T. I. Chuvaeva

Radiative properties of Nd-doped transparent glass-ceramics in the lithium aluminosilicate system

Abstract We have studied the optical (absorption, luminescence) properties of Nd3+-doped transparent glass-ceramics obtained in the lithium aluminosilicate system. Phase composition of the glass-ceramics has been determined to affect their luminescent characteristics. We have found out that concentration quenching of neodymium fluorescence decreases upon recrystallization of β-eucryptite solid solutions (SS) into β-spodumene ones. The effect is due to partitioning of the rare-earth ions into the crystalline phase. For glass-ceramics nucleated with titania we have shown using the Judd–Ofelt theory that introduction of P2O5 in the glass-ceramic composition has a favorable effect on radiative parameters increasing noticeably the stimulated emission cross-section for the transition 4 F 3/2 → 4 I 11/2 . The influence of crystalline phase value fraction on quantum efficiency estimated from fluorescence decay measurements has been elucidated as well. It has been shown that reasonable values of the fluorescence quantum yield can be obtained provided that thermal shock resistance of materials remains sufficiently high. We have also carried out laser operation testing in a setup with flash lamp pumping. The observed spread in values of lasing characteristics can be attributed to insufficient optical quality of materials due to phase separation effects.

Structural states of Ni(II) in glasses and glass-ceramic materials of the lithium-aluminium-silicate system

Abstract Optical absorption spectra of Ni(II) have been investigated in glass-ceramic materials of the lithium-aluminium-silicate system at various phase transition stages. Four types of absorption spectra have been revealed. The Ni(II) absorption bands have been attributed based on the calculation of the energy level diagram. It has been shown that in the case of heat treatment at the initial stage of phase decomposition, there arise aluminium titanium nickel regions that decompose in subsequent heating into titanium phases and aluminium-nickel spinel microcrystals. In crystallizing the main phase, i.e. solid solutions of β-eucryptite, Ni(II) is built into the structure of this phase replacing lithium isomorphously.

On the phase separation and crystallization of glasses in the MgO-Al2O3-SiO2-TiO2 system

The changes in the structure and phase composition of glasses in the MgO–Al2O3–SiO2–TiO2 system (at different TiO2 contents and ratios MgO : Al2O3) upon their heat treatment in the temperature range 700–960°C are investigated by small-angle X-ray scattering (SAXS) technique and X-ray powder diffraction analysis. The influence of gallium oxide additives on the phase separation and crystallization is analyzed. It is demonstrated that the heat treatment results in the phase separation, which occurs through the spinodal decomposition mechanism. A regular structure formed upon phase separation is retained after the completion of crystallization in inhomogeneity regions. The interference effects due to the regularity in the distribution of nanocrystals in the vitreous matrix bring about a decrease in the light scattering intensity and provide transparency of glass-ceramic materials.

A Raman Spectroscopic Study of Phase Transformations in Titanium-Containing Magnesium Aluminosilicate Glasses

The evolution of the Raman spectra of the 20MgO · 20Al2O3 · 60SiO2 glass with 10 mol % TiO2 and glasses doped with 0.5 wt % NiO or 2.5 and 5.0 mol % Ga2O3 is investigated in the course of heat treatment resulting in phase separation and crystallization. The composition and structure of the amorphous and crystalline phases (precipitating in the course of phase separation) and the residual glass are analyzed on the basis of Raman scattering, small-angle X-ray scattering (SAXS), and X-ray powder diffraction data. The size of nanoparticles of the precipitated phases is determined by low-frequency Raman spectroscopy, and the results obtained are compared with the nanoparticle size derived from the SAXS data.

Low-frequency Raman scattering of magnesium aluminosilicate glasses and glass-ceramics

Abstract The evolution of low-frequency ( −1 ) Raman scattering of titania-containing magnesium aluminosilicate (20MgO:20Al 2 O 3 :60SiO 2 with 10 mol% TiO 2 ) glasses with heat-treatment has been studied. It was found that in spectra of heat-treated glasses the intensity of the boson peak is rapidly diminished. Under certain conditions of heat-treatment schedules the boson peak disappears while the sample remains amorphous. It implies that sometimes the boson peak is missing from spectra of amorphous materials. In Raman spectra of heat-treated glasses that show the disappearance of the boson peak a new narrow highly polarized low-frequency band appears and increases in intensity. This band has been associated with a spheroidal mode of elastic vibrations in nanometer sized particles of aluminomagnesium phase formed upon phase decomposition of the glass. Its frequency changes with heat-treatment in the temperature range 750–950°C, from 50 to 17 cm −1 reflecting an increase in the sizes of corresponding nanometer particles. The composition and structure of amorphous and crystalline phases formed upon heat-treatment and also of residual glass as well were analyzed using the Raman scattering data, small angle X-ray scattering (SAXS) and X-ray diffraction (XRD) analysis.

Structural transformations of nanometer sized crystals in CoO-doped β-eucryptite-based glass-ceramics

Abstract Structural transformations of nanometer sized crystals in CoO-doped β-eucryptite-based glass-ceramics were studied by optical spectra and X-ray diffraction analysis. Four types of Co(II) absorption spectra were revealed. In the course of amorphous phase separation Co(II) ions enter into the aluminotitanate amorphous phase. Nanometer sized crystals of CoAl 2 O 4 , TiO 2 and CoTiO 3 precipitate at elevated temperature. The temperature reversible order–disorder phenomenon was revealed in spinel crystals. Nanometer sized disordered crystals of β-eucryptite solid solution precipitate after heat treatment at 720°C. At a crystallization temperature higher than 900°C, an ordering of these crystals takes place which is confirmed by the absorption spectra and the temperature dependence of lattice parameters of the unit cell.

The structure of luminescence centers of neodymium in glasses and transparent glass-ceramics of the Li2O-Al2O3-SiO2 system

Abstract The structure of luminescence centers of Nd3+ in lithium aluminosilicate glasses and glass-ceramics has been studied. A correlation was obtained between spectroscopic parameters, the character of phase transformations in the course of conversion to glass-ceramics and the precipitated phases. In crystallization of β-eucryptite solid solutions, the greater part of Nd3+ ions remains in a residual glassy phase. Under re-crystallization of these solid solutions into β-spodumene solid solutions, Nd3+ ions enter the β-spodumene structure. The conclusion was drawn regarding the chemical properties of the second coordination sphere of the Nd3+ ions.

Structural states of Co(II) in β-eucryptite-based glass-ceramics nucleated with ZrO2

Abstract The structure of a Co(II) ion lithium aluminosilicate glass and glass-ceramics nucleated with ZrO 2 was studied by optical spectra and X-ray diffraction analysis. Three types of Co(II) absorption spectra were revealed. A type I spectrum arises from the absorption of six-coordinated and four-coordinated Co(II) in an initial glass. A type II spectrum appears when, in the course of crystallization of β-eucryptite solid solution, Co(II) ions enter into microcrystals of this phase, mainly, in sixfold positions, isomorphously substituting for Li(I) ions. At crystallization temperature higher than 950°C, the ordering of microcrystals of β-eucryptite solid solution takes place, which leads to Co(II) coordination change to fourfold positions, which leads to a type III spectrum. The Co(II) structural change in β-eucryptite solid solution was confirmed by the temperature dependence of lattice parameters of the unit cell.

Crystallization Kinetics of Sodium-Niobium Silicate Glasses

The crystallization of glasses in the Na2O-Nb2O5-SiO2 system in the temperature range 690–750°C is investigated by measuring the material density and X-ray powder diffraction analysis. It is found that the kinetics of precipitation of the NaNbO3 crystalline phase can be described by the Avrami equation with the Avrami exponentn ≈ 1. The model describing the crystallization kinetics is proposed. The crystallization of amorphous inhomogeneity regions, which are formed as a result of phase separation, is considered in the model. The consideration refers to the crystallization process governed by the nucleation of crystals in these regions. The calculated Avrami exponents are close to unity. The proposed model does not contradict the available data on the X-ray small-angle scattering that have been obtained upon crystallization of the samples and qualitatively explains a nonmonotonic variation in the material transparency during crystallization.

Kinetics of the Ostwald ripening in lithium aluminosilicate glasses containing TiO2 and ZrO2

A structural investigation is undertaken on lithium aluminosilicate glasses containing a titanium oxide or titanium and zirconium oxides. The structural transformations in the course of Ostwald ripening observed after the increase in temperature of the glasses preliminarily heat-heated to the attainment of a stable two-phase structure (catalyst phase) are studied by the small-angle X-ray scattering (SAXS) technique. In all the cases, an increase in temperature leads to the loss of stability and a partial dissolution of the phase precipitated at a lower temperature. The low-temperature heat treatment (660°C) of lithium aluminosilicate glasses containing TiO2 and ZrO2 brings about the precipitation of aluminotitanate phase that reaches the stable state upon prolonged heat treatments for 5000–6000 h. As the temperature increases, this phase becomes unstable, which results in its dissolution and the precipitation of a new phase containing titanium and zirconium oxides in the form of zirconium titanate microcrystals. Changes in the phase composition in the course of Ostwald ripening upon isothermal treatments of glasses containing TiO2 and ZrO2 are associated with different mobilities of structural elements involving titanium or zirconium ions.