Mesostructure of xerogels of hydrated zirconium dioxide

The fractal properties of xerogels of hydrated zirconium dioxide ZrO₂, which are synthesized by precipitation from a zirconyl nitrate solution at various pH values, are studied. Small-angle neutron scattering data and low-temperature nitrogen adsorption data indicate the strong effect of the acidity of the medium on the fractal dimension of xerogels and the size of constituent monomer particles. It is shown that this effect is also characteristic of xerogels of hydrated hafnium dioxide HfO₂, which is a chemical analogue of hydrated zirconium dioxide.     

Spectroscopy of optical centers of Eu<Superscript>3+</Superscript> ions in partially stabilized and stabilized zirconium crystals

The structure of the optical centers of Eu³⁺ ions in tetragonal (ZrO₂)_1–x–y (Y₂O₃) _x (Eu₂O₃) _y (х = 2.7–3.6; y = 0.1) and cubic (ZrO₂)_1–x–y (Y₂O₃) _x (Eu₂O₃) _y (х = 8–38; y = 0.1–0.5) crystals of solid solutions on the basis of zirconium dioxide is studied using the methods of optical and Raman-scattering spectroscopy. Characteristic optical centers of Eu³⁺ ions with different crystalline environments are revealed in the above compounds.     

Raman scattering spectroscopy of inclusions of carbon in Al<Subscript>2</Subscript>O<Subscript>3</Subscript> films and its solid solutions with HfO<Subscript>2</Subscript>

We apply Raman scattering spectroscopy to study the nature of carbon inclusions in Al₂O₃ and (HfO₂) _x (Al₂O₃)_{1 ? x} films deposited using volatile complex compounds. Raman spectra of the films under investigation contain D and G vibrational modes, which indicate that carbon clusters of the sp ² configuration tend to form in the films. We estimate the size of clusters from the integrated intensity ratio I _D /I _G and find it to be in the range of 14–20 Å. The content of hydrogen in carbon clusters is calculated from the height of the photoluminescence pedestal and is found to vary from 14 to 30 at % depending on the regime of the film’s synthesis.     

Optical properties of multilayer structures

The possibility of using the ellipsometry method for investigation of the optical properties of multilayer films and structures is shown. The optical properties of structures HfO₂/SiO₂/Si, HfO₂/Si, ZrO₂/Si, Ta₂O₅/Si, and Al₂O₃/Si are studied. It is found that a layer of hafnium silicate is formed at the interface between the HfO₂ film and Si. Annealing of the structures in oxygen shows that oxides studied are oxygen-permeable and that the thickness of SiO₂ at the film-substrate interface increases. The growth rate of SiO₂ layers depends on the chemical nature of an oxide. Al₂O₃ films are impermeable for oxygen diffusion. The production of layers of alloys (Al₂O₃) _x (HfO₂)_{1 ? x} is optimized, which allows one to obtain layers with a homogeneous distribution of elements over the thickness.     

Pressure-induced superconductivity and structural transitions in Ba(Fe<Subscript>0.9</Subscript>Ru<Subscript>0.1</Subscript>)<Subscript>2</Subscript>As<Subscript>2</Subscript>

Electrical transport and structural characterizations of isoelectronically substituted Ba(Fe_0.9Ru_0.1)₂As₂ have been performed as a function of pressure up to ～ 30 GPa and temperature down to ～ 10 K using designer diamond anvil cell. Similar to undoped members of the AFe₂As₂ (A = Ca, Sr, Ba) family, Ba(Fe_0.9Ru_0.1)₂As₂ shows anomalous a-lattice parameter expansion with increasing pressure and a concurrent ThCr₂Si₂ type isostructural (I4/mmm) phase transition from tetragonal (T) phase to a collapsed tetragonal (cT) phase occurring between 12 and 17 GPa where the a is maximum. Above 17 GPa, the material remains in the cT phase up to 30 GPa at 200 K. The resistance measurements show evidence of pressure-induced zero resistance that may be indicative of high-temperature superconductivity for pressures above 3.9 GPa. The onset of the resistive transition temperature decreases gradually with increasing pressure before completely disappearing for pressures above ～ 10.6 GPa near the T-cT transition. We have determined the crystal structure of the high-T _c phase of Ru-doped BaFe₂As₂ to remain as tetragonal (I4/mmm) by analyzing the X-ray diffraction pattern obtained at 10 K and 9.7 ± 0.7 GPa, as opposed to inferring the structural transition from electrical resistance measurement, as in a previous report [S.K. Kim, M.S. Torikachvili, E. Colombier, A. Thaler, S.L. Bud’ko, P.C. Canfield, Phys. Rev. B 84, 134525 (2011)].     

Optical properties of lithium niobate and lithium tantalate crystals with impurities and defects

The photoinduced and Raman scattering in lithium niobate and lithium tantalate crystals with impurities and defects have been studied. An exciting laser beam propagated either along the ferroelectric Z axis or perpendicular to it. The conditions for exciting transverse and longitudinal polar optical modes in Raman spectra are established. The regularities of the excitation of Raman spectra in several polarization geometries (X(ZZ)Y, Z(XX, Y Y)Z, Z(XX, Y Y)Z, X(ZX)Y, X(ZX)X and X(ZX)X) have been investigated. Additional (extra) spectral lines are interpreted as a manifestation of a biphonon enhanced by the Fermi resonance and the result of violation of selection rules for pseudoscalar modes of the A ₂ type due to the reduction of the point symmetry group caused by the presence of impurities and defects in real crystals. The conditions for exciting coherent longitudinal and transverse modes in lithium niobate and lithium tantalate single crystals upon stimulated Raman scattering are analyzed. The temperature evolution of the spectra recorded in the X(ZZ)Y geometry near the ferroelectric phase transition point is explained based on the concept of effective soft mode and analysis of the isofrequency opalescence effect. Strong photoluminescence is found in copper-doped lithium niobate crystals.     

Raman spectroscopy of SrB<Subscript>4</Subscript>O<Subscript>7</Subscript> single crystals in the temperature range 300–1273 K

The polarized Raman spectra of SrB₄O₇ (SBO) single crystals are studied in detail in the temperature range of 300–1273 K. The TO, LO, and IO phonon lines of A₁, A₂, B₁, and B₂ symmetries of rhombic SBO at 300 K are identified. The behavior of the Raman spectra of SBO crystals is studied upon heating up to their melting. The relation of Raman spectra with the structure of boron–oxygen fragments, as well as the transformation of spectra in the process of melting of SBO crystals, is discussed.     

Chemical and phase compositions of multilayer nanoperiodic <Emphasis Type="Italic">a</Emphasis>-SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/ZrO<Subscript>2</Subscript> structures subjected to high-temperature annealing

The chemical and the phase compositions of multilayer nanoperiodic SiO _x /ZrO₂ structures prepared by vacuum evaporation from separated sources and subjected to high-temperature annealing have been studied by X-ray photoelectron spectroscopy with a layer-by-layer etching. It is found that, under deposition conditions used, the silicon suboxide layers had the stoichiometric coefficient x ~1.8 and the zirconium-containing layers were the stoichiometric zirconium dioxide. It was found, using X-ray photoelectron spectroscopy, that annealing of the multilayer structures at 1000°C leads to mutual diffusion of the components and chemical interaction between ZrO₂ and SiO _x with predominant formation of zirconium silicate at heteroboundaries of the structures. The SiO _x layers of the annealed nanostructures contained ~5 at % elemental silicon as a result of the phase separation and the formation of fine silicon nanocrystals.     

Atomic and Electronic Structures of Metal-Rich Noncentrosymmetric ZrO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

The atomic and electronic structures of metal-rich noncentrosymmetric zirconium oxide synthesized by the ion beam sputtering of a metallic target in an oxygen atmosphere has been studied by X-ray photoelectron spectroscopy, Raman scattering, spectral ellipsometry, and quantum-chemical simulation. It has been established that ZrO_{x < 2} consists of ZrO₂, metallic Zr, and zirconium suboxides ZrO_y. The stoichiometry parameter of ZrO_y has been estimated. It has been shown that the optical properties of ZrO_{x < 2} are determined by metallic Zr. A model of fluctuation of the width of the band gap and a potential for electrons and holes in ZrO_{x < 2} based on spatial fluctuations of the chemical composition has been proposed.     

Sol-gel method and luminescence properties of the ZrO<Subscript>2</Subscript>:Eu<Superscript>3+</Superscript> phosphors with different charge compensation

Eu³⁺-doped ZrO₂ phosphors with different charge compensators (Li⁺, Na⁺, K⁺) were prepared by the sol-gel method. The properties of the as-obtained samples are characterized by X-ray diffraction, scanning electron microscope, photoluminescence spectra, and decay curve. The results show that ZrO₂:Eu³⁺ phosphors with different charge compensation are mixed phase of tetragonal and monoclinic phase, and the volume fraction of tetragonal phase of ZrO₂:Eu³⁺/Na⁺ phosphor is bigger than the other phosphors. The phosphors can emit strong red light at 606~616 nm (⁵D₀ → ⁷F₂) excited by ultraviolet light (395 nm). Compared with two charge compensation patterns in the ZrO₂:Eu³⁺, it has been found that ZrO₂:Eu³⁺ phosphors used Na⁺ as charge compensator show greatly enhanced red emission under 395 nm excitation and longer luminescence lifetime.     

Structure,phase composition,and spectral luminescence properties of partially stabilized zirconium dioxide crystals doped with Yb<Superscript>3+</Superscript> ions

Investigation of partially stabilized zirconium dioxide crystals via transmission electron microscopy has revealed a developed twin structure therein. The following compositions of the above crystals have been selected for this study: 97.2 mol % ZrO₂–1.0 mol % Y₂O₃–1.8 mol % Yb₂O₃; 97.2 mol % ZrO₂–2.0 mol % Y₂O₃–0.8 mol % Yb₂O₃; 97.2 mol % ZrO₂–2.5 mol % Y₂O₃–0.3 mol % Yb₂O₃; and 96.3 mol % ZrO₂–3.4 mol % Y₂O₃–0.3 mol % Yb₂O₃. X-ray diffraction analysis of these crystals indicate the presence of transformable (t) and nontransformable (t') tetragonal phases. Optical spectroscopy measurements of ZrO₂–Y₂O₃–Yb₂O₃ crystals with tetragonal and cubic structures have highlighted in Yb³⁺-doped zirconium dioxide samples the formation of the optical centers of the Yb³⁺ ions is observed, whose crystal surrounding is similar to those in cubic zirconium dioxide crystals.     

Raman scattering in sodium nitrite crystals near the phase transition

Optical Raman spectra of a ferroelectric sodium nitrite crystal have been detected in a wide spectrum range at various temperatures, including the region of the ferroelectric phase transition. A manifestation of a transverse soft polar mode of the A ₁(z) type responsible for the ferroelectric phase transition has been discovered in the spectrum at room temperature. This mode has been found to become overdamped even far from the ferroelectric phase transition temperature. This mode also appears as a central peak under heating. It has been found that the pseudoscalar mode of the A ₂ type has the highest intensity in the Raman spectrum of sodium nitrite. The frequency corresponding to the maximum intensity of this mode in the Raman spectrum varies from 130 cm^–1 at 123 K to 106 cm^–1 at T = 513 K. A fair agreement of the experimental data for the A ₁(z) mode with the Lyddane–Sachs–Teller relation has been established. The polariton curves for the A ₁(z) polar mode and the dispersion curves for axinons has been plotted.     

Superposition of M<Subscript>5</Subscript>X<Subscript>5</Subscript> superstructures and X-ray diffraction in TiO<Subscript>1.0</Subscript> titanium monoxide

The interpretation of diffraction spectra of ordered high-temperature phases of solid solutions and strongly nonstoichiometric compounds is discussed. It has been shown that variations of the intensities of superstructure reflections, which cannot be explained within simple ordering models, can be due to the superposition of superstructures with different symmetries in the matrix of the basis crystal structure. Using an example of atom–vacancy ordering in TiO_1.0 titanium monoxide, a model of the order–order transition state formed by the superposition of low-temperature monoclinic (space group A2/m (C2/m)) and high-temperature cubic (space group Pm3?m) M₅X₅ superstructures has been proposed. It has been shown that the transition state is thermodynamically equilibrium and should be implemented instead of the M₅X₅ cubic superstructure. The transition state model can be considered as an M_(5–i)X_(5–i) superstructure (i = 1, 14/18, 11/18) with the monoclinic symmetry (space group P1m1).     

Optical studies of phase transitions in the (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>Ti(O<Subscript>2</Subscript>)F<Subscript>5</Subscript> crystal

Polarization-optical study of twinning and measurements of the Raman spectra and birefringence in oxyfluoride (NH₄)₃Ti(O₂)F₅ were carried out over the temperature range 90–350 K. Phase transitions were detected at temperatures T ₀₁ = 266 K (second-order transition) and T ₀₂ = 225 K (first order). It is assumed that the crystal symmetry is changed as follows: Fm3m ? I4/mmm ? I4/m. Anomalies of the spectral parameters are established in the frequency range of internal vibrations of ammonium ions and Ti(O₂)F₅ complexes. An analysis of the results shows that the transition at T ₀₁ is likely due to small shifts of the tetrahedral groups from their position on the triad axis and that the transition at T ₀₂ is due to fluorine-oxygen ordering of Ti(O₂)F₅ complexes.     

Band structure of (Sr<Subscript>3</Subscript>Sc<Subscript>2</Subscript>O<Subscript>5</Subscript>)Fe<Subscript>2</Subscript>As<Subscript>2</Subscript> as a possible basis phase of new FeAs superconductors

The results of the ab initio FLAPW-GGA computations of the band structure of the recently synthesized layered tetragonal (space group I4/mmm) arsenide (Sr₃Sc₂O₅)Fe₂As₂ as a possible basis phase of a new group of FeAs superconductors are presented. For (Sr₃Sc₂O₅)Fe₂As₂, the energy bands, electron state density distributions, Fermi surface topology, low-temperature electron specific heat, molar Pauli paramagnetic susceptibility, and effective atomic charges have been determined. These results are discussed compared to similar data for the layered tetragonal crystals LaFeAsO, SrFeAsF, SrFe₂As₂, and LiFeAs that are the basis phases of the recently discovered high-temperature (T _C ～ 26–56 K) 《1111》, 《122》, and 《111》 FeAs superconductors.     

Order–order transition structural state in titanium monoxide TiO<Subscript>1.0</Subscript>

A new class of defect structures in which point defects of a crystal lattice simultaneously occupy sites of two different superstructures is proposed. The formation of these structural modifications is due to a second-order order–order phase transition that does not occur to the end. The allowable relation between the long-range order parameters in the structural modification formed by a combination of the monoclinic (M ₅ X ₅)_mon (space group C2/m (A2/m)) and the cubic (M ₅ X ₅)_cub (space group Pm3m) of the superstructures is studied using an atom–vacancy ordering in titanium monoxide TiO_1.0. The thermodynamic calculations show that the proposed structural modification is equilibrium and must form instead of assumed high-temperature cubic phase (Ti₅O₅)_cub.     

In situ Raman spectroscopic study of interaction between phengite and water under <Emphasis Type="Italic">P</Emphasis>–<Emphasis Type="Italic">T</Emphasis> conditions typical of subduction zones

In situ Raman spectroscopy is used to study the layered mineral phengite K(Al, Mg)₂(Si, Al)₄O₁₀(OH)₂ in water under the high temperature and pressure conditions typical of cold subduction zones (T up to 373°C and P = 12.5 GPa). High pressure and temperature were created in a resistively heated diamond anvil cell. Raman spectra show the high P–T stability of phengite. No reversible polymorphic transitions (overhydration or notable amorphization) are observed.     

Intermediate phases in [111]- and [001]-oriented PbMg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>O<Subscript>3</Subscript>–29PbTiO<Subscript>3</Subscript> single crystals

Phase transformations in [111]- and [001]-oriented PbMg_1/3Nb_2/3O₃–29PbTiO₃ single crystals have been studied using dielectric and optical measurements before and after applying an electric field. It is shown that the subsequence of phase transitions rhombohedral (R)—tetragonal (T)—cubic (C) phases is observed in nonpolarized samples of both orientations as temperature increases. In the [111]-oriented crystal, an additional intermediate monoclinic phase (it is possible, M _a ) is induced after preliminary polarization at room temperature and the R–M _a –T–C phase transitions are observed on heating. In the [001]-oriented crystal, after its polarization, the monoclinic phase forms instead of the rhombohedral phase even at room temperature and the M _a –T–C transitions occur on heating. The results are discussed from the point of view of the existence polar nanoregions with different local symmetries in a glasslike matrix.     

Effect of the Lattice and Spin-Phonon Contributions on the Temperature Behavior of the Ground State Splitting of Gd<Superscript>3+</Superscript> in SrMoO<Subscript>4</Subscript>

The temperature behavior of the EPR spectra of the Gd³⁺ impurity center in single crystals of SrMoO₄ in the temperature range T = 99–375 K is studied. The analysis of the temperature dependences of the spin Hamiltonian b ₂ ⁰ (T) = b₂(F) + b₂(L) and P ₂ ⁰ (T) = P₂(F) + P₂(L) (for Gd¹⁵⁷) describing the EPR spectrum and contributing to the Gd³⁺ ground state splitting ΔE is carried out. In terms of the Newman model, the values of b₂(L) and P₂(L) depending on the thermal expansion of the static lattice are estimated; the b₂(F) and P₂(F) spin-phonon contributions determined by the lattice ion oscillations are separated. The analysis of b ₂ ⁰ (T) and P ₂ ⁰ (T) is evidence of the positive contribution of the spin-phonon interaction; the model of the local oscillations of the impurity cluster with close frequencies ω describes well the temperature behavior of b₂(F) and P₂(F).     

Effect of gallium alloying on the structure,the phase composition,and the thermoelastic martensitic transformations in ternary Ni–Mn–Ga alloys

The effect of gallium alloying on the structure, the phase composition, and the properties of quasibinary Ni₅₀Mn_50–zGa_z (0 ? z ? 25 at %) alloys is studied over a wide temperature range. The influence of the alloy composition on the type of crystal structure in high-temperature austenite and martensite and the critical martensitic transformation temperatures is analyzed. A general phase diagram of the magnetic and structural transformations in the alloys is plotted. The temperature–concentration boundaries of the B2 and L2₁ superstructures in the austenite field, the tetragonal L1₀ (2M) martensite, and the 10M and 14M martensite phases with complex multilayer crystal lattices are found. The predominant morphology of martensite is shown to be determined by the hierarchy of the packets of thin coherent lamellae of nano- and submicrocrystalline crystals with planar habit plane boundaries close to {011}_B2. Martensite crystals are twinned along one of the 24 \(24\left\{ {011} \right\}{\left\langle {01\bar 1} \right\rangle _{B2}}\) “soft” twinning shear systems, which provides coherent accommodation of the martensitic transformation–induced elastic stresses.