Features of acoustic wave propagation near the structural phase transition in the La<Subscript>0.875</Subscript>Sr<Subscript>0.125</Subscript>MnO<Subscript>3</Subscript> manganite

Features of the phase transition from the disordered state to the ordered orbital state in a La_0.875Sr_0.125MnO₃ single crystal, caused by the cooperative Jahn-Teller effect, have been investigated. A significant change in the acoustic wave parameters in the entire range of cooperative distortion of the structure is revealed. Application of an external magnetic field shifts the structural phase transition to low temperatures.     

Magnetoresistance of a Two-Dimensional TbTe<Subscript>3</Subscript> Conductor in the Sliding Charge-Density Wave Regime

The magnetoresistance of a TbTe₃ two-dimensional conductor with a charge-density wave (CDW) has been measured in a wide temperature range and in magnetic fields of up to 17 T. At temperatures well below the Peierls transition temperature and in high magnetic fields, the magnetoresistance exhibits a linear dependence on the magnetic field caused by the scattering of normal charge carriers by “hot” spots of the Fermi surface. In the sliding CDW regime in low magnetic fields, a qualitative change in the magnetoresistance has been observed associated with the strong scattering of carriers by the sliding CDW.     

High-frequency ultrasonic studies of the structural phase transition in an La<Subscript>0.875</Subscript>Sr<Subscript>0.125</Subscript>MnO<Subscript>3</Subscript> single crystal

The specific features of a phase transition from a disordered orbital state to an ordered orbital state in an La_0.875Sr_0.125MnO₃ single crystal are investigated using acoustic methods at a frequency f = 500 MHz. The phase transition is accompanied by a distortion of MnO₆ octahedra due to the cooperative Jahn-Teller effect and is a first-order phase transition, as judged from the sharp change observed in the damping of acoustic pulses, the acoustic wave velocity, and the temperature hysteresis. It is revealed that the parameters of the acoustic waves change significantly throughout the temperature range of existence of the cooperatively distorted structure. In an external magnetic field, the structural phase transition is shifted toward lower temperatures.     

Low-dimensional exchange interactions,structural deformation,and antiferromagnetic ordering in lithium manganite LiMnO<Subscript>2</Subscript>

Structure deformation in lithium manganite LiMnO₂ has been analyzed. It is shown that suppression of the Jahn-Teller effect with a decrease in temperature is related to striction effects, which are caused by intercluster exchange interactions. It is concluded that low-dimensional interactions play an important role in the formation of magnetic properties of lithium manganite.     

Manifestations of Phase Transitions and the Thermooptical-Memory Effect in the Absorption Spectra of (N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>)<Subscript>2</Subscript>CuCl<Subscript>4</Subscript> Crystals

The manifestations of phase transitions and the spatial modulation of the structure of the (N(CH₃)₄)₂CuCl₄ crystal in its absorption spectra have been investigated. It has been established that the thermooptical-memory effect manifesting itself in the absorption spectra of this crystal entirely corresponds to the model of ordering of defects and impurities in a sample with a modulated structure. It is shown that, as a result of stabilization of the crystal studied in the incommensurate phase, its symmetry and the symmetry of the metal-halogen complex in it lower depending on the defect wave. This manifests itself as a marked shift of the intraionic-absorption bands and an increase in their intensity. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 3, pp. 386–390, May–June, 2005.     

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.     

Shapiro steps in the torsion of a quasi-one-dimensional conductor TaS<Subscript>3</Subscript>

Features have been observed in the current dependences of the torsion angle, φ(I), for samples of a quasi-one-dimensional conductor TaS₃ irradiated by a high-frequency (HF) field. The features appear at the same current values as the Shapiro steps on the current-voltage characteristics, i.e., correspond to the synchronization of the motion of a charge-density wave (CDW) by the HF field. The shape of the features in the φ(I) dependences indicates a decrease in the inhomogeneous deformation of the CDW under the synchronization conditions. The investigation of torsion appears to be a highly sensitive method for determining the spatial coherence of the CDW.     

Effect of thermal treatment on the thermoelectric properties of Sb<Subscript>0.9</Subscript>Bi<Subscript>1.1</Subscript>Te<Subscript>2.9</Subscript>Se<Subscript>0.1</Subscript> solid solution thin films

The effect thermal treatment in a vacuum has on the thermoelectric properties of Sb_0.9Bi_1.1Te_2.9Se_0.1 solid solution thin films obtained via ion-beam sputtering in an argon atmosphere is considered. It is established that the specific resistance and thermopower are determined by the type and concentration of intrinsic point defects of the Sb_0.9Bi_1.1Te_2.9Se_0.1 solid solution. The power factor values are found to be comparable to those of nanostructured materials based on (Bi,Sb)₂(Te,Se)₃ solid solutions.     

Electron and electron-phonon effects in the quasi-two-dimensional molecular conductor θ-(BETS)<Subscript>4</Subscript>HgBr<Subscript>4</Subscript>(C<Subscript>6</Subscript>H<Subscript>5</Subscript>Cl): Optical studies at 300–15 K

This paper reports on a study of the polarized reflectance and optical conductivity spectra of the quasi-two-dimensional molecular conductor θ-(BETS)₄HgBr₄(C₆H₅Cl) within the 700–6500-cm^?1 region at 300–15 K and within the 9000–40 000 cm^?1 region at 300 K performed along two principal directions in the crystal plane parallel to the conducting layers of the BETS molecules. The IR spectra obtained at 300 K follow a close-to-Drude behavior, with strong broad features (1200–1400 cm^?1) due to electron-vibrational (vibronic) coupling (VC) superposed on the high Drude background. As the temperature is lowered in the range 180–80 K, in the spectra there appears a Lorentz term with ω_t=2900 cm^?1, as well as three additional VC-induced bands in the 800–1180-cm^?1 region, which disappear as the temperature is decreased further. The results obtained indicate the existence of unstable structural distortions along the two principal directions in the crystal, which are accompanied by the formation of a commensurate charge-density wave.     

Magnetostructural phase separation and giant isotope effect in R<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript>

Results of neutron diffraction studies of R_0.5Sr_0.5MnO₃ manganites (R = Sm, Nd_0.772Tb_0.228, and Nd_0.544Tb_0.456) performed to reveal the microscopic origins of the giant oxygen isotope effect recently discovered in Sm_0.5Sr_0.5MnO₃ are presented. It is shown that two crystalline phases differing in the type of Jahn-Teller distortions of oxygen octahedra and in the type of magnetic ordering coexist at low temperatures in all the studied compositions. A scenario for the observed phase transitions is suggested based on the diffraction data. It is found that the percolation transition from the metallic to insulating state in compositions with Sm upon substitution of ¹⁸O for ¹⁶O is associated with a sharp (from 65 to 13%) decrease in the volume of the ferromagnetic metallic phase.     

Luminescence and scintillation properties of Y<Subscript>3</Subscript>A<Subscript>l5</Subscript>O<Subscript>12</Subscript>:Ce single crystals and single-crystal films

Luminescence and scintillation properties of Y₃A_l5O₁₂:Ce single crystals grown from the melt by the Czochralski and horizontal directed crystallization methods in various gas media and Y₃A_l5O₁₂:Ce single-crystal films grown by liquid-phase epitaxy from a melt solution based on a PbO-B₂O₃ flux have been comparatively analyzed. The strong dependence of scintillation properties of Y₃A_l5O₁₂:Ce single crystals on their growth conditions and concentrations of Y_Al antisite defects and vacancy defects has been established. Vacancy defects are involved in Ce³⁺ ion emission excitation as the centers of intrinsic UV luminescence and trapping centers. It has been shown that Y₃A_l5O₁₂:Ce single-crystal films are characterized by faster scintillation decay kinetics than single crystals and a lower content of slow components in Ce³⁺ ion luminescence decay during high-energy excitation due to the absence of Y_Al antisite defects in them and low concentration of vacancy defects. At the same time, the light yield of Y₃A_l5O₁₂:Ce single-crystal films is comparable to that of single crystals grown by directed crystallization due to the quenching effect of the Pb²⁺ ion impurity as a flux component and is slightly lower (∼25%) than the light yield of single crystals grown by the Czochralski method.     

P2-type Na<Subscript>0.67</Subscript>Mn<Subscript>0.6</Subscript>Fe<Subscript>0.4-x-y</Subscript>Zn<Subscript>x</Subscript>Ni<Subscript>y</Subscript>O<Subscript>2</Subscript> cathode material with high-capacity for sodium-ion battery

The P2-type Na_0.67Mn_0.6Fe_0.4O₂ (NaMnFe), Na_0.67Mn_0.6Fe_0.3Zn_0.1O₂ (NaMnFeZn), and Na_0.67Mn_0.6Fe_0.2Zn_0.1Ni_0.1O₂ (NaMnFeZnNi) are prepared using an acetate decomposition reaction and developed as promising cathode materials for high-capacity sodium-ion batteries. The XRD patterns show that Zn²⁺ and Ni²⁺ ions are successfully incorporated into the lattice of the Na-Mn-Fe-O system, and the P2-type structure remains unchanged after substitution. The charging/discharging tests exhibit that the Na_0.67Mn_0.6Fe_0.4O₂, Na_0.67Mn_0.6Fe_0.3Zn_0.1O₂, and Na_0.67Mn_0.6Fe_0.2Zn_0.1Ni_0.1O₂ electrodes have the capacities of 200.4, 182.0, and 202.2 mAhg^?1, respectively. The Na_0.67Mn_0.6Fe_0.4O₂ electrode has a higher initial capacity but faster capacity decay. When partially substituting Zn and Ni for Fe, the Na_0.67Mn_0.6Fe_0.3Zn_0.1O₂ and Na_0.67Mn_0.6Fe_0.2Zn_0.1Ni_0.1O₂ electrodes exhibit lower reversible capacity but improved cycling stability (88.3 and 93.4% capacity retention over 100 cycles). The greatly improved electrochemical performance of the Na_0.67Mn_0.6Fe_0.2Zn_0.1Ni_0.1O₂ electrode apparently belongs to the contribution of the Zn²⁺ and Ni²⁺ substitution, which facilitates to alleviate the Jahn-Teller distortion of Mn and suppresses the polarization.     

Electrophysical properties and the structure of the YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> compound thermally restored after low-temperature decomposition

The electrophysical properties and structure of the nonstoichiometric high-temperature superconductor YBa₂Cu₃O _y restored at T = 930–950°C after low-temperature decomposition (T = 200°C) into phases different in the oxygen content have been studied. It has been shown that, unlike heat treatments at T ≤ 900°C, the superconducting properties are almost completely restored for 3–5 h during grain recrystallization, which is impossible at lower temperatures. After short-term annealing at T = 930–950°C (for 1–2 h), the ceramic material still contains a significant number of structural defects, most likely, in cation sublattices. These defects can contribute to the pinning of magnetic vortices, which substantially increases the critical current density in magnetic fields up to 2 T as compared to ceramic materials produced by the conventional technology.     

Fine structure and magnetism of the cubic oxide compound Ni<Subscript>0.3</Subscript>Zn<Subscript>0.7</Subscript>O

The fine structure and spin system of the cubic oxide Ni_0.3Zn_0.7O compound prepared from the initial hexagonal phase by quenching a sample with a high temperature and applying an external hydrostatic pressure to it have been studied using magnetic measurements, synchrotron and X-ray diffraction. It has been revealed that the diffraction patterns of this compound contain a system of weak diffuse maxima with the wave vectors q = (1/6 1/6 1/6)2π/a and (1/3 1/3 1/3)2π/a, along with strong Bragg peaks of the cubic phase. It has been shown that the origin of the diffuse peaks is due to longitudinal and transverse displacements of ions with respect to symmetric crystallographic directions of the {111} type. The reasons for the ion displacement and specific features of the structure of the spin system of the strongly correlated oxide Ni_0.3Zn_0.7O compound have been briefly discussed.     

High-temperature superconductors YBaCu<Subscript>2</Subscript>O<Subscript>5</Subscript> and Tl<Subscript>1.5</Subscript>BaCa<Subscript>2</Subscript>Cu<Subscript>2.5</Subscript>O<Subscript>8</Subscript> with a decreased content of heavy metals (Ba and Tl)

New high-temperature superconductors YBaCu₂O₅ and Tl_1.5BaCa₂Cu_2.5O₈ have been synthesized. They can be considered as a result of the removal of barium cuprate BaCuO₂ from YBa₂Cu₃O₇ and barium cuprate-tallate BaCu_0.5Tl_0.5O₂ from Tl₂Ba₂Ca₂Cu{ia3}O₁₀. The synthesized compounds contain a smaller amount of toxic Ba and Tl. Investigation of electrical conductivity has demonstrated that the values of T _c of synthesized materials are close to T _c of starting materials. The Meissner effect observed for these materials has confirmed their belonging to high-temperature superconductors.     

Formation of three-dimensional arrays of magnetic clusters NiO,Co<Subscript>3</Subscript>O<Subscript>4</Subscript>, and NiCo<Subscript>2</Subscript>O<Subscript>4</Subscript> by the matrix method

A method has been proposed for the formation of three-dimensional arrays of isolated magnetic clusters NiO, Co₃O₄, and NiCo₂O₄ in the sublattice of pores in the matrix of bulk synthetic opals through a single impregnation of the pores with melts of nickel and cobalt nitrate crystal hydrates and their thermal degradation. The method makes it possible to controllably vary the degree of filling of pores in the matrix with oxides within 10–70 vol %. The composition and structure of the synthesized materials, as well as the dependences of their static magnetic susceptibility on the magnetic field strength, have been investigated.     

Enhanced electrochemical performance of lithium ion battery cathode Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>/C

Li-ion battery cathode material lithium-vanadium-phosphate Li₃V₂(PO₄)₃ was synthesized by a carbon-thermal reduction method, using stearic acid, LiH₂PO₄, and V₂O₅ as raw materials. And stearic acid acted as reductant, carbon source, and surface active agent. The effect of its content on the crystal structure and electrochemical performance of Li₃V₂(PO₄)₃/C were characterized by XRD and electrochemical performance testing, respectively. The results showed that the content of carbon source has no significant effect on the crystal structure of lithium vanadium phosphate. Lihtium vanadium phosphate obtained with 12.3% stearic acid demonstrated the best electrochemical properties with a typical discharge capacity of 119.4 mAh/g at 0.1 C and capacity retention behavior of 98.5% after 50 cycles. And it has high reversible discharge capacity of 83 mAh/g at 5 C with the voltage window of 3 to 4.3 V.     

Atomic and Electronic Structures of Intrinsic Defects in Ta<Subscript>2</Subscript>O<Subscript>5</Subscript>: Ab Initio Simulation

The atomic and electronic structure of intrinsic point defects in orthorhombic tantalum oxide has been studied by numerical simulation within the density functional theory. It has been shown that all defects responsible for metal enrichment of Ta₂O₅ serve as electron and hole traps. Under conditions of strong oxygen depletion and at a metal–insulator interface, which are characteristic of resistive memory elements, interstitial tantalum atoms compete with an oxygen vacancy in the formation of a conducting filament. Interstitial oxygen atoms are not involved in charge transport. Tantalum substituting oxygen can be considered as a combination of the oxygen vacancy and interstitial tantalum. The analysis of the calculated thermal and optical energies of trap ionization shows that the oxygen vacancy is a key defect for charge transport in Ta₂O₅.