Structural studies of phase transitions in crystalline and liquid halides (ZnCl<Subscript>2</Subscript>, AlCl<Subscript>3</Subscript>) under pressure

The results of investigating the phase diagrams of ZnCl₂ and AlCl₃ halides, as well as the structure of the shortrange order of the corresponding melts under pressures up to 6.5 GPa, by the method of energy-dispersive x-ray diffraction are reported. When a ZnCl₂ crystal is compressed, a phase transition occurs from the γ phase (HgI₂ structure type) to the δ phase (distorted CdI₂ structure, WTe₂ type). The structural studies of the liquid state of ZnCl₂ and AlCl₃ indicate that the intermediate-range order decreases rapidly in the tetrahedral network of both melts as the pressure increases to 1.8 and 2.3 GPa for ZnCl₂ and AlCl₃, respectively. With further compression, the transitions in both melts occur with a change in the structure of the short-range order and with an increase in the coordination number. In this case, the transition in AlCl₃ occurs at ≈4 GPa and is a sharp first order transition, whereas the transition in ZnCl₂ occurs more smoothly in a pressure range of 2–4 GPa with a maximum intensity near 3 GPa. Thus, the AlCl₃ and ZnCl₂ compounds exemplify the existence of two phenomena—gradual decay of intermediate-range structural correlations and a sharper liquid-liquid coordination transition.     

The transport and thermal properties of glassy LiPO<Subscript>3</Subscript>/crystalline Al<Subscript>2</Subscript>O<Subscript>3</Subscript> (ZrO<Subscript>2</Subscript>) composite electrolytes

Glassy LiPO₃/crystalline Al₂O₃ and glassy LiPO₃/crystalline ZrO₂ (0–12.5 vol.% of oxide fillers) composite solid electrolytes have been prepared by glass matrix softening. Their thermal and transport properties have been investigated by differential scanning calorimetry (DSC) and impedance spectroscopy methods. The addition of ZrO₂ leads to a decrease in the crystallization temperature of LiPO₃ glass. It was found that the conductivity behavior depends on the nature of the dispersed addition. In the case of the Al₂O₃ addition, the increase in the electrical conductivity is observed. The ionic conductivity of the LiPO₃/10% Al₂O₃ composite reaches 5.8 × 10^?8 S/cm at room temperature. In contrast, the conductivity in the LiPO₃/ZrO₂ composite system decreases.     

Phase stability and elasticity of Sc<Subscript>2</Subscript>O<Subscript>3</Subscript> at high pressure

An investigation into the high-pressure behavior of Sc₂O₃ was conducted using first-principle calculations based on density functional theory within the generalized gradient approximation. Nine candidate phases were considered and simulated, C-, B-, A-RES, Rh₂O₃(II), Gd₂S₃, U₂S₃, Al₂Er₃, corundum and PPv respectively. Our results demonstrate phase transitions of Sc₂O₃ from C- to B-RES phase at 15 GPa, then to Gd₂S₃ phase at 18 GPa. Elastic constants of Sc₂O₃ present softening from about 270 GPa to 337 GPa, associated with a semiconductor-to-metal crossover. The Gd₂S₃-type Sc₂O₃ is both mechanically and dynamical stable at least up to 302 GPa supported by the mechanically stable criteria and the phonon spectrum.     

Structural and magnetic phase transformations in the BiFeO<Subscript>3</Subscript>-CaFe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> system

The crystal structure and magnetic properties of the Bi_{1 ? x} Ca _x Fe_{1 ? x/2}Nb _x/2O₃ system were studied. It is shown that, at x ≤ 0.15, the unit-cell symmetry of solid solutions is rhombohedral (space group R3c). Solid solutions with x ≥ 0.3 have an orthorhombic unit cell (space group Pbnm). The rhombohedral compositions are antiferromagnetic, while the orthorhombic compositions exhibit a small spontaneous magnetization due to Dzyaloshinski?-Moriya interaction. In CaFe_0.5Nb_0.5O₃, the Fe³⁺ and Nb⁵⁺ ions are partially ordered and the unit cell is monoclinic (space group P2₁/n). In the concentration range 0.15 < x < 0.30, a two-phase state (R3c + Pbnm) is revealed.     

Structural disorder in the paraelectric phase of the Fe<Subscript>3</Subscript>B<Subscript>7</Subscript>O<Subscript>13</Subscript>Br boracite

A structural model of the cubic paraelectric phase of a Fe₃B₇O₁₃Br crystal belonging to the boracite family has been developed using the data obtained by single-crystal X-ray diffraction with due regard for the results of extended X-ray absorption fine structure (EXAFS) spectroscopy. It has been shown that the best agreement between the data obtained by these two methods is achieved within a model assuming a disorder in the arrangement of both the Fe and Br atoms and a high degree of correlation of their displacements. It has been found that, during the phase transition from the rhombohedral ferroelectric phase to the cubic paraelectric phase, no significant transformation of the structure is observed on a local level. In this case, a change in the macroscopic symmetry occurs predominantly as a result of the variation in the set of possible spatial orientations of stable structural fragments, which is characteristic of order-disorder phase transitions.     

Synthesis and characterization of hollow nanoparticles of crystalline Gd<Subscript>2</Subscript>O<Subscript>3</Subscript>

Although Gd₂O₃ (gadolinia) nanoparticle is the subject of intense research interest due to its magnetic property as well as controllable emission wavelengths by doping of various lanthanide ions, it is known to be difficult to prepare monodisperse crystalline gadolinia nanoparticles because it requires high temperature thermal annealing process to enhance the crystallinity. In this article, we demonstrate the synthesis of hollow nanoparticles of crystalline Gd₂O₃ by employing poly(N-vinylpyrrolidone) (PVP) to stabilize the surface of Gd(OH)CO₃·H₂O nanoparticles and to successively form SiO₂ shell as a protecting layer to prevent aggregation during calcinations processes. Silica shells could be selectively removed after calcinations by a treatment with basic solution to give hollow nanoparticles of crystalline Gd₂O₃. The formation mechanism of hollow nanoparticles could be suggested based on several characterization results of the size and shape, and crystallinity of Gd₂O₃ nanoparticles by TEM, SEM, and XRD.     

Kinetics of the transient optical absorption in Li<Subscript>2</Subscript>B<Subscript>4</Subscript>O<Subscript>7</Subscript> and LiB<Subscript>3</Subscript>O<Subscript>5</Subscript> lithium borate crystals

This paper reports on a study of the kinetics of electron tunneling transport between electron and hole centers in Li₂B₄O₇ and LiB₃O₅ lithium borate crystals under the conditions where the mobility of one of the partners in the recombination process is thermally stimulated. A mathematical model has been proposed to describe all specific features in the relaxation kinetics of the induced optical density observed in Li₂B₄O₇ (LTB) and LiB₃O₅ (LBO) nonlinear optical crystals within a broad time interval of 10⁻⁸−1 s after a radiation pulse. The results of calculations have been compared with experimental data on transient optical absorption (TOA) of LTB and LBO crystals in the visible and ultraviolet spectral regions. The nature of the radiation defects responsible for TOA and the dependence of the TOA decay kinetics on temperature, excitation power, and other experimental conditions have been discussed.     

Metallic liquid hydrogen and likely Al<Subscript>2</Subscript>O<Subscript>3</Subscript> metallic glass

Dynamic compression has been used to synthesize liquid metallic hydrogen at 140 GPa (1.4 million bar) and experimental data and theory predict Al₂O₃ might be a metallic glass at ∼ 300 GPa. The mechanism of metallization in both cases is probably a Mott-like transition. The strength of sapphire causes shock dissipation to be split differently in the strong solid and soft fluid. Once the 4.5-eV H-H and Al-O bonds are broken at sufficiently high pressures in liquid H₂ and in sapphire (single-crystal Al₂O₃), electrons are delocalized, which leads to formation of energy bands in fluid H and probably in amorphous Al₂O₃. The high strength of sapphire causes shock dissipation to be absorbed primarily in entropy up to ∼400 GPa, which also causes the 300-K isotherm and Hugoniot to be virtually coincident in this pressure range. Above ∼400 GPa shock dissipation must go primarily into temperature, which is observed experimentally as a rapid increase in shock pressure above ∼400 GPa. The metallization of glassy Al₂O₃, if verified, is expected to be general in strong oxide insulators. Implications for Super Earths are discussed.     

Luminescence and electronic excitations in crystals K<Subscript>2</Subscript>Al<Subscript>2</Subscript>B<Subscript>2</Subscript>O<Subscript>7</Subscript> with defects

This paper reports on the results of the comprehensive study of the dynamics of electronic excitations in K₂Al₂B₂O₇ (KABO) crystals, obtained by low-temperature luminescence vacuum ultraviolet spectroscopy with nanosecond time resolution upon photoexcitation by synchrotron radiation. For the first time, the data have been obtained on the photoluminescence (PL) decay kinetics, PL spectra with time resolution, PL excitation spectra with time resolution, and reflection spectra at 7 K; the intrinsic nature of PL at 3.28 eV has been established; luminescence bands of defects have been separated in the visible and ultraviolet spectral regions; an intense long-wavelength PL band has been detected at 1.72 eV; channels of the formation and decay of electronic excitations in K₂Al₂B₂O₇ crystals have been discussed.     

Internal friction of Li<Subscript>2</Subscript>B<Subscript>4</Subscript>O<Subscript>7</Subscript> single crystals

This paper reports on the results of measurements of the internal friction Q^?1 and the shear modulus G of Li₂B₄O₇ single crystals along the crystallographic directions [100] and [001] in the temperature range 300–550 K for strain amplitudes of (2–10)×10^?5 at infralow frequencies. The anomalies observed in Q^?1 and G in the temperature range 390–410 K are due to thermal activation of the mobility of lithium cations and their migration from one energetically equivalent position to another. A jump in the internal friction background is revealed in the vicinity of the Q^?1 and G anomalies for the Li₂B₄O₇ crystal. The magnitude of this jump depends on the crystallographic direction.     

Structural Aspects of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>/CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> Magnetic Nanoparticles According to X-Ray and Neutron Scattering

Structural aspects of powders containing magnetic nanoparticles Fe₃O₄/CoFe₂O₄ with the anticipated “core-shell” structure are considered by means of comparative analysis with individual particles of Fe₃O₄, CoFe₂O₄ in accordance of data obtained from X-ray powder diffraction and small-angle scattering of X-ray (synchrotron) radiation and neutrons. It is shown that magnetic particles in the powders under study have a strong polydispersity and form complex aggregates. Characteristic sizes of the crystallites, as well as a ratio of magnetite to cobalt-ferrite in the composition of the Fe₃O₄/CoFe₂O₄ particles were evaluated from the analysis of the diffraction peaks. Аnalyzing the data on small-angle scattering, the dimensional characteristics of particles and aggregates, as well as the volume fraction of the last ones in the powders, have been obtained. Fractal dimensions of aggregates are determined. A significant difference is observed in the scattering on Fe₃O₄/CoFe₂O₄ particles and the total scattering consisting of partial contributions to scattering on individual magnetite (Fe₃O₄) and cobalt-ferrite (CoFe₂O₄) powders, which does not exclude the formation of the “core-shell” structure.     

Edge absorption spectra of crystalline and glassy PbGeS<Subscript>3</Subscript>

Edge absorption spectra of crystalline and glassy lead thiogermanate (PbGeS₃) have been measured in the temperature range from 77 to 470 K. It is shown that the dependence of the absorption coefficient on the photon energy for the glassy and crystalline states in the polarization E ∥ c is described by the Urbach rule. For the crystal in the polarization E ∥ b, at T < 300 K, an almost parallel shift of the intrinsic absorption edge to lower energies occurs with an increase in temperature, whereas at T ≥ 300 K, the Urbach absorption edge is observed. The parameter σ₀, related to the electron-phonon coupling constant, and the energy ?ω_ph of the effective phonons involved in the formation of the absorption edge of crystalline PbGeS₃ are determined from the temperature dependence of the parameter of the absorption edge slope. The contributions of the dynamic and static disorders to the diffusion of the absorption edge of crystalline PbGeS₃, as well as the topological disorder of glassy PbGeS₃, have been estimated.     

Gas sensor based on nanosize In<Subscript>2</Subscript>O<Subscript>3</Subscript>:Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> film

Nanosize films of In₂O₃:Ga₂O₃ (96:4 weight %) have been deposited on a glassceramic substrate by the method of rf magnetron sputtering. The surfaces of fabricated films were studied with use of a scanning electron microscope; sizes of grains were determined and the thicknesses of films were measured. In order to prepare a gas-sensitive structure, a thin catalytic palladium layer and ohmic comb contacts were deposited on the In₂O₃:Ga₂O₃ film surface by the method of ion-plasma sputtering. The sensitivity of sensors based on the glassceramic/In₂O₃:Ga₂O₃ (96:4 weight %)/Pd structure to different concentrations of propane and butane gas mixture, as well as to methane was investigated at temperatures of working substance from 250 to 300°C.     

Interaction of carbon monoxide with In<Subscript>2</Subscript>O<Subscript>3</Subscript> and In<Subscript>2</Subscript>O<Subscript>3</Subscript>-Au nanocomposite

This is an IR spectroscopic study of the interaction of CO with In₂O₃ and the nanocomposite In₂O₃-Au. A mechanism for low-temperature detection of CO on nanocomposite In₂O₃-Au can be determined from these data. This process includes catalytic oxidation of CO through formation of intermediate complexes involving hydroxyl groups of In₂O₃.     

Spectral-luminescent investigations of glasses and glass-ceramic materials in Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-GeO<Subscript>2</Subscript>-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> system

Glasses of 2Bi₂O₃-3GeO₂-xFe₂O₃ composition, where x = 0–1.5, are obtained under oxidizing and reducing conditions. Glass-ceramic materials are produced by the thermal treatment of the glasses, the properties of which, as well as those of the original glasses, are studied by the methods of X-ray phase analysis and optical and luminescent spectroscopy. It is found that the Fe³⁺/Fe²⁺ ion ratio in the samples changes depending on the synthesis conditions of the original glasses and crystallization process.     

First-principles study of high pressure phase transformations in Li<Subscript>3</Subscript>N

The lattice dynamics of lithium nitride (Li₃N) under high pressure are extensively investigated to probe its phase transformations by using the pseudopotential plane-wave method within the density functional theory. A new second order α↦α^′-Li₃N phase transition is identified for the first time. The newly proposed α^′-phase possesses a hexagonal symmetry with four ions in the unit cell having a space group of P-3m1. Further enthalpy and phonon calculations support the existence of this phase, which stabilizes in a narrow pressure range of 2.8 – 3.6 GPa at zero temperature. Upon further compression, transitions to denser packed phases of β-and γ-Li₃N are typical first order. The analysis of the electronic densities of states suggests that all the high pressure modifications of Li₃N are insulators and, interestingly, the typical behavior of compression is to broaden the band gap.     

Structural,Electronic, and Optical Properties of Cubic Y<Subscript>2</Subscript>O<Subscript>3</Subscript>: First-Principles Calculations

Structural, electronic, and optical properties of cubic Y₂O₃ were studied using the plane-wave ultrasoft pseudopotential technique based on the first-principles density-functional theory (DFT). The ground-state properties were calculated and these results were in good agreement with the previous work. Furthermore, in order to understand the optical properties of cubic Y₂O₃, the complex dielectric function, refractive index, extinction coefficient, optical reflectivity, absorption coefficient, energy-loss function, and complex conductivity function were calculated, which were in favorable agreement with the theoretical and experimental values. We explained the origin of the absorption peaks using the theories of crystal-field and molecular-orbital bonding and investigated the relation between electronic structure and optical properties.     

Conductivity and spectroscopic studies of Li<Subscript>2</Subscript>O-V<Subscript>2</Subscript>O<Subscript>5</Subscript>-B<Subscript>2</Subscript>O<Subscript>3</Subscript> glasses

Lithium vanadium-borate glasses with the composition of 0.3Li₂O–(0.7-x)B₂O₃–xV₂O₅ (x?=?0.3, 0.325, 0.35, 0.375, 0.4, 0.425, 0.45, and 0.475) were prepared by melt-quenching method. According to differential scanning calorimetry data, vanadium oxide acts as both glass former and glass modifier, since the thermal stability of glasses decreases with an increase in V₂O₅ concentration. Fourier transform infrared spectroscopy data show that the vibrations of [VO₄] structural units occur at V₂O₅ concentration of 45 mol%. It is established that the concentration of V⁴⁺ ions increases exponentially with the growth of vanadium oxide concentration. Direct and alternative current measurements are carried out to estimate the contribution both electronic and ionic conductivities to the value of total conductivity. It is shown that the electronic conductivity is predominant in the total one. The glass having the composition of 0.3Li₂O-0.275B₂O₃-0.475V₂O₅ shows the highest electrical conductivity that has the value of 7.4?×?10^?5 S cm^?1 at room temperature.     

Investigation of polyamorphism in compressed B<Subscript>2</Subscript>O<Subscript>3</Subscript> glass by the direct measurement of the density

Precise measurements of the relative volume change of vitreous B₂O₃ have been performed by the strain-gauge technique at hydrostatic pressures up to 9 GPa. The features of the strain-gauge technique are analyzed and the specificity of the measurements of “relaxed” and “unrelaxed” bulk moduli is discussed. Smeared anomalies of compressibility (at P > 0.5 GPa and P > 5 GPa) and logarithmic relaxation of the glass density are observed. A significant (by several times!) difference has been revealed between the relaxed bulk modulus of glass obtained from the volume measurements and the unrelaxed modulus estimated from the Brillouin spectroscopic data. The measurements of the relative volume change under compression together with the previous structure investigations and computer simulation results reveal the basic features of the phase transitions in B₂O₃ glass. Both direct and reverse transitions are smeared in pressure. The residual densification in glass is not associated with change in the short-range order.