The giant dielectric tunability effect in bulk La2NiMnO6 around room temperature

The effect of applied dc bias electric field on dielectric permittivity in bulk La₂NiMnO₆ is investigated in this paper. It is found that a small bias field of 40 V/cm can greatly reduce the dielectric permittivity around the room temperature, compared to the much larger electric field that is required for conventional ferroelectric materials. The observed giant dielectric tunability is retained over a broad range of around room temperature and is most likely related to the charge ordering of Ni²⁺ and Mn⁴⁺ ions, further confirming the existence of electronic ferroelectricity in La₂NiMnO₆.     

Superconductivity well above room temperature in compressed MgH<Subscript>6</Subscript>

It has been suggested that hydrogen-rich systems at high pressure may exhibit notably high super-conducting transition temperatures. One of the more interesting theoretical predictions was that hydrogen sulfide can be metallized and the high-temperature superconducting state can be induced. A record critical temperature (203 K) was later confirmed for H3S in an experiment. In this paper, we investigated, within the framework of the Eliashberg formalism, the properties of compressed MgH6, which is expected to be a very good candidate for room-temperature superconductivity. This applies particularly to the pressure range from 300 to 400 GPa, where the transition temperature is close to 400 K. Moreover, the estimated thermodynamic properties and the resulting dimensionless ratios exceed the predictions of the Bardeen–Cooper–Schrieffer theory. This behavior is attributed to the strong electron–phonon coupling and retardation effects existing in hydrogen-dominated materials under high pressure.     

Large successive magnetocaloric effects around room temperature in Ni<Subscript>50</Subscript>Mn<Subscript>34</Subscript>In<Subscript>15</Subscript>Al alloy

Two successive magnetocaloric effects consisting of inverse magnetocaloric effect around martensitic transition and negative magnetocaloric effect around magnetic transition of austenitic phase have been observed in Ni₅₀Mn₃₄In₁₅Al alloy. Large inverse magnetic entropy change ΔS_m ( ~ 21.3 J kg^?1 K^?1), small thermal and magnetic hysteresis of martensitic transition give rise of large net refrigerant capacity ( ~ 152.3 J kg^?1) under a magnetic field of 50 kOe, which is comparable with that ( ~ 157.9 J kg^?1) of second-order transition. The large combined magnetocaloric effects make the Ni₅₀Mn₃₄In₁₅Al alloy as a promising candidate material for room temperature magnetic refrigeration.     

Magnetodielectric coupling by exchange striction in Y<Subscript>2</Subscript>Cu<Subscript>2</Subscript>O<Subscript>5</Subscript>

We have studied the magnetodielectric response of Y₂Cu₂O₅, the so-called blue phase in the Y₂O₃-CuO-BaO phase diagram. Based on symmetry principles, we predict and demonstrate magneto-dielectric coupling on a single crystal sample. We report an anomaly in the dielectric constant at the ordering temperature of the Cu spins. We probe the magnetic field-induced phase transitions between four different magnetic phases using magneto-capacitance measurements, demonstrating relatively strong magnetodielectric coupling. We observe an increase in dielectric constant in the spin-flip phase where there exists spontaneous magnetization. We construct a detailed magnetic phase diagram. The magnetodielectric coupling is analyzed in terms of striction induced by symmetric superexchange and optical phonon frequency shifts.     

Effect of porosity and pore morphology on the low-frequency dielectric response in sintered ZrO<Subscript>2</Subscript>—8 mol% Y<Subscript>2</Subscript>O<Subscript>3</Subscript> ceramic compact

Effect of porosity and pore size distribution on the low-frequency dielectric response, in the range 0.01–100 kHz, in sintered ZrO₂—8 mol% Y₂O₃ ceramic compacts have been investigated. Small-angle neutron scattering (SANS) technique has been employed to obtain the pore characteristics like pore size distribution, specific surface area etc. It has been observed that the real and the imaginary parts of the complex dielectric permittivity, for the specimens, depend not only on the porosity but also on the pore size distribution and pore morphology significantly. Unlike normal Debye relaxation process, where the loss tangent vis-à-vis the imaginary part of the dielectric constant shows a pronounced peak, in the present case the same increases at lower frequency region and an anomalous non-Debye type relaxation process manifests.     

Magnetoreflection and magnetooptical Kerr effect in La<Subscript>2/3</Subscript>Ba<Subscript>1/3</Subscript>MnO<Subscript>3</Subscript> films at room temperature

Results of studies of magnetooptical Kerr effect and magnetoreflection of natural light in La_2/3Ba_1/3MnO₃/SrTiO₃ films of different thickness are presented. The Kerr effect was shown to be the most prominent in visible and near IR range; magnetoreflection was found to achieve its maximum of about 10% in the mid-IR range near the room temperature. Physical mechanisms defining the value and sign of the effects and the influence of the thin-film state on the magnetooptical properties are discussed. Magnetoreflection is estimated in the framework of the magnetorefractive effect theory.     

High-temperature conductivity and structure of Y<Subscript>2</Subscript>(WO<Subscript>4</Subscript>)<Subscript>3</Subscript> ceramics

The magnitude and character of conductivity were studied for Y₂(WO₄)₃ ceramics synthesized by the ceramic (from oxides) and organic-nitrate procedures. Investigation of the dependence \(\sigma \left( {{\alpha _{{o_2}}}} \right)\) and measurements of the ion transport numbers of charge carriers by the EMF method showed that Y₂(WO₄)₃ is basically an ion conductor. The conductivity is largely determined by the sample preparation conditions related to the dependence of the specific surface area and powder grain size on the synthetic procedure. The maximum high-temperature conductivity of Y₂(WO₄)₃ was 2.51 × 10^–4 S/cm, which roughly corresponds to the conductivities of Sc₂(WO₄)₃ and In₂(WO₄)₃ measured under the same conditions. It was confirmed that Y₂(WO₄)₃ crystallizes as a mixed monoclinic-orthorhombic structure at 1000°C. The character of water incorporation in hydrated Y₂(WO₄)₃ crystals was studied by thermogravimetry and diffuse reflectance IR spectroscopy. A qualitative model of water intercalation was suggested.     

Optical phonons in Rb<Subscript>2</Subscript>TeBr<Subscript>6</Subscript> and Cs<Subscript>2</Subscript>TeBr<Subscript>6</Subscript> crystals

Raman scattering in Rb₂TeBr₆ and Cs₂TeBr₆ crystals is studied. The phonon spectra of the crystals are calculated using the factor group method. The number of Raman-active modes, their symmetries, and selection rules are found. Observed Raman spectrum lines are identified with atomic vibration modes of the crystal.     

Characterization of Y<Subscript>2</Subscript>BaCuO<Subscript>5</Subscript> nanoparticles synthesized by nano-emulsion method

Nanoscale yttrium–barium–copper oxide (Y₂BaCuO₅, Y211) particles were synthesized using the emulsion method and the solution method. The basic water-in-oil (w/o) emulsion system consisted of n-octane (continuous oil phase), cetyltrimethylammonium bromide (cationic surfactant), butanol (cosurfactant) and water. The composition of the emulsion system was varied and characterized by measuring the conductivity of the solutions and droplet size. The droplet size of emulsion was determined by using the dynamic light scattering method. The water content, cosurfactant content, and surfactant/n-octane ratio affected the droplet size which was in the range of 3–8 nm, and hence the w/o emulsion system was referred to as a nano-emulsion system. A model was used to verify the droplet size. The influence of salt (Y₂(NO₃)₃) content on the droplet size was investigated and the addition of salt reduced the droplet size. The effects of reaction time and temperature on the Y211 particle sizes were also investigated. The particles were characterized using the TEM, SEM, and XRD. Nanoparticles produced by the nano-emulsion method were calcined at 850°C to form the Y211 phase as compared to solid state processing temperature of 1050°C. Based on the TEM analysis, the average diameter of the Y211 particles produced using the nano-emulsion method was in the range of 30–100 nm. The effect of adding 15% Y211 nanoparticles to the superconductor YBCO-123 as flux pinning centers, was investigated, and the transition temperature was reduced by 3 K.     

Spectral characteristics of a Yb-doped Y<Subscript>2</Subscript>O<Subscript>3</Subscript> ceramic laser

We report the experimental observation of random wavelength emission and intensity-dependent central-wavelength shift in a diode-pumped Yb³⁺-doped Y₂O₃ ceramic laser. We show experimentally that, like conventional lasers, the emission of the laser has fixed well-defined transverse modes; however, its instantaneous emission wavelengths change randomly with time. The central wavelength of the laser emission also shifts with the intracavity light intensity. A model was developed to describe the spectral behavior of Yb³⁺-doped lasers. We show that the observed random wavelength emission and central lasing wavelength shift of the laser could be well explained based on the strong reabsorption of light in the gain medium. PACS 42.55.Rz; 42.60.Mi; 42.55.Xi     

Study on the spin-states of cobalt-based double-layer perovskite Sr<Subscript>2</Subscript>Y<Subscript>0.5</Subscript>Ca<Subscript>0.5</Subscript>Co<Subscript>2</Subscript>O<Subscript>7</Subscript>

The spin-states of cobalt based perovskite compounds depend sensitively on the valence state and local crystal environment of Co ions and the rich physical properties arise from strong coupling among charge, spin, and orbital degrees of freedom. While extensive studies have been carried out in the past, most of them concentrated on the isotropic compound LaCoO₃. In this paper, using the unrestricted Hartree-Fock approximation and the real-space recursion method, we have investigated the competition of various magnetically ordered spin-states of anisotropic double-layered perovskite Sr₂Y_0.5Ca_0.5Co₂O₇. The energy comparison among these states shows that the nearest-neighbor high-spin-intermediate-spin ferromagnetically ordered state is the relevant magnetic ground state of the compound. The magnetic structure and sizes of magnetic moments are consistent with the recent experimental observation.     

Anisotropy of the dielectric permittivity of Sn<Subscript>2</Subscript>P<Subscript>2</Subscript>S<Subscript>6</Subscript> measured with light-induced grating techniques

We apply, for the first time to our knowledge, photorefractive grating spectroscopy to obtain not-yet-known data on the anisotropy of the dielectric permittivity of Sn₂P₂S₆. Two independent techniques are used, one based on measurements of the amplitude of the space-charge field grating as a function of grating spacing and the other based on measurements of the grating decay time, also as a function of grating spacing. Both techniques provide close values for the anisotropy, which appears to be well pronounced, a ratio ε_xx/ε_zz≈4 is revealed for two of the three independent components of the dielectric tensor. Our data also allow us to conclude that the charge mobility is nearly isotropic in the same plane, μ_xx/μ_zz≈1. Received: 2 December 2002 / Published online: 26 March 2003 RID="*" ID="*"Corresponding author. Fax: +380-44/265-2359, E-mail: odoulov@iop.kiev.ua     

Magnetic properties of the Gd<Subscript>2</Subscript>V<Subscript>0.67</Subscript>Mo<Subscript>1.33</Subscript>O<Subscript>7</Subscript> and Y<Subscript>2</Subscript>VMoO<Subscript>7</Subscript> pyrochlore compounds

It is demonstrated that 50% substitution of vanadium for molybdenum in the pyrochlore lattice of the complex oxide Y₂(V _x Mo_{1 ? x} )₂O₇ results in a transition from the spin-glass ground state (at x = 0) to the ferromagnetic state in Y₂VMoO₇ (a = 10.1645(2) Å, T _C = 55 K). The Gd₂V_0.67Mo_1.33O₇ compound (a = 10.2862(3) Å) is a ferromagnet with T _C (84 K) exceeding that of undoped Gd₂MnO₂O₇.     

Investigation of luminescence spectra of the Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> and Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>:Ce single-crystal films

The luminescence spectra of single-crystal films and bulk crystals of yttrium-aluminum garnet Y₃Al₅O₁₂ and Ce³⁺-activated Y₃Al₅O₁₂ were investigated. It was shown that the room-temperature luminescence intensity of the Ce³⁺-free single-crystal Y₃Al₅O₁₂ film was considerably lower than that of the bulk crystals, while the luminescence intensity of the Ce³⁺ ions in the Y₃Al₅O₁₂:Ce films was considerably higher than that one for the corresponding bulk crystal.     

On the room temperature multiferroic BiFeO<Subscript>3</Subscript>: magnetic,dielectric and thermal properties

Magnetic dc susceptibility between 1.5 and 800 K, ac susceptibility and magnetization, thermodynamic properties, temperature dependence of radio and audio-wave dielectric constants and conductivity, contact-free dielectric constants at mm-wavelengths, as well as ferroelectric polarization are reported for single crystalline BiFeO₃. A well developed anomaly in the magnetic susceptibility signals the onset of antiferromagnetic order close to 635 K. Beside this anomaly no further indications of phase or glass transitions are indicated in the magnetic dc and ac susceptibilities down to the lowest temperatures. The heat capacity has been measured from 2 K up to room temperature and significant contributions from magnon excitations have been detected. From the low-temperature heat capacity an anisotropy gap of the magnon modes of the order of 6 meV has been determined. The dielectric constants measured in standard two-point configuration are dominated by Maxwell-Wagner like effects for temperatures T > 300 K and frequencies below 1 MHz. At lower temperatures the temperature dependence of the dielectric constant and loss reveals no anomalies outside the experimental errors, indicating neither phase transitions nor strong spin phonon coupling. The temperature dependence of the dielectric constant was measured contact free at microwave frequencies. At room temperature the dielectric constant has an intrinsic value of 53. The loss is substantial and strongly frequency dependent indicating the predominance of hopping conductivity. Finally, in small thin samples we were able to measure the ferroelectric polarization between 10 and 200 K. The saturation polarization is of the order of 40 μC/cm², comparable to reports in literature.     

Influence of K<Subscript>2</Subscript>Ti<Subscript>6</Subscript>O<Subscript>13</Subscript> on dielectric and barrier properties of polymer

Polymer composite comprising polyvinylidene fluoride (PVDF) and potassium hexatitante (K₂Ti₆O₁₃) was synthesized by solution casting. The effect of K₂Ti₆O₁₃ on surface, thermal, and electrical properties of polymer composite were investigated. The addition of K₂Ti₆O₁₃ with polymer leads to thermal degradation and transition of polymer composite from semi-crystalline to amorphous phase. The optimum results of contact angle for different loading wt% of K₂Ti₆O₁₃ were directly correlated with the surface morphology. Our experimental results confirmed the incorporation of K₂Ti₆O₁₃ in polymer by SEM micrographs. The evaluated dielectric properties (ε' = 424; tan δ = 2.14 at 130 °C and 100 Hz frequency for 20 wt% loading of K₂Ti₆O₁₃) for polymer composite are higher in compared to pure polymer. The enhancement in dielectric constant and changing the surface properties of polymer composite can be used for the development of electrochemical storage device applications.     

Investigation of the dielectric,optical and photorefractive properties of Sb-doped Sn<Subscript>2</Subscript>P<Subscript>2</Subscript>S<Subscript>6</Subscript> crystals

In this work we report results on electro-physical, optical and photorefractive investigations for Sb-doped Sn₂P₂S₆ crystals. The crystals are obtained by two methods: the vapour-transport technique and the Bridgman technique using stoichiometric Sn₂P₂S₆ composition with different amounts of antimony in the initial compound. The good optical quality of the crystals obtained with the Bridgman technique is underlined. The dependences of the photorefractive two-beam coupling coefficient and the grating build-up time are investigated at the wavelength of 633 nm. It is found that the sample doped with 1.5% of Sb is characterized by an optimal combination of the main photorefractive parameters exhibiting a fairly high two-beam coupling coefficient (up to 20 cm⁻¹) and a short response time (1.3 ms) that is the shortest among all the previously studied Sn₂P₂S₆ crystals in the red spectral region.     

Structural and electronic properties of Y<Subscript>2</Subscript>CrS<Subscript>4</Subscript> from first-principles study

We systematically study the structural, electronic, and magnetic properties of chromium sulfide Y₂CrS₄ by using density-functional theory. We find that antiferromagnetic order is more energetically favorable than ferromagnetic state and near the Fermi level the main occupation is from Cr 3d states.