Production of Sr-deficient bismuth tantalates from microwave-hydrothermal derived precursors: Structural and dielectric properties

Strontium bismuth tantalates were produced for the first time from microwave-hydrothermal precursors at , for 2 h. Structural and dielectric properties were investigated by X-ray diffraction and complex impedance spectroscopy. A high ferroelectric-paraelectric transition temperature of was observed, together with two different dielectric regimes for the ac electrical conductivity below T_c. The activation energies were calculated as 0.155 and 0.531 eV, and are related to conduction by oxygen vacancies. It was concluded that the low activation energies showed by these materials could contribute to their fatigue-free nature.     

Dielectric relaxation in single crystal NH4H2PO4 in the high-temperature regime

The dispersion curves of the dielectric response in single crystal NH₄H₂PO₄ were obtained in the radio frequency range and below the high-temperature transition at T_p−160 °C. The results reveal dielectric relaxation at low frequency, which is about 10⁵ Hz at 70 °C, and it shifts to higher frequencies (∼3×10⁶ Hz) as the temperature increases. The relaxation frequency was determined from the peak obtained in the imaginary part of the permittivity as well as from the derivative of the real part of the permittivity. The activation energy E_a=0.55 eV, obtained from the relaxation frequency is very close to that derived from the dc conductivity. We suggest that this dielectric relaxation could be due to the proton jump and phosphate reorientation that cause distortion and change the local lattice polarizability inducing dipoles like      

Giant magnetoimpedance and colossal ac magnetoresistance of a Cu coil wound on La0.67Sr0.33MnO3

We observed the giant magnetoimpedance and colossal ac magnetoresistance for a Cu coil wound on La_0.67Sr_0.33MnO₃ under low dc magnetic fields. Even though the dc magnetoresistance for La_0.67Sr_0.33MnO₃ plate is only −2.4% under H=12 kOe, a Cu coil wound on La_0.67Sr_0.33MnO₃ plate exhibits a colossal ac magnetoresistance of −93% at 10 MHz and a giant magnetoimpedance of −59% in a wide frequency range of 500 kHz-10 MHz under a longitudinal field . The transverse magnetoimpedance is weaker than the longitudinal one. The giant magnetoimpedance and colossal ac magnetoresistance for a Cu coil wound on La_0.67Sr_0.33MnO₃ are connected with the variation of permeability induced by dc magnetic field.     

On photoluminescence in HgGa2S4

In this paper, we investigate the kinetics of photoluminescence in excited crystals of HgGa₂S₄ which have recently been proposed for implementing tunable luminescent devices. From photoluminescence experiments, performed at various temperatures and excitation powers, it appears that two kinds of radiative recombination processes take place during crystal excitation. These originate two bands in emission spectra which were resolved by means of a fitting procedure. The dependencies of these bands on temperature and excitation power density are explained by means of a specific kinetic model. A broad band, peaking at about 1.8 eV, is ascribed to electron-hole tunnel recombinations occurring in associated donor-acceptor pairs, according to a Prener-Williams scheme. The second narrow band, peaking at about 2.3 eV, is ascribed to electron-hole recombinations occurring in centres presenting short () and long-life () excited states. At room temperature, owing to thermally activated relaxation from short- to long-life states, these centres saturate under relatively low excitation powers. The tunability of photoluminescence is a consequence of competition between monomolecular and bimolecular recombination processes.     

Spin-peierls transition in dimerized stacks of anion-radical salt (N-Me-2,5-di-Me-Pz)(TCNQ)2, (Pz is pyrazine)

An anion-radical salt (ARS) (N-Me-2,5-di-Me-Pz)(TCNQ)₂, where Pz is pyrazine, was synthesized and its crystal structure was resolved. X-ray diffraction experiments on single crystals were performed. Heat capacity was measured in the temperature range from 2 to 300 K. Magnetisation and magnetic susceptibility were measured in the temperature range from 2 to 300 K and the low-temperature part was measured in magnetic fields from 5 mT to 5 T. The experimental results were explained in terms of dimerized Heisenberg spin chain model. Numerical calculations were performed and compared with experimental data.     

The magnetocaloric effect and low temperature specific heat of SmNi

The magnetocaloric effect (MCE) in a magnetic SmNi sample was evaluated from magnetization and heat capacity measurements. The MCE phenomena in the vicinity of magnetic phase transitions in terms of magnetic entropy change, , and adiabatic temperature change, , are reported. Isothermal magnetization measurements at several temperatures around the transition were carried out and used for versusT calculations. A similar dependence of the magnetic entropy change was evaluated from heat capacity C_p(T) measurements under zero field and 5 T. The SmNi system provides magnetic refrigerants that induce an adiabatic cooling of about during the magnetization process with a field of 5 T in the temperature range of 35-45 K. The temperature dependence of C_p(T) is analyzed in terms of the magnetic and the lattice contributions.     

Nuclear magnetic resonance study of the relaxation mechanisms of the Be, Al, and Si nuclei in Cr-doped Be3Al2Si6O18 crystals

The variations with temperature of the relaxation mechanisms of the ⁹Be, ²⁷Al, and ²⁹Si nuclei in emerald (Be₃Al₂Si₆O₁₈:Cr³⁺) single crystals were investigated by using a pulse NMR spectrometer. The values of the spin-lattice relaxation rates for the three nuclei are different, and these differences are attributed to the differences between their Larmor frequencies and their local nuclear environments. The relaxation rates of the ⁹Be and ²⁷Al nuclei undergo no abrupt changes within the temperature range 180-400 K, which indicates that there are no phase transitions within this temperature range. The spin-lattice relaxation rates of ⁹Be and ²⁷Al were found to be proportional to temperature. Therefore, the nuclear spin-lattice relaxation processes of these two nuclei proceed via single-phonon processes.     

Influence of magnetic ordering on the resistivity anisotropy of α-MnS single crystal

The resistivity and the optical absorbtion spectra of single crystal α-MnS are studied in the temperature range 80-300 K along two directions [100] and [111]. Strong anisotropy of the resistivity, and the shift of absorbtion spectra band edge below are explained in terms of model involving delocalized holes in 3d-band manganese ions interacting with localized spins by using the sd-model.     

Fluorescence and solvatochromism of a merocyanine dye with a high quadratic polarizability in solutions and polymer films

The influence of medium polarity on the spectroscopic and photophysical properties of 2-[(2E,4E)-6-(1,3,3-trimethyl-2,3-dihydro-1H-2-indolyliden)-2,4-hexadienyliden]malononitrile (THDM) in solutions and polymer matrices is studied at room temperature and 77 K under conditions of steady-state and pulsed laser excitation. A large bathochromic shift of the absorption spectra observed upon an increase in the solvent polarity is caused by a strong increase of the dipole moment in the ground state () on going to an excited Franck-Condon state (). Based on the solvatochromic data, the quadratic polarizability was calculated to be , which is close to the experimentally determined value . A strong narrowing of the fluorescence spectra in comparison with the absorption spectra at room temperature is observed upon an increase in the solvent polarity caused by a decrease in the bond length alternation parameter and by weakening of vibronic interactions in the singlet excited state. The mirror symmetry of the absorption and fluorescence spectra of THDM in ethanol at 77 K is explained by the increase of the electrostatic interactions between the solvent and merocyanine molecules. The dynamic fluorosolvatochromism of THDM in the picosecond range is caused by molecules reorientations of the polar environment occurring during a time period consistent with the dielectric relaxation time of these molecules.     

Relaxor behavior of K0.5La0.5Bi2Ta2O9 ceramics

Potassium lanthanum bismuth tantalate (K_0.5La_0.5Bi₂Ta₂O₉), a new relaxor ferroelectric was synthesized via the solid-state reaction route. X-ray structural studies along with Rietveld refinement confirmed it to be an n=2 member of the Aurivillius family of oxides and the refined lattice parameters are , and . The appearance of 1/2{h00} and 1/2{hk0} type superlattice reflections in the electron diffraction patterns reflected the presence of ordered polar regions. A broad dielectric peak associated with frequency dependent dielectric maximum temperature was observed. The value of the diffuseness parameter γ=1.93, obtained from the fit of a modified Curie-Weiss law established the relaxor nature of the title compound. The dielectric relaxation obeyed the Vogel-Fulcher relation wherein and . The relaxor behavior was attributed to the local polar ordering on A-sites.     

Dielectric spectroscopy on Bi3Zn2Sb3O14 ceramic: an approach based on the complex impedance

The dielectric properties and loss of Bi_1.5ZnSb_1.5O₇ a poor-semiconducting ceramic were investigated by impedance spectroscopy, in the frequency range from 5 Hz to 13 MHz. Electric measurements were performed from 100 to 700 °C. Pyrochlore type phase was synthesized by the polymeric precursor method. Dense ceramic with 97% of the theoretical density was prepared by sintering via constant heating rate. The dielectric permittivity dependence as a function of frequency and temperature showed a strong dispersion at frequency lower than 10 kHz. The losses exhibit slight dependence with the frequency at low temperatures presenting a strong increase at temperatures higher than 400 °C. A decrease of the loss magnitude occurs with increasing frequency. Relaxation times were extracted using the dielectric functions Z″(ω) and M″(ω). The plots of the relaxation times τ_Z′ and τ_M″ as a function of temperature follow the Arrhenius law, where a single slope is observed with activation energy values equal to 1.38 and 1.37 eV, respectively.     

Temperature-induced line broadening, line narrowing and line shift in the luminescence of nanocrystalline ZnS:Mn

Emission spectra of nanocrystalline ZnS:Mn²⁺ have been recorded as a function of temperature between 4 and . For the two emission bands observed (a defect-related emission band around and a Mn²⁺-related emission band around ), the temperature dependence of the width and the position of the emission bands has been analyzed. The shift and the broadening of the Mn²⁺ emission can be satisfactorily explained by theoretical models and parameters for electron-phonon coupling that are similar to those for bulk ZnS. For the defect-related emission, the shift to lower energies follows the decrease in bandgap of bulk ZnS with increasing temperature. The width of the defect-related emission decreases as the temperature is raised. This anomalous behavior is explained by inhomogeneous broadening at low temperatures.     

Non-linear dielectric effect in the isotropic phase of antiferroelectric liquid crystals

We have theoretically studied the non-linear dielectric effect in the isotropic phase of antiferroelectric liquid crystals on the basis of phenomenological theory. We find an analytical expression for the non-linear dielectric effect in the isotropic phase of the transition. The temperature dependence of the non-linear dielectric effect is presented in the isotropic phase of the transition.     

Electrical and dielectric properties of the Ca2MnO4−δ system

The materials were prepared in two steps: the preparation of the parent compound Ca₂MnO₄ by the sol-gel method followed by the creation of oxygen vacancies δ. The morphology has been studied by scanning electron microscopy (SEM) technique and has revealed that the free surface morphology becomes more densified when increasing the oxygen vacancies δ. The electrical properties have been studied using dc conductivity and ac impedance spectroscopy techniques. From the dc conductivity measurements it was found that, at high temperatures, the conduction mechanism is controlled by small polarons. The activation energy increases with δ, indicating an increase in the height of the potential barriers. The dielectric properties were studied using the modulus formalism, showing a non-Debye relaxation process, which is thermally activated presenting activation energy independent of δ.     

Magnetic and dielectric properties of Ni0.8Co0.1Cu0.1Fe2O4+PZT composites

Magnetoelectric composites of Ni_0.8Co_0.1Cu_0.1Fe₂O₄ and Lead Zirconate Titanate (PZT) were prepared by using conventional ceramic method. The measured values of saturation magnetization (M_s) and magnetic moments (μ_B) are in accordance with the volume fraction of ferrite content in the composite. The dielectric constant of the composites decreases with frequency. The plots of dielectric constant () against temperature (T) show a peak at their respective transition temperatures. The ME output was measured by varying dc bias magnetic field. A large ME output signal of 776 mV/cm was observed for 35% ferrite +65% ferroelectric composite. The magnetoelectric (ME) response is found to be dependent on the content of ferrite phase.     

Structural and dielectric properties of Bi modified PLZT ceramics

Polycrystalline samples of bismuth (Bi) doped lead lanthanum zirconate titanate (PLZT) with the general formula Pb_0.9(La_1−zBi_z)_0.1(Zr_0.65Ti_0.35)_0.975O₃, where [z=0.0, 0.3, 0.5, 0.7] near the morphotropic phase boundary (MPB) has been synthesized by a solid solution mixing technique. Some aspect of crystal structure of the compound at room temperature was studied using X-ray diffraction (XRD) technique. Detailed studies of dielectric constant (ε) and dielectric loss () obtained both as a function of frequency (100 Hz-100 kHz) at room temperature (RT) and temperature (RT-600 K) at 10 kHz suggest that compounds undergo ferroelectric-paraelectric phase transition of diffuse type. Both ac and dc conductivity have been studied over a wide range of temperature. The activation energy (E_a) of the samples was calculated from the plot of ac conductivity vs. inverse of absolute temperature. The temperature variation of resistivity shows that the compounds have negative temperature coefficient of resistance (NTCR).     

Determination of electrical and solar cell parameters of FTO/CuPc/Al Schottky devices

A Schottky structure is fabricated using CuPc sandwiched between fluorinated tin oxide (FTO) and aluminium electrodes. The electrical properties of the device are measured at room temperature. Permittivity of the device is calculated from capacitance measurements. The saturation current density, , diode ideality factor, n=3.02 and barrier height, are determined for the Schottky juction. Reverse bias versus is interpreted in terms of Schottky emission. Solar cell parameters are determined from the J-V characteristics. Power conversion efficiency, η of 0.0024% is obtained for the cell. Band gap energy of the material is determined from UV-visible absorption spectrum.