Synthesis,Crystal Structure,and Magnetic Properties of the YbFeTi<Subscript>2</Subscript>O<Subscript>7</Subscript> Compound

We report on the synthesis conductions and results of experimental investigations of the crystal structure and magnetic properties of a new magnetic compound YbFeTi₂O₇. According to the X-ray diffractometry data, the crystal structure of the investigated compound is described by the rhombic space group Pcnb with unit cell parameters of a = 9.8115(1) Å, b = 13.5106(2) Å, and c = 7.31302(9) Å and atomic disordering in the distribution of iron ions Fe³⁺ over five structural sites. The magnetic measurements in the lowtemperature region revealed a kink in the temperature dependence of the magnetic moment and its dependence on the sample magnetic prehistory. The experimental results obtained suggest that with a decrease in temperature the sample passes from the paramagnetic state to the spin-glass-like magnetic state characterized by a freezing temperature of T _f = 4.5 K at the preferred antiferromagnetic exchange coupling in the sample spin system. The chemical pressure variation upon replacement of rare-earth ion R by Yb in the RFeTi₂O₇ system does not change the crystal lattice symmetry and magnetic state.     

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.     

Elastic,magnetic, and magnetoelectric properties of the CaBaCo<Subscript>4</Subscript>O<Subscript>7</Subscript> multiferroic

The structural, elastic, magnetic, and magnetoelectric properties of the CaBaCo₄O₇ multiferroic are experimentally studied and compared with the properties of the related YBaCo₄O₇ cobaltite, where Y³⁺ ions substitute for Ca²⁺ ions. Unlike the frustrated YBaCo₄O₇ magnet, the softening of Young’s modulus and the hysteresis in the ΔE(T)/E ₀ curve of ferrimagnetic CaBaCo₄O₇ in the paramagnetic region are weak, and the anomaly during the magnetic transition increases by almost an order of magnitude. This difference can point to different characters of the development of a long-range magnetic order in these two cobaltites. The distortion of the crystal structure that removes the frustrations of exchange interactions is found to correlate with the magnetic behavior of the cobaltites under study. The magnetization curves of the Ca cobaltite have two steps below 15 K, which can point to the presence of a metastable state in a high magnetic field. The study of the longitudinal and transverse magnetoelectric effects in a pulsed magnetic field demonstrates that their magnitudes are maximal near T _C and change their character from linear to quadratic during passage through this temperature.     

The Structure of Electronic States and Optical Properties of Cr<Subscript>80</Subscript>Al<Subscript>20</Subscript> Compound

Ellipsometric studies of the optical properties of the intermetallic Cr₈₀Al₂₀ compound in the spectral range of 0.22–15 μm have been performed. The self-consistent calculation of the electronic structure and density of the electronic states of the material has been carried out. The spectrum of interband light absorption is interpreted on the basis of comparative analysis of theoretical and experimental dispersion dependences of optical conductivity. A number of characteristics of conduction electrons have been determined.     

Preparation and electrochemical properties of Li<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>12</Subscript>/Ti<Subscript>4</Subscript>O<Subscript>7</Subscript> composite for lithium-ion batteries

In order to improve the rate capability of Li₄Ti₅O₁₂, Ti₄O₇ powder was successfully fabricated by improved hydrogen reduction method, then a dual-phase composite Li₄Ti₅O₁₂/Ti₄O₇ has been synthesized as anode material for lithium-ion batteries. It is found that the Li₄Ti₅O₁₂/Ti₄O₇ composite shows higher reversible capacity and better rate capability compared to Li₄Ti₅O₁₂. According to the charge-discharge tests, the Li₄Ti₅O₁₂/Ti₄O₇ composite exhibits excellent rate capability of 172.3 mAh g^?1 at 0.2 C, which is close to the theoretical value of the spinel Li₄Ti₅O₁₂. More impressively, the reversible capacity of Li₄Ti₅O₁₂/Ti₄O₇ composite is 103.1 mAh g^?1 at the current density of 20 C after 100th cycles, and it maintains 84.8% of the initial discharge capacity, whereas that of the bare spinel Li₄Ti₅O₁₂ is only 22.3 mAh g^?1 with a capacity retention of 31.1%. The results indicate that Li₄Ti₅O₁₂/Ti₄O₇ composite could be a promising anode material with relative high capacity and good rate capability for lithium-ion batteries.     

Energy Band Structure and Optical Properties of LiNaSO<Subscript>4</Subscript> Crystals

The energy band structure of a LiNaSO₄ has been calculated within the framework of density functional theory for space group P31c. It has been established that bandgap width E_g in the GGA approximation is 5.49 eV. The valence band top is generally formed of oxygen p-electrons, while the conduction band bottom is composed of lithium and sodium s-states. The effect of the cationic substitution Na → K → Rb → NH₄ on the electron structure of LiBSO₄ group crystals is considered. Based on the calculated dielectric functions, the spectral dependences of the refractive indices and extinction coefficient of a crystal were calculated and compared with experimental data.     

Structure,charge ordering and physical properties of Yb<Subscript>2</Subscript>Fe<Subscript>3</Subscript>O<Subscript>7</Subscript>

The structural and physical properties of the layered Yb₂Fe₃O₇ have been extensively investigated. Transmission electron microscopy (TEM) observations at room temperature reveal the presence of diffuse zigzag-type streaks at 1/3(h h l) running along the c* axis direction, suggesting the presence of a charge ordered state with a shorter coherence length in comparison with that in Lu₂Fe₃O₇. The measurements of magnetization demonstrate that the replacement of Lu³⁺ by the magnetic Yb³⁺ ion in this layered system could result in visible effects on the low-temperature magnetic properties: the ferrimagnetic phase transition temperature decreases and an additional magnetic anomaly possibly attributed to antiferromagnetic coupling between Yb and Fe layers appears at around 50 K. Analysis of the dielectric properties shows that the Yb₂Fe₃O₇ material in general has a large dielectric constant of about 5000 at room temperature, and a broader relaxation time distribution in comparison with ErFe₂O₄.     

Structure,lattice dynamics,and exchange interaction in Lu<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript>, Y<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript>: an ab initio approach

Ab initio calculations of the crystal structure and fundamental vibrations of vanadium pyrochlores Lu₂V₂O₇ and Y₂V₂O₇ are performed. The calculations are performed in the framework of the density functional theory (DFT) with the use of hybrid functionals. The ions involved in the vibrations are determined by the isotope substitution method. Values of the isotropic exchange interaction constant were calculated. Theoretical results for the crystal structure parameters, the vibrational frequencies, and the isotropic exchange interaction parameter are compared with the experimental data.     

Calculations of Structural,Electronic, Chemical Bonding,and Optical Properties of Orthorhombic CsAlTiO<Subscript>4</Subscript>

Structural parameters, electronic, chemical bonding and optical properties of orthorhombic CsAlTiO₄ are studied using the plane-wave ultrasoft pseudopotential technique based on the first-principles density-functional theory (DFT). The equilibrium lattice constants, bulk modulus and electronic structure are obtained. To our knowledge, no data are available in literature of orthorhombic CsAlTiO₄ with Pnma space group for comparison. Electronic and chemical bonding properties have been studied from the calculations of band structure, density of states and charge densities. The complex dielectric functions are calculated and we have explained the origins of spectral peaks.     

Luminescence characteristics of the Pr<Superscript>3+</Superscript> ion in SrB<Subscript>4</Subscript>O<Subscript>7</Subscript> and SrB<Subscript>6</Subscript>O<Subscript>10</Subscript>

Spectral and kinetic characteristics of the luminescence and luminescence excitation spectra of polycrystalline SrB₄O₇:Pr (1%) and SrB₆O₁₀:Pr (1%) samples are studied at 150–170 K. The samples show an intense luminescence band in the vicinity of 405 nm (¹ S ₀→¹ I ₆ transitions of Pr³⁺) and shorter wavelength bands also assigned to transitions from the ¹ S ₀ level. The main luminescence decay constant is ～2×10^?7 s. The excitation spectra of the ¹ S ₀ luminescence in these crystals are significantly different. The SrB₄O₇:Pr crystal shows three well-resolved bands at 6.14, 6.55, and 6.91 eV in the region of the 4f ²→4f ¹5d transitions and a complex structure in the region of interband transitions (7.1–20 eV), whereas the SrB₆O₁₀:Pr crystal shows a weakly structured band at 6.31 eV and no excitation in the region of the interband transitions. The physical mechanisms that may be responsible for the observed features of the spectra are 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.     

Electronic excitations in BeAl<Subscript>2</Subscript>O<Subscript>4</Subscript>, Be<Subscript>2</Subscript>SiO<Subscript>4</Subscript>, and Be<Subscript>3</Subscript>Al<Subscript>2</Subscript>Si<Subscript>6</Subscript>O<Subscript>18</Subscript> crystals

Low-temperature (T = 7 K) time-resolved selectively photoexcited luminescence spectra (2–6 eV) and luminescence excitation spectra (8–35 eV) of wide-bandgap chrysoberyl BeAl₂O₄, phenacite Be₂SiO₄, and beryl Be₃Al₂Si₆O₁₈ crystals have been studied using time-resolved VUV spectroscopy. Both the intrinsic luminescence of the crystals and the luminescence associated with structural defects were assigned. Energy transfer to impurity luminescence centers in alexandrite and emerald was investigated. Luminescence characteristics of stable crystal lattice defects were probed by 3.6-MeV accelerated helium ion beams.     

Elevated electrochemical property of LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> originated from nano-sized Mn<Subscript>3</Subscript>O<Subscript>4</Subscript>

By employment of nano-sized pre-prepared Mn₃O₄ as precursor, LiMn₂O₄ particles have been successfully prepared by facile solid state method and sol-gel route, respectively. And the reaction mechanism of the used precursors of Mn₃O₄ is studied. The structure, morphology, and element distribution of the as-synthesized LiMn₂O₄ samples are characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). Compared with LiMn₂O₄ synthesized by facile solid state method (SS-LMO), LiMn₂O₄ synthesized by modified sol-gel route (SG-LMO) possesses higher crystallinity, smaller average particle size (~175 nm), higher lithium chemical diffusion coefficient (1.17 × 10^?11 cm² s^?1), as well as superior electrochemical performance. For example, the cell based on SG-LMO can deliver a capacity of 85.5 mAh g^?1 at a high rate of 5 °C, and manifests 88.3% capacity retention after 100 cycles at 0.5 °C when cycling at 45 °C. The good electrochemical performance of the cell based on SG-LMO is ascribed mainly to its small particle size, high degree of dispersion, and uniform element distribution in bulk material. In addition, the lower polarization potential accelerates Li⁺ ion migration, and the lower atom location confused degree maintains integrity of crystal structure, both of which can effectively improve the rate capability and cyclability of SG-LMO.     

High-temperature heat capacity of stannates Pr<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript> and Nd<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript>

Oxide compounds Pr₂Sn₂O₇ and Nd₂Sn₂O₇ have been obtained by solid-phase synthesis. The effect of temperature on the heat capacity of Pr₂Sn₂O₇ (360–1045 K) and Nd₂Sn₂O₇ (360–1030 K) has been studied using differential scanning calorimetry. The thermodynamic properties of the compounds (changes in enthalpy, entropy, and the reduced Gibbs energy) have been calculated by the experimental data of C_p = f(T).     

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.     

Theoretical Study of the Structural,Electronic, Chemical Bonding and Optical Properties of the A2<Subscript>1</Subscript><Emphasis Type="Italic">am</Emphasis> Orthorhombic SrBi<Subscript>2</Subscript>Ta<Subscript>2</Subscript>O<Subscript>9</Subscript>

The structural parameters, density of states, electronic band structure, charge density, and optical properties of orthorhombic SrBi₂Ta₂O₉ have been investigated using the plane-wave ultrasoft pseudopotential technique based on the first-principle density functional theory (DFT). The calculated structural parameters were in agreement with the previous theoretical and experimental data. The band structure showed an indirect (S to Γ) band gap with 2.071 eV. The chemical bonding along with population analysis has been studied. The complex dielectric function, refractive index, and extinction coefficient were calculated to understand the optical properties of this compound, which showed an optical anisotropy in the components of polarization directions (100), (010), and (001).     

Specific features of the electrical conductivity of V<Subscript>4</Subscript>O<Subscript>7</Subscript> single crystals

The electrical conductivity of V₄O₇ single crystals has been measured over a wide temperature range, including both the region of existence of the metallic phase and the region of the metal-insulator transition. It has been shown that the low conductivity of metallic V₄O₇ is due to the strong electron-electron correlation, whose role increases with decreasing temperature as the phase-transition temperature is approached. The temperature dependence of the conductivity of the insulator phase of V₄O₇ is explained in terms of the theory of hopping conduction taking into account the influence of atomic thermal vibrations on the resonance integral.     

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.     

Synthesis,sintering, and conductivity of Mn<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript>

Studies on the sintering of manganese pyrovanadate depending on the temperature and the crystallite size show that we are prevented from obtaining a bulk ceramic sample by the anisotropic growth of grains. Investigation of the electrical properties of Mn₂V₂O₇ in the temperature range of 250–800°C reveals the activation energy at which bulk conductivity is 0.62 eV.     

Temperature dependences of the second-harmonic generation in LiKB<Subscript>4</Subscript>O<Subscript>7</Subscript> single crystals

We have discovered interesting temperature anomalies of the second-harmonic generation (SHG) in novel LiKB₄O₇ single crystals. We have found that the maximal SHG signal exists at a temperature equal to about 230–240 K. The maximally achieved second-order susceptibility was equal to about 1.4 pm/V at a 1064-nm laser wavelength and was achieved at the sizes of crystallites of less than 80 nm. At sizes of about 1200 nm, the second-order optical properties are commensurable with the second-order susceptibilities of the bulk crystallites and are equal to about 0.6 pm/V. The observed size sensitivity of the effect may reflect the substantial role of the nanoconfined effects.