S. Yu. Stefanovich

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

Abstract 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 Tg. 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 Tg. 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.

Whitlockite solid solutions Ca9−x M x R(PO4)7 (x = 1, 1.5; M = Mg, Zn, Cd; R = Ln, Y) with antiferroelectric properties

Whitlockite solid solutions Ca9−xMxR(PO4)7 (M = Mg, Zn, Cd; R = Ln, Y) were synthesized as powders and ceramics using solid-phase synthesis. Dielectric investigations and second harmonic generation (SHG) tests showed that ferroelectric (FE) phase transitions existing in samples with x = 0 change to antiferro-electric (AFE) transitions between two centrosymmetrical phases in samples with x = 1 or 1.5. The calcium-ion solid electrolyte conductivity in Ca9−xMxR(PO4)7 at high temperatures appears either as a result of an antiferroelectric-paraelectric (AFE-PE) phase transition (for x = 1) or as a result of a separate phase transition near 1173 K (for x = 1.5). The appearance of dielectric properties in whitlockites is discussed with reference to the features of their polar and centrosymmetrical structures.

KTiOPO4 precipitation from potassium titanium phosphate glasses, producing second harmonic generation

Abstract Glass formation in the K 2 O–TiO 2 –P 2 O 5 –SiO 2 system, and the structure and crystallization behavior of glasses having compositions near the stoichiometry of KTiOPO 4 (KTP) have been examined by differential thermal analysis (DTA), X-ray diffraction (XRD), fast Fourier transform infrared (FTIR) spectroscopy and second harmonic generation (SHG). Pure glasses may be obtained adding an appropriate amount of P 2 O 5 as well as of SiO 2 to the KTP composition. Transparent and homogeneous glasses were synthesized for the 56(K 2 O·2TiO 2 ·P 2 O 5 )·44P 2 O 5 (KTP–44P) and the 90(K 2 O·2TiO 2 ·P 2 O 5 )·10SiO 2 (KTP–10Si) molar compositions. In contrast, for the 80(K 2 O·2TiO 2 ·P 2 O 5 )·20SiO 2 (KTP–20Si) and for higher SiO 2 contents, the obtained glasses were partially opaque and phase separated. KTP glasses with lower P 2 O 5 content cannot be obtained as they crystallize during quenching. The KTP–10Si glass shows the only exothermic peak connected to KTP phase crystallization whereas the KTP–44P and the KTP–20Si glasses devitrify in two steps. Firstly the same unidentified phase is formed in both these glasses and the KTP phase is formed only at higher temperatures. Transparent and opalescent KTP glasses exhibiting SHG activity may be produced by careful heat treatments at temperatures just above T g . The origin of SHG in transparent glasses is supposed to be connected with either precipitation of KTiOPO 4 nanocrystallites or very early stages of liquid-type phase separation.

Synthesis and Properties of La2(Mo1 – xMx )2O9 (M = Nb, Ta) Ionic Conductors

La2(Mo1 – xMx )2O9 (M = Nb, Ta; 0 < x ≤ 0.2) solid solutions were prepared, and their physicochemical and electrical properties were studied. Second harmonic generation measurements indicate that the solid solutions have a noncentrosymmetric structure and undergo a structural phase transition accompanied by a sharp increase in ionic conductivity, similar to that of La2Mo2O9 . The transition temperature is found to decrease with increasing Nb or Ta content. The introduction of 5 wt % Nb increases the 800°C conductivity of the material.

High energy density in silver niobate ceramics

Solid-state dielectric energy storage is the most attractive and feasible way to store and release high power energy compared to chemical batteries and electrochemical super-capacitors. However, the low energy density (ca. 1 J cm−3) of commercial dielectric capacitors has limited their development. Dielectric materials showing field induced reversible phase transitions have great potential to break the energy storage density bottleneck. In this work, dense AgNbO3 ceramic samples were prepared successfully using solid state methods. Ferroelectric measurements at different temperatures reveal evidence of two kinds of polar regions. One of these is stable up to 70 °C, while the other remains stable up to 170 °C. The associated transition temperatures are supported by second harmonic generation measurements on poled samples and are correlated with the occurrence of two sharp dielectric responses. The average unit cell volume is seen to increase with increasing DC field and has been interpreted in terms of increasing levels of structural disorder in the system. At a high electric field the structure becomes ferroelectric with high polarization. This field induced transition exhibits a recoverable energy density of 2.1 J cm−3, which represents one of the highest known values for lead-free bulk ceramics.

Potassium bromo-borate K3[B6O10]Br—A new nonlinear optical material

A new polar hexaborate, K3[B6O10]Br (space group R3m), is synthesized under hydrothermal conditions. The radical anion is composed of macallisterite hexaborate blocks 6[3Δ + 3T] formed by boron triangles and boron tetrahedra. The blocks are joined together through vertices into a [B6O10]∞∞∞2− zeolite-like framework of a new type. Large K+ cations and Br− anions are located in extended channels that are aligned parallel to rhombohedral translations of the crystal lattice. The structure of the sublattice formed by large ions consists of BrK6 bromo-centered octahedra, which are linked together through vertices into a hexagonally distorted perovskite framework. The perovskite framework is inserted into the boron-oxygen framework so that these two frameworks do not intersect. The nonlinear optical properties of powdered samples of the K3[B6O10]Br compound are investigated using the second harmonic generation method. It is found that the K3[B6O10]Br compound exhibits a high nonlinear optical activity that exceeds the activities of the majority of borate compounds and is characteristic of haloid borates of the hilgardite family.

Grain-oriented surface crystallization of lanthanum borosilicate and lanthanum borogermanate glasses

Abstract Under appropriate conditions (high temperature gradient, rapid heating and following prolonged isothermal treatment, suppression of bulk crystallization, polished glass surface, etc.) the La 2 O 3 -B 2 O 3 - X O 2 ( X = Si, Ge) systems glasses are crystallized in preference on the surface with formation of perfect textures based on needle-shaped stillwellite-like crystals of the LaBXO 5 composition. Lanthanum borogermanate (LBG) glasses exhibit more intense texture-forming ability than lanthanum borosilicate (LBS) ones. Glass ceramic textures with a degree of grain orientation up to 0.9 in the LBG system are obtained. The pyroelectric coefficient values of the best of synthesized LBG textures are as high as 0.5–1.5 nC cm −2 K −1 in a wide temperature range. A poor crystallization ability of the LBS glasses at temperatures less than 1000°C supposed to be used in preparing both thin polar crystalline films on the glass surface for nonlinear optical applications and ferroelectric (pyroelectric) glass ceramics by the powder technology.

Formation of LiNbO3 powders and thin-films by hydrolysis of metal alkoxides

Crystalline LiNbO3 was prepared by simultaneous hydrolysis of lithium and niobium alkoxides (in the mole ratio Li : Nb = 1 :1) followed by heat treatment. Crystallization of LiNbO3 occurs in a wide temperature region, 350 to 700° C, directly from an amorphous hydrolysis product containing residual alkoxy and hydroxy groups. Heat treatment at 700° C results in the formation of fully crystalline LiNbO3 powder with completely formed ferroelectric properties. Hydrolysis of metal alkoxide solutions is successfully used to obtain high-quality LiNbO3 crystalline thin films.

Stillwellite glass-ceramics with ferroelectric properties

Abstract The process of crystallization of LaBGeO 5 and PrBGeO 5 phases with stillwellite structure from the glassy state is studied. Exotherms on differential thermal analysis curves for lanthanum borogermanate glasses are interpretated by using the X-ray diffraction and the second harmonic generation method data. It is shown that acentric nuclei of the stillwellite phase are formed at the early stages of crystallization. There is not any metastable phase, prior to stillwellite precipitation. Glass-ceramic samples with high stillwellite content were synthesized, and their properties were measured and compared with the properties of a single crystal of LaBGeO 5 .

Surface and bulk stillwellite textures in glasses of the La2O3–B2O3–GeO2 system

Abstract Both surface and bulk textures penetrating the whole volume of the glass may be formed in glasses of the La 2 O 3 –B 2 O 3 –GeO 2 system near the stoichiometry of LaBGeO 5 stillwellite. These textures cause non-linear optic, ferroelectric and pyroelectric properties of crystallized glasses. The present work represents SEM study of surface and bulk grain-oriented crystallization of stillwellite glasses for different conditions of synthesis of both planar surface (non-linear optic) and bulk (ferro/pyroelectric) textures based on oriented needle-shaped crystals of LaBGeO 5 stillwellite.