorine concentrations at which glasses can be prepared by mold casting for Ba: 2B ratios in the ranges 1.0–2.2 and 0.2–0.5. The process atmosphere has been shown to have a significant effect on the glass composition because of the chlorine release from the melt. We have determined the characteristic temperatures of the glasses and identified the crystalline phases that precipitate during cooling of the melt and heat treatment of the glasses.

Crystallization behavior of hafnium fluoride-based fluorochloride glasses

The crystallization mechanism of chlorine-containing fluorohafnate glasses with the compositions (65–56)HfF4 · (20–x)BaF2 · xBaCl2 · 4LaF3 · (3–y)AlF3 · yInF3 · (5–18)NaF has been studied by X-ray diffraction and differential thermal analysis. The results demonstrate that characteristic features of the glasses are a decrease in glass transition temperature and the precipitation of fine-particle crystalline chloride phases at temperatures below the crystallization temperature of their fluoride analogs. We have studied the effect of the Cl/F ratio in the glass batch and the melt cooling rate on the crystallization behavior of the glasses and determined the heat treatment temperature and time for the formation of transparent glassceramic materials. The luminescence properties of Er3+-doped fluoride and fluorochloride glasses and glassceramics prepared from them have been investigated in the 1.5-µm range.

Scintillating Fluorohafnate Glasses

Data are presented on the roentgenoluminescence and gamma-radiation resistance of Ce3+-doped fluorohafnate glasses prepared under different redox conditions and containing different additions. The glasses are shown to contain several types of luminescence centers and radiation-induced color centers, whose concentrations are governed by the preparation conditions and the nature of the dopants. The radiation resistance and light output of the glasses can be notably enhanced by optimizing the preparation conditions and introducing certain additions.

Eu2+ and Gd3+ Distributions in Fluorohafnate Glasses Studied by Electron Paramagnetic Resonance

We have studied electron paramagnetic resonance spectra of europium- and gadolinium-activated ZBLAN-type fluorohafnate glasses, using the composition 58HfF4 · 20BaF2 · 2LaF3 · 3AlF3 · 17NaF as an example. The ratio of the concentration of free Eu2+ and Gd3+ ions to that of ions in clusters has been quantitatively evaluated for the first time. The percentage of free ions has been shown to increase significantly with decreasing activator concentration. At activator concentrations of 1.25 mol % EuF2 and 1 mol % GdF3, the activator ions predominantly form clusters and only a small fraction of Eu2+ and Gd3+ are present as individual ions, whereas at 0.1 mol % EuF2 the concentration of free ions is comparable to that of ions in the clusters.