On the effect of ordering on vibrational spectrum of relaxor ferroelectric PbSc1/2Ta1/2O3 in paraphase

The effect of the ordering of Sc³⁺ and Ta⁵⁺ ions on the behavior of the phonon subsystem in the relaxor ferroelectric PbSc_1/2Ta_1/2O₃ from the numerous AB′ _x B″ _x?1O₃ family has been studied. The nature of an additional contribution to the light-scattering spectra has been analyzed on the experimental data on Raman scattering in PbSc_1/2Ta_1/2O₃, SrAl_1/2Ta_1/2O₃, SrAl_1/2Nb_1/2O₃, and BaMg_1/3Ta_2/3O₃ compounds and inelastic neutron scattering in PbSc_1/2Ta_1/2O₃. It is shown that broadening of the lines in the Raman spectra of PbSc_1/2Ta_1/2O₃ crystals is caused by additional scattering by phonons from various points of the Brillouin zone.     

Infrared transmission of (R2O OR R’O)–(TiO2, Nb2O5 OR Ta2O5)-Al2O3 glasses

The new families of aluminate glasses obtained by the present authors from their melts in the systems K₂O–Ta₂O₅–Al₂O₃, Na₂O–K₂O–Ta₂O₅–Al₂O₃, K₂O –Cs₂O– Ta₂O₅–Al₂O₃, K₂O–Nb₂O₅–Al₂O₃, Na₂Oz.sbnd;K₂O–TiO₂–Al₂O₃, BaO–TiO₂–Al₂O₃, BaO–ZrO₂–TiO₂–Al₂O₃ and Na₂O–K₂O–BaO–ZrO₂–Ta₂O₅–TiO₂ –Al₂O₃ showed high transmissions of visible and infrared (IR) radiation ranging from 0.4 to about 6 μm, as well as high refractive indices up to 2.0. Their physical and chemical properties such as glass-forming ability, softening temperature, hardness and hygroscopicity were comparable to conventional silicate glasses. These properties are useful for IR applications. The cause of the high IR transmission of the aluminate glasses was interpreted in terms of the masses of the constituent cations and the single bond strengths of the cations with oxygen ions.     

Formation of new lanthanide oxide glasses by laser spin melting and free-fall cooling

Glass spherules were made from molten droplets spun off the ends of laser-heated ceramic rods and solidified by free-falling cooling. Glasses of several hundred μm diameter were formed from the pure lanthanide oxides Sc₂O₃, Y₂O₃, Sm₂O₃, Gd₂O₃, Yb₂O₃ and Lu₂O₃; glass formation decreased with addition of CaO. Stable glasses were also produced from binary La₂O₃ compositions with 25, 50 and 75 wt % of each of the oxides Y₂O₃, Yb₂O₃, Nb₂O₅ and Ta₂O₅, and from the ternary compositions 40 wt % La₂O₃ + 20 wt % CaO + 40 wt % Al₂O₃ or Ga₂O₃. The indices of refraction of all of the glasses and the Abbe numbers of several were measured.     

Vertical Bridgman growth and optical properties of doped Bi12SiO20 crystals

The core-free BSO crystals with high optical quality, doped with NiO, Sb₂O₃, V₂O₅, Nb₂O₅, Ta₂O₅, and P₂O₅, have been grown by the vertical Bridgman (VB) method for the first time. The “interface” segregation coefficients of the above-mentioned dopants in BSO with values of 0.014, 0.162, 0.704, 0.039, 0.077, and 0.976, respectively, have been approximately evaluated according to the diffusive solute transport regime with the assumption of negligible natural convection in the melt. The transmission spectra of various doped BSO crystals have been measured in the range from 300 to 700 nm. A blue shift of the transmission edge in Ni-, Sb-, and P-doped BSO and a red shift in Nb- and V-doped BSO have been found as compared to undoped BSO. The modification in the absorption amplitude implies that the photorefractive performances of BSO crystals may be improved through doping.     

Study of cation ordering and magnetic phase transitions in ternary Fe-containing perovskite oxides by Mössbauer spectroscopy

Nuclear gamma-resonance (NGR) spectra were studied in ceramics of ternary perovskite-type oxides Pb(Fe_2/3W_1/3)O₃, Pb(Fe_1/2Ta_1/2)O₃, and Pb(Fe_1/2Nb_1/2)O₃ and in the Pb(Fe_1/2Sb_1/2)O₃ oxide synthesized under pressure. A singlet corresponding to compositional-ordering regions was observed in the NGR spectra for Pb(Fe_1/2Sb_1/2)O₃. Weak doublets associated with paramagnetic regions (in which the degree of cation ordering is higher than the volume average) were observed in the spectra for Pb(Fe_2/3W_1/3)O₃ and Pb(Fe_1/2Nb_1/2)O₃ at temperatures 30–70 K below the antiferromagnetic phase transition point.     

<Emphasis Type="Italic">Ab Initio</Emphasis> calculation of elastic and electromechanical constants of langasite family crystals

Elastic and electromechanical (piezoelectric) constants for six langasite family crystals Ca₃TaGa₃Si₂O₁₄, Sr₃TaGa₃Si₂O₁₄, Ba₃TaGa₃Si₂O₁₄, La₃Ta_0.5Ga_5.5O₁₄, La₃Nb_0.5Ga_5.5O₁₄, La₃Ga₅SiO₁₄ are calculated within the density-functional perturbation theory.     

Comparison of the Spectral Characteristics of Chromium-Containing Crystals of the Calcium Gallogermanate Type with Their Structural Features

The molecular refractions and characteristics of the circular dichroism spectra of unactivated and chromium_activated La₃Ga_5.5Ta_0.5O₁₄, La₃Ga_5.5Nb_0.5O₁₄, La₃Ga₅GeO₁₄, La₃Ga₅SiO₁₄, Sr₃Ga₂Ge₄O₁₄, and Ca₃Ga₂Ge₄O₁₄crystals are compared with their structural parameters. Correlations between the molecular refractions and the structural distortions responsible for the optical activity of crystals and correlations between the characteristics of circular dichroism bands of defects and chromium ions and the interatomic distances in polyhedra and their distortions are found. The calcium gallogermanate structures exhibit a unique correspondence between the electronic configuration of host ions and the local symmetry.     

Structure and properties of ceramic solid solutions Li x Na1 − x Ta y Nb 1 − y O3 (x = 0–0.05, y = 0–0.04)

The structure of Li_0.03Na_0.97Ta _y Nb_{1 ? y} O₃ ceramic solid solutions and the processes of its disordering with a change in composition and temperature have been studied by full-profile X-ray diffraction analysis and Raman spectroscopy. It was established that at room temperature the structure of Li_0.03Na_0.97Ta_0.05Nb_0.95O₃ solid solutions is close to the NaNbO₃ structure and has a space group P21ma with four formula units per unit cell. It was shown that the oxygen octahedra in the Li _x Na_1?x Ta_0.1Nb_0.9O₃ solid solutions (0, 0.15, 0.03, 0.04, 0.05) are slightly distorted and their geometry is identical to that of octahedra in the NaNbO₃ structure. The temperature dependence of the intensities of lines in the Raman spectrum of Li_0.03Na_0.97Ta_0.4Nb_0.6O₃ solid solutions corresponding to vibrations of Li⁺ and Na⁺ cations in the cuboctahedral structural voids revealed a strong disordering in the alkali metal sublattice at a temperature above 322°C. This may indicate the essential stepwise increase in the Li⁺ cation mobility and may result in a change in the kinetic and energetic characteristics of ion charge transport.     

Atomic layer deposition of Al2O3, ZrO2, Ta2O5, and Nb2O5 based nanolayered dielectrics

Ta₂O₅, Ta-Nb-O, Zr-Al-Nb-O, and Zr-Al-O mixture films or solid solutions were grown on Si(1 0 0) substrates at 300 °C by atomic layer deposition. The equivalent oxide thickness of Ta₂O₅ based capacitors was between 1 and 3 nm. In Zr-Al-O films, the high permittivity of ZrO₂ was combined with high resistivity of Al₂O₃ layers. The permittivity, surface roughness and interface charge density increased with the Zr content and the equivalent oxide thickness was between 2.0 and 2.5 nm. In the Zr-Al-Nb-O films the equivalent oxide thickness remained at 1.8-2.0 nm.     

Investigation into the structure of ceramic Li x Na1−x Ta0.1Nb0.9O3 solid solutions by X-ray analysis and Raman spectroscopy

The structure of ceramic solid solutions Li _x Na_{1 ? x} Ta_0.1Nb_0.9O₃ has been investigated by full-profile X-ray diffraction analysis and Raman spectroscopy. It is shown that their structure for all x values is similar to the NaNbO₃ structure. The structure of NaTa_0.1Nb_0.9O₃ solid solutions (x = 0) is characterized by the sp. gr. Pbcm, and the structure of Li _x Na_{1 ? x} Ta_0.1Nb_0.9O₃ solid solutions with x = 0.05, 0.1, and 0.16 is characterized by the sp. gr. P2₁ ma. The unit-cell parameters decrease with an increase in x. At x = 0, doubled values of the parameter c and, correspondingly, the unit-cell volume are observed in the antiferroelectric phase. This fact is confirmed by the form of the vibrational spectrum in the region of stretching modes of oxygen atoms along the polar axis.     

The Growth and Properties of K1-yLiyTa1-xNbxO3 Crystal

Potassium lithium tantalate niobate (K_1-yLi_yTa_1-xNb_xO₃, KLTN) crystals with tetragonal phase at room temperature were grown by the flux pulling method. Two-wave coupling energy transfer has been realized in this crystal. The two-wave coupling gain coefficient measured is 0.46 cm⁻¹, and the sign of the dominant carrier is positive. As compared with potassium tantalate niobate (KTN), the dielectric properties of KLTN changed significantly.     

The absorption and circular dichroism spectra of langasite family crystals doped with chromium ions

The absorption and circular dichroism spectra of langasite family crystals, La₃Ga₅SiO₁₄, La₃Ga₅GeO₁₄, Ca₃Ga₂Ge₄O₁₄, Sr₃Ga₂Ge₄O₁₄ (red), Sr₃Ga₂Ge₄O₁₄ (green), La₃Ta_0.5Ga_5.5O₁₄, and La₃Nb_0.5Ga_5.5O₁₄, which were doped with chromium ions, have been investigated in the range of 240–850 nm. It is shown that chromium ions are incorporated into the structure of the investigated crystals both in the octahedrally (Cr³⁺ ion in 1a octahedron) and tetrahedrally (Cr⁴⁺ ion in 2d tetrahedron) coordinated positions. The ion ratio Cr³⁺/Cr⁴⁺ changes in a wide range in the crystals studied.     

Crystallographic and dielectric properties of flux grown (B′B″: InNb, InTa, YbNb, YbTa and MgW) single crystals

Single crystals of PbIn_1/2Nb_1/2O₃ (PIN), PbIn_1/2Ta_1/2O₃ (PIT), PbYb_1/2Nb_1/2O₃ (PYN), PbYb_1/2Ta_1/2O₃ (PYT) and PbMg_1/2W_1/2O₃ (PMW) have been grown by the flux method. The PbO-based solvents were used. Transparent, light yellow and arrow like shaped PIN and PIT crystals of the perovskite structure were obtained. Small amounts of red and of octahedron habit PIN and PIT crystals of the pyrochlore type were simultaneously grown. In the case of PYN, PYT and PMW only the crystals of the perovskite structure have been grown. The transparent and brown PYN and PYT crystals of octahedron habit were obtained. The transparent, light yellow and of octahedron or truncated octahedron shape PMW crystals were grown. The crystals were characterised by X-ray and dielectric studies. They showed that as-grown PIN crystals are nearly disordered, exhibit the rhombohedral distortion of the pseudo-perovskite unit cell and reveal relaxor behaviour. The partially ordered PIT crystals show monoclinic distortion and undergo antiferroelectric–paraelectric phase transition. The PYN, PYT and PMW single crystals, characterised by chemical order in the B′/B″ ion sublattice, exhibit orthorhombic symmetry and undergo the first-order antiferroelectric–paraelectric phase transitions.     

Optical activity and crystalline structure of crystals of the langasite family

The dependences of the refraction on the structural parameters are considered for the La₃Ga_5.5Nb_0.5O₁₄, La₃Ga_5.5Ta_0.5O₁₄, La₃Ga₅SiO₁₄, Ca₃Ga₂Ge₄O₁₄, and Sr₃Ga₂Ge₄O₁₄ crystals of the langasite family. It is shown that the angle of deviation of the 1a octahedron faces that are normal to the optical axis from 60° is one of the main sources of optical activity of these crystals. The interaction of the cations belonging to the 3e dodecahedron and 1a octahedron, cations of the 3e dodecahedron and 2d tetrahedron, and the repulsion of O²⁻ ions are believed to be the basic interactions affecting the angle value. The dependences of the angle on the crystal-chemical characteristics of the considered crystals are analyzed. The role of the relative sizes of the structural polyhedra is demonstrated.     

Synthesis and crystal growth of sillenite phases in the Bi2O3 ‐ TiO2 ‐ Nb2O5 system

The synthesis of Bi₂O₃‐Nb₂O₅ sillenite phase (BNbO) and the solubility of this phase with Bi₁₂TiO₂₀ was investigated by solid‐state reaction synthesis and niobium doped Bi₁₂TiO₂₀ (BTO:Nb) crystals were grown by the Top Seeded Solution Growth (TSSG) technique. The structures of polycrystalline compounds were checked by X‐ray powder diffraction method at room temperature. The correct composition of the sillenite phase stabilized with niobium was determined as Bi₁₂[Nb_0.17Bi_0.83]O_19.7 (BNbO) with unit cell parameter a = 10.261(2) Å. The system BTO‐BNbO is poorly soluble, but niobium doped BTO crystals were grown from the liquid composition 10Bi₂O₃ : xTiO₂ : (1‐x)/2 Nb₂O₅, with x = 0.95 and 0.90. A niobium concentration limit in the liquid phase is established in order to grow BTO:Nb with good crystalline quality. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structure type of hexagonal tantalum bronzes with variable composition K6Ta6 + ZO15F6(F, O)y: Ta(5 ? δ)+ bronzes and Ta5+ compounds

The structure type of hexagonal compounds with the variable composition K₆Ta_{6 + z} O₁₅F₆ (F, O)_y, where Z ≤ 1 and y ≤ 3 (sp. gr. P6/m, a ∼ 13.12 Å, c ∼ 3.86 Å) has been studied. Based on the structural data for a crystal of the Ta⁵⁺-containing compound and two crystals of Ta^{(5 − δ)+}-containing bronzes formed at the cathode during electrolysis of salt melts containing Ta⁺ cations, it was established that the bronze formation is associated with the interstitial defects of intercalated Ta cations. The scheme of reduction of Ta cations at the cathode is suggested, and the formulas of the compounds are obtained with due regard for partial Ta reduction to various integral oxidation degrees. The crystals of the colorless transparent Ta⁵⁺-containing compound of the composition K₆Ta _6.27 ⁵⁺ O₁₅F_7.4, brown semitransparent partly reduced Ta⁺-containing bronze of the composition K₆Ta _6.33 ⁵⁺ Ta _0.55 ⁺ O₁₅F_8.2, and dark gray metal-like completely reduced Ta⁺-containing bronze of the composition K₆Ta ₆ ⁵⁺ Ta⁺O₁₅F₇ are studied experimentally. __________ Translated from Kristallografiya, Vol. 49, No. 1, 2004, pp. 75–91. Original Russian Text Copyright ? 2004 by Arakcheeva, Chapuis, Grinevich, Shamrai. Dedicated to the 80th Birthday of L.A. Shuvalov     

Czochralski growth and constitutional studies on single crystals of potassium lithium niobate (KLN)

The ferroelectric phase of potassium lithium niobate K₃Li_2−xNb_5+xO₁₅ (KLN) is a very promising material for the conversion of infrared light to light in the visible region. However, growing of single crystals is known to be complicated due to the considerable anisotropy of the growth rate and the thermal expansion behaviour. The single crystals of KLN, Mg²⁺‐doped KLN, as well as the mixed crystals of potassium lithium tantalate niobate K₃Li₂(Nb_1−xTa_x)₅O₁₅ (KLTN) were grown by the Czochralski technique. The chemical analyses of the samples were performed by atomic absorption spectroscopy (AAS) and X‐ray fluorescence analysis (XRF). The element concentrations along the single crystals were measured by the electron microprobe analysis (EMPA) to clarify the segregation phenomena in the grown crystals. The elements distribution coefficients were also calculated. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and study of PbFe1/2Ta1/2O3 single crystals

Single crystals of the composition PbFe_1/2Ta_1/2O₃ are grown by the method of mass crystallization from flux. It is established that, unlike the PbFe_1/2Ta_1/2O₃ ceramic, the synthesized single crystals possess pronounced relaxor properties: the maximum of the dielectric constant is diffuse and its temperature, T _m, increases by more than 70 K with an increase in the frequency from 10² to 10⁶ Hz. It is assumed that the unusual properties of the PbFe_1/2Ta_1/2O₃ crystals are caused by mesoscopically inhomogeneous compositional ordering and comparatively high conductivity providing favorable conditions for the appearance of the volume-charge and thermal electron polarization.     

Optical and structural properties of Sr-Nb-phosphate glasses

This research studied optical and structural properties of SrO-Nb₂O₅-P₂O₅ glasses containing 0 ~ 25 mol% Nb₂O₅ and 35 ~ 60 mol% SrO using spectra of UV-Vis, FTIR and XPS. Our aim was to work new moldable glasses which are colorless with high refractive index and transmittance, and their correlation with glass structure. Nb₂O₅ plays the role as an intermediate of glass formation, and transforms the role to network-former or glass-modifier depending on composition. Refractive index increases monotonically with Nb₂O₅ content at the expense of transmittance, and specific coloring which occurs in ternary compositions with the ratio P₂O₅/SrO > 1. This effect arises from much increased [Nb³⁺ and Nb⁴⁺] fraction, as quantitatively deconvoluted in Nb3d XPS spectra, in the ligand [NbO₆]_octahedral which is supported by FTIR and O1s XPS spectra. As the ratio P₂O₅/SrO ≤ 1, Nb₂O₅ is more of a network former and [NbO₆]_octahedral structure turns into [NbO₄]_tetrahedron with partial Nb-O…(Sr) bonding turns into (Nb-O-Nb)_tetrahedron, coloring disappears.     

Structure and properties of potassium niobato-borophosphate glasses

Bulk glasses of the series (1 ? x)[0.5K₂O–0.1B₂O₃–0.4P₂O₅]–xNb₂O₅ with x = 0–45.7 mol% Nb₂O₅ were prepared by slow cooling in air and investigated by Raman, ³¹P, and ¹¹B MAS NMR spectroscopy. The incorporation of Nb₂O₅ into the parent borophosphate glass results in a substantial increase in the glass transition temperature and chemical durability of glasses. Raman spectra showed that Nb atoms form distorted NbO₆ octahedra, which are isolated at low Nb₂O₅ content, whereas at higher Nb₂O₅ content they form clusters. ¹¹B NMR spectra of the glasses revealed the interaction between Nb₂O₅ and BO₄ tetrahedral units, which results in a partial transformation of tetrahedral BO₄ units to trigonal BO₃ units and the formation of mixed B(OP)_4?n(ONb)_n units.