Amorphous nanostructuring in potassium niobium silicate glasses by SANS and SHG: a new mechanism for second-order optical non-linearity of glasses

Potassium niobium silicate (KNS) glasses xK₂OxNb₂O₅(1−2x)SiO₂ with x=0.167; 0.182; 0.200; 0.220 and 0.250 have been subjected to prolonged heat treatments in a wide temperature range above T_g. As a result, glasses exhibiting liquid-type phase separation phenomena have been isolated. Moreover for each glass composition, the temperature zones have been determined to produce transparent, opalescent or opaque materials which have been studied by X-ray diffraction (XRD), small-angle neutron scattering (SANS) and second harmonic generation (SHG) techniques. SANS data unambiguously point at nanostructuring of KNS glasses in the scale of 5-20 nm under appropriate heat treatments near T_g. In contrast to initial KNS glasses, nanostructured glasses exhibit SHG activity. For earliest stages of phase separation SHG-active glasses are characterized by fully amorphous XRD patterns. Further development of phase separation in glasses with increasing of their opalescence leads to diminishing SHG, and subsequently partial crystallization takes place giving opaque materials. Since relative maximum of SHG efficiency corresponds to non-crystalline nanostructured glasses, such new transparent second-order non-linear media may be of both scientific and practical interest. With regard to non-crystalline structure of nano-inhomogeneities, SHG mechanism in the glasses is supposed to be due to a combination of third-order non-linearity with a spatial modulation of linear polarizability.     

Nanosized structural transformation and nonlinear optical properties of lithium niobium germanate glasses

Glass formation in Li₂O-Nb₂O₅-GeO₂ (LNG) system, the structure and crystallization behavior of glasses that have compositions near the ratio Li₂O/Nb₂O₅ ∼ 1 corresponding to stoichiometry of ferroelectric phase LiNbO₃ were examined by differential thermal analysis, X-ray diffraction, small-angle neutron scattering and second harmonic generation (SHG). LNG glasses were subjected to heat treatments at temperatures in the range between T_g and temperature of the first exothermic peak in order to initiate nonlinear optical activity by nanoheterogeneity formation. Transparent nanostructured glasses with second-order optical nonlinearity were obtained for compositions characterized by the Li₂O/Nb₂O₅ molar ratio ranging from 0.83 to 1.2 and GeO₂ 40-45 mol%. As prolonged heat treatments of nanostructured glasses result in crystallization of ferroelectric LiNbO₃ the origin of SHG in transparent LNG glasses is supposed to be connected predominantly with polarity of nanoheterogeneities formed at the initial stage of phase separation.     

The origin of nanostructuring in potassium niobiosilicate glasses by Raman and FTIR spectroscopy

The structure of potassium niobium silicate glasses in a wide compositional range has been studied by means of Raman and FTIR spectroscopy. The glasses spectra were compared to those of the KNbSi₂O₇ and K₃Nb₃O₆Si₂O₇ polycrystalline samples, obtained by crystallization of glasses of the same composition. It was found that the structure of such glasses is formed by SiO₄ tetrahedra and distorted NbO₆ octahedra. The amount of highly distorted (edge-sharing, non-bridging oxygens) octahedra results essentially unchanged from the glass composition. By contrast, the fraction of octahedra with a lower distortion degree (corner-sharing, bridging oxygens) increases with the Nb₂O₅ content. Raman and FTIR investigations indicate that during long heat treatments at temperatures near T_g, in the 23K₂O · 27Nb₂O₅ · 50SiO₂ glass, a structural change occurs regarding the amorphous matrix with a decrease of the niobium octahedra distortion. This can be related to a segregation process producing niobium rich regions nanometric in size. In the first heat treatment (2 h) the glass remains amorphous while for more prolonged heat treatments, nanocrystals of an unidentified phase are formed. In the same time the changes of the amorphous matrix hinder further crystallization.     

Second-order optical non-linearity initiated in Li2O–Nb2O5–SiO2 and Li2O–ZnO–Nb2O5–SiO2 glasses by formation of polar and centrosymmetric nanostructures

Amorphous nanoheterogeneities of the size less than 100 Å have been formed in glasses of the Li₂O–Nb₂O₅–SiO₂ (LNS) and Li₂O–ZnO–Nb₂O₅–SiO₂ (LZNS) systems at the initial stage of phase separation and examined by transmission electron microscopy, small-angle X-ray and neutron scattering. Both LNS and LZNS nanoheterogeneous glasses exhibit second harmonic generation (SHG) even when they are characterized by fully amorphous X-ray diffraction (XRD) patterns. Chemical differentiation and ordering of glass structure during heat treatments at appropriate temperatures higher T_g lead to drastic increase of SHG efficiency of LNS glasses contrary to LZNS ones in the frame of amorphous state of samples. Following heat treatments of nanostructured glasses result in crystallization of ferroelectric LiNbO₃ and non-polar LiZnNbO₄ in the LNS and LZNS glasses, respectively. Taking into account similar polarizability of atoms in LNS and LZNS glasses, the origin of the principal difference in the second-order optical non-linearity of amorphous LNS and LZNS samples is proposed to connect predominantly with the internal structure of formed nanoheterogeneities and with their polarity. Most probably, amorphous nanoheterogeneities in glasses may be characterized with crystal-like structure of polar (LiNbO₃) phase initiating remarkable SHG efficiency or non-polar (LiZnNbO₄) phase, which do not initiate SHG activity. It gives an opportunity to vary SHG efficiency of glasses in a wide rage without remarkable change of their transparency by chemical differentiation process at the initial stage of phase separation when growth of nanoheterogeneities is ‘frozen’. At higher temperatures, LiNbO₃ crystals identified by XRD precipitate in LNS glasses initiating even more increase of SHG efficiency but visually observable transparency is impaired.     

Structure and crystallization of potassium titanium phosphate glasses containing B2O3 and SiO2

Transparent glasses composition of which can be expressed by the formula: (100−x) · (K₂O · 2TiO₂ · P₂O₅) · x(K₂O · 2B₂O₃ · 7SiO₂), where x=5, 10, 15 and 20 mol% (KTP-xKBS), were obtained by melt quenching technique. The structure and crystallization behavior of these glasses have been examined by Fourier transform infrared spectroscopy, differential thermal analysis and X-ray diffraction. In spite of their nominal composition, the studied glasses exhibit a similar oxygen polyhedra distribution. However, significant differences were found in the trigonal BO₃ units amount. During DTA runs all the examined glasses devitrify in two steps. In the former, very small crystals of an unknown crystalline phase are produced. In KTP-5KBS and KTP-10KBS glasses anatase phase was also detected. Attempts were made in order to identify the unknown phase (UTP) for which a AB₃(XO₄)₂(OH)₆ Crandallite-type structure was proposed where the A, B and X sites were occupied by K, Ti and/or Al, and P, respectively. In the second devitrification step the crystallization of the KTiOPO₄ phase occurs while the UTP phase previously formed disappears. Isothermal heat treatments performed at temperature just above T_g have allowed one to obtain transparent crystal-glass nanocomposites, formed by crystalline nanostructure of the UTP phase uniformly dispersed in the amorphous matrix.     

Control of the thermal properties of slow bioresorbable glasses by boron addition

This work studied the properties of glasses with the molar composition 63.8SiO₂-(11.6-x)Na₂O-(0.7 + x)B₂O₃-19.2CaO-3MgO-1.5Al₂O₃-0.2P₂O_5, in which x = 0, 1, 2, 3. These glasses are of interest for the development of slowly dissolving fibers to be incorporated in composites for medical applications. The thermal properties were recorded using hot stage microscopy, differential thermal analysis, and heat treatments in the range of 800°-1000 °C. The glass crystallization behavior was determined based on calculated values of the activation energy of crystallization and the Johnson-Mehl-Avrami exponent. The structural units in the glass network were identified using infrared spectroscopy. Finally, in vitro dissolution was tested in SBF solution.The addition of B₂O₃ increased the glass transition temperature and reduced the working temperature. When heat treated at 900 °C, the glass with the smallest amount of B₂O₃ formed two crystalline phases: magnesium silicate MgSiO₃ and wollastonite CaSiO₃. In the other compositions, only CaSiO₃ was observed after heat treatment at 950 °C. All the glasses crystallized preferentially from the surface. Changes in the liquidus and crystallization temperatures were related to changes in the glass structure. The formation of [BO₃] units led to glasses with improved resistance to crystallization and decreased liquidus temperature. In the glasses with 2.7 and 3.7 mol% B₂O₃, [BO₃] units were transformed into [BO₄] units. The formation of [BO₄] led to an increase in fragility and a decrease in resistance to crystallization. All the glasses dissolved slowly in simulated body fluid.     

NMR studies on rapidly solidified SiO2Al2O3 and SiO2Al2O3Na2O-glasses

²⁷Al and ²⁹Si MAS NMR studies were performed on roller-quenched SiO₂Al₂O₃-glasses with Al₂O₃ contents ranging from 10 to 60 mol% and on SiO₂Al₂O₃Na₂O glasses containing 10 mol% Al₂O₃ and 2.5 to 10 mol% Na₂O. Pure aluminium silicate glasses show NMR peaks at 0, 30 and 60 ppm. The frequency distribution of the different Al-sites is not affected by the glass composition. In glasses of the system SiO₂Al₂O₃Na₂O the 30 ppm peak decreases to zero as the Na₂O content increases. The 30 ppm peak is assigned to distorted triclustered AlO-tetrahedra, rather than to fivefold coordinated Al. Triclustering of tetrahedra may provide for charge neutrality in glasses with molar excess of Al₂O₃ over Na₂O. As charge balance is increasingly achieved by addition of alkali ions, the tendency of tetrahedral triclustering is reduced, reflected by the disappearance of the 30 ppm peak in glasses containing ≥ 7.5 mol% Na₂O.     

Structure and non-linear optical properties of BaO–TiO2–SiO2 glass containing Ba2TiSi2O8 crystal

The BaO–TiO₂–SiO₂ crystallized glass containing Ba₂TiSi₂O₈ crystal phase was examined using X-ray diffraction (XRD) measurement and Maker fringe technique. It has been found that the crystallized glasses with a composition of close to the stoichiometric Ba₂TiSi₂O₈ showed bulk crystallization, whereas other crystallized glasses with non-stoichiometric composition of Ba₂TiSi₂O₈ showed surface crystallization. The parameter ΔT, the difference in temperature between the crystallization onset and the glass transition temperature, strongly affects the crystallization behaviors of the present BTS glasses. The 30BaO–20TiO₂–50SiO₂ transparent surface crystallized glass with strong c-oriented Ba₂TiSi₂O₈ phase showed its second-order non-linear optical constant, d₃₃ = 6.1 pm/V.     

Influence of CoO addition on phase separation and crystallization of glasses of the ZnO–Al2O3–SiO2–TiO2 system

The evolution of structure, phase composition and spectroscopic properties of CoO-doped (up to 5 mol%) titania-containing zinc aluminosilicate glasses with their heat-treatment has been studied using Raman scattering, small angle X-ray scattering, X-ray diffraction analysis and optical absorption spectra. Addition of cobalt oxide was observed to facilitate amorphous phase separation of the parent glass and gahnite, ZnAl₂O₄, crystallization. Cobalt oxide entered phases formed during low-temperature heat-treatments (720 °C), i.e., amorphous phase, enriched in ZnO, Al₂O₃ and TiO₂ and crystalline phase of gahnite. The absorption of these glass-ceramics was defined mainly by tetrahedral Co²⁺ ions located in gahnite nanocrystals. As the temperature was increased further, traces of anosovite solid solution appeared and then decomposed. Even after high-temperature heat-treatments, a certain portion of Co²⁺ ions remained in amorphous zinc aluminotitanate phase and in octahedral sites of inversed gahnite spinel. In glass-ceramics, the residual high silica amorphous phase contained a small quantity of [TiO₄] centers, which content was smaller in Co:ZAS samples as compared with non-doped glass-ceramics.     

Structural investigations of nitrided Nb2O5 and Nb2O5–SiO2 sol–gel derived films

Sol–gel derived xNb₂O₅–(100 ? x)SiO₂ films (where x = 100, 80, 60, 50, 40, 20, 0 mol%) were nitrided at various temperatures (800 °C, 900 °C, 1000 °C, 1100 °C and 1200 °C). The structural transformations occurring in the films as a result of ammonolysis were studied using X-ray diffraction (XRD), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The XRD results have shown that the temperatures below 1100 °C were too low to obtain a pure NbN phase in the samples. The AFM observations indicate that the formation of the NbN phase and the size of NbN grains are related to the silica content in the layer. NbN grains become more regular and larger as the niobium content increases. The maximum grain size of about 100 nm was observed for x = 100. Preparation of the Nb₂O₅–SiO₂ sol–gel derived layers and the subsequent nitridation is a promising method of inducing crystalline NbN in amorphous matrices. It follows from the XPS results that a small amount of Nb₂O₅ remains in the films after nitridation at 1200 °C and that nitrogen reacted not only with Nb₂O₅ but also with SiO₂.     

Crystallization of 2(Ca,Sr,Ba)O-TiO2-2SiO2 composition glasses

Crystallization was examined for glasses having chemical composition of 2(Ca,Sr,Ba)O-TiO₂-2SiO₂ in which the CaO/SrO/BaO molar ratio varied. Powdered glass samples were pelleted into disks and sintered at 950 °C for 2 h. The major phase precipitated in the sintered samples was (Ca,Sr,Ba)₂TiSi₂O₈ and minor phase of perovskite such as CaTiO₃ or SrTiO₃ increased with CaO content in the samples containing more than 40 mol% of CaO in total CaO+SrO+BaO. Three regions having different slopes were found in linear relationships between SrO mol% and exothermal peak temperature on DSC curves or d[0 0 2] values determined by powder XRD method. These facts suggested that the major phase precipitated in each region was a solid solution containing a different amount of CaO, SrO, BaO and that these compositions varied depending on SrO content in the sample. The micro-crystalline structure, which could be useful in fabricating a dielectric dense body, was observed for samples containing 30-70 mol% of SrO.     

The crystallization mechanism in high strength glass-ceramics

The addition of NiO to a MgO Al₂ Al₂O₃ SiO₂ basic glass has an influence on the phase separation and crystallization mechanism. The differentiation of the components already detectable in the amorphous state continues in the crystallization process of the nickel-rich glasses. The formation of Ni Al-spinel in the bulk restricts the precipitation of high quartz crystals to the skin. The result is a high strength glass-ceramic with clearly distinguishable bulk and surface regions.     

Glass formation and structure of glasses in the ZnO―Bi2O3―WO3―MoO3 system

The glass formation region in the ternary ZnO―Bi₂O₃―WO₃ system is determined by melt quenching technique (cooling rates 10¹-10² K/s). New original glasses are obtained in a narrow concentration range with high WO₃ content (60-75 mol%). Homogeneous glasses of the composition (100 − x)[0.2ZnO·0.3Bi₂O₃·0.5WO₃]xMoO₃, were obtained between 20 and 60 mol% MoO₃. Characterization of the amorphous samples was made by x-ray diffraction (XRD), differential thermal analysis (DTA) and infrared spectroscopy (IR). The thermal stability of glasses decreases with the increasing of MoO₃ content. The glass transition temperature, T_g, varies between 340-480 °C, while the crystallization temperature, T_x, varies between 388-531 °C. The tungstate glasses possess higher crystallization temperature (T_x over 500 °C) in comparison with the other vanadate and molybdate non-traditional glasses. The glass network is realized by transformation of three-dimensional structure of WO₃ into a layered one, consisting mainly of WO₆ units. We supposed that the network of quaternary glasses is built up by MoO₄, MoO₆ and WO₆. At low concentration ZnO and Bi₂O₃ facilitate the disorder in the supercooled melts, while at high concentration stimulate crystallization processes. These oxides belong to the intermediate ones.     

Dielectric analysis of tungsten-phosphoniobate 20A2O-30WO3 -10Nb2O5 -40P2O5 (A = Li, Na) glass-ceramics

New phosphate glasses of the quaternary system A₂O-Nb₂O₅-WO₃-P₂O₅, where X = Li and Na were prepared by the melt-quenching method. The introduction of WO₃ in the glass composition was based on the proposal of analysing the effect of the diminishing of the molar amount of the alkaline oxide and thus decreasing the molar ratio between network modifiers and network formers (M/F).In the present work we present the preparation of 20A₂O-30WO₃-10Nb₂O₅-40P₂O₅ (A = Li, Na) transparent glasses. These glasses were heat-treated in air, at 550 °C and 650 °C for 4 h. The structure of the obtained samples was studied by X-ray powder diffraction (XRD) and Raman spectroscopy and the morphology by scanning electron microscopy (SEM). The dc (σ_dc), ac (σ_ac) conductivity and dielectric spectroscopy measurements were performed in the function of the temperature and were related with the structural changes of the glass structures.     

Evaluation of chemical durability,thermal properties and structure characteristics of Nb–Sr-phosphate glasses by Raman and NMR spectroscopy

Thermal properties, water durability and structure of Nb₂O₅–SrO–P₂O₅ glasses containing 0–25 mol% Nb₂O₅ and 35–60 mol% SrO were explored aiming to develop high refractive index optical glasses. Structure studied using Raman and NMR spectra reveals that by increasing Nb₂O₅ content, niobium plays the role as intermediate. Nb⁵⁺ tends to break P–O–P and O–P–O bonds forming [NbO₆] structure. Thus fractions of Q³ and Q² decrease, while Q¹ fraction increases. Furthermore the Q⁰ fraction replaces the lessened Q³ fraction. As P₂O₅ content is reduced to 30 mol%, partial [NbO₆]_octa turns into [NbO₄]_tetra and partial (Nb–O)_short-octa becomes (Nb–O)_short-tetra bond to stabilize the glass structure. Glass-transition and softening-temperatures of the glasses increase by increasing SrO and Nb₂O₅ contents. Thermal expansion coefficient increases by increasing SrO while decreases with Nb₂O₅ content. Water durability is enhanced as increasing Nb₂O₅ and SrO contents. Properties of the glasses correlate well with the worked out structure.     

Influence of various alkali and divalent metal oxides on phase transformations in NiO-doped glasses of the Li2O–Al2O3–SiO2–TiO2 system

Regularities of phase transformations in glasses of the Li₂O–Al₂O₃–SiO₂–TiO₂ system doped with up to 2.5 mol% of alkali- and divalent metal oxides were studied by X-ray diffraction analysis, Raman scattering and optical spectroscopy. Ni(II) ions were used as spectral probes of phase transformations because Ni(II)-ions enter the inhomogeneous regions formed during the phase separation and crystallization, and their absorption spectra change with heat-treatment temperature reflecting formation of aluminotitanate amorphous regions, spinel nanosized crystals and β-quartz solid solutions, consequently.It was demonstrated that the technological additives do not change the sequence of the phases' formation but accelerate the liquid phase separation and crystallization. Addition of MgO and ZnO leads to increasing the temperature range of spinel precipitation. Addition of CaO, BaO and PbO results in increasing the light scattering of prepared glass-ceramics.In selection of the technological additives for decreasing the melting temperature of glass-ceramics for optical and photonic applications the influence of the additives on the structure and optical properties of the prepared material should be considered.     

Effect of BaO on the crystallization kinetics of glasses along the Diopside-Ca-Tschermak join

We report on the effect of BaO on the crystallization kinetics of glasses in the diopside (CaMgSi₂O₆)-Ca-Tschermak (CaAl₂SiO₆) system. Partial substitution (i.e. 5%, 10% and 20%) of Ba²⁺ for Ca²⁺ was attempted in composition CaMg_0.8Al_0.4Si_1.8O₆, in three different glasses while partial substitution of B³⁺ for Al³⁺ was made in the fourth glass. Structural investigations on the glasses have been made by density measurements, molar volume and Infra-red spectroscopy (FTIR). Non-isothermal crystallization kinetic studies have been employed to study the mechanism of crystallization in all the four glasses. The Avrami parameter for the glass powders is ∼2, indicating the existence of intermediate mechanism of crystallization. Crystallization sequence in the glasses has been followed by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and FTIR. Augite crystallized out being the dominant phase in all the glass-ceramics, while different polymorphs of BaAl₂Si₂O8 were present as secondary or minor phases.     

Electric conductivity of glasses in the system Na2O/CaO/SiO2/Fe2O3

The addition of polyvalent transition metal ions to the usually insulating traditional soda-lime-silica glasses can lead to semiconducting properties. We report on synthesis of glasses and glass-ceramics in a soda-lime-silicate based system containing Fe₂O₃ in the concentration range from 5 to 30 mol%. Two sub-systems were considered, in one of them the ratio [Na₂O]/[Fe₂O₃] was varied while in the other one, the ratio [SiO₂]/[Fe₂O₃] was changed. The phase composition of the synthesized products was characterized by X-ray diffraction and energy dispersive X-ray analysis, while the electrical properties were studied by impedance spectroscopy. Partially crystallized non-reduced samples are semiconducting even at room temperature while the glassy samples (both reduced and non-reduced) exhibit semiconducting properties at temperatures equal or larger than 100 °C. An attempt is done to predict the physical approximation explaining the conduction process in the glasses.     

Crystallization of bismuth oxide nano-crystallites in a SiO2–PbO–Bi2O3 glass matrix

SiO₂–PbO–Bi₂O₃ glasses having the composition of 35SiO₂–xPbO–(65 ? x)Bi₂O₃ (where x = 5, 20 and 45; in mol%) have been prepared using the conventional melting and annealing method. Differential scanning calorimetry (DSC) was employed to characterize the thermal behavior of the prepared glasses in order to determine their crystallization temperatures (T_cr). It has been found that T_cr decreases with the decrease of Bi₂O₃ content. The amorphous nature of the prepared glasses as well as the crystallinity of the produced glass–ceramics were confirmed by X-ray powder diffraction (XRD) analysis. SiPbBi₂O₆ glass nano-composites, comprising bismuth oxides nano-crystallites, were obtained by controlled heat-treatment of the glasses at their (T_cr) for 10 h. Transmission electron microscopy (TEM) of the glass nano-crystal composites demonstrates the presence of cubic Bi₂O₃ nano-crystallites in the SiPbBi₂O₆ glass matrix. Nano-crystallites mean size has been determined from XRD line width analysis using Scherrer's equation as well as from TEM; and the sizes obtained from both analyses are in good agreement. These sizes varied from about 15 to 170 nm depending on the chemical compositions of parent glasses and, consequently, their structure. Interestingly, replacement of the Bi₂O₃ by PbO in the glass compositions has pronounced effect on the nature, morphology and size of the formed nano-crystallites. Decrease of the Bi₂O₃ content increases the size of the nano-crystallites, and at the lowest Bi₂O₃ extreme, namely 20 mol%, introduces minority of the monoclinic Bi₂O₄ in addition to the cubic Bi₂O₃. The crystallization mechanism is suggested to involve a diffusion controlled growth of the bismuth oxide nano-crystallites in the SiPbBi₂O₆ glass matrix with the zero nucleation rate.     

Transparent phosphosilicate glasses containing crystals formed during cooling of melts

Effect of P₂O₅–SiO₂ substitution on spontaneous crystallization of SiO₂–Al₂O₃–P₂O₅–Na₂O–MgO melts during cooling was studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and rotation viscometry. Results show that addition of P₂O₅ leads to amorphous phase separation (APS), i.e., phosphate- and silicate-rich phases. It is due to the tendency of Mg²⁺ to form [MgO₄] linking with [SiO₄]. Molar substitution of P₂O₅ for SiO₂ enhances the network polymerization of silicate-rich phase in the melts, and thereby the spontaneous crystallization of cubic Na₂MgSiO₄ is also enhanced during cooling of the melts. In addition, the sizes of the local crystalline and separated glassy domains are smaller than the wavelength of the visible light, and this leads to the transparency of the obtained glasses containing crystals.