Influence of the composition of TlGa1 − x Er x Se2 crystals on their dielectric characteristics and parameters of localized states

Frequency dependences of the real (?′) and imaginary (?″) parts of the complex permittivity, the dielectric loss tangent (tanδ), and the ac conductivity (σ_ac) in frequency range f = 5 × 10⁴?3.5 × 10⁷ Hz have been investigated for TlGa_{1 ? x} Er _x Se₂ crystals of various compositions. It has been established that the relaxation dispersion of ?′ and ?″ takes place for the studied crystals. The influence of the erbium content in the crystals on their dielectric coefficients has been studied. The ac conductivity of the TlGa_{1 ? x} Er _x Se₂ single crystals in the high-frequency range obeys the law σ_ac ～ f ^0.8, which is characteristic of the hopping mechanism of charge transfer over the states localized in the vicinity of the Fermi level. Parameters of the states localized in the band gap of TlGa_{1 ? x} Er _x Se₂ and the influence of the composition of the crystals on these parameters have been evaluated.     

Optical properties of (F2 +) H centers in NaCl:OH− crystals

Abstract

In this work we present new data on the formation and destruction of the laser active (F₂ ⁺)     

Influence of temperature and Pr-doping on the dielectric properties of Ca2−xPrxMnO4 compounds and structural transition effects

Complex perovskite oxides Ca_2?xPr_xMnO₄ (x = 0–0.5) compounds were synthesized by a solid-state reaction technique. A tetra–ortho structural transition was observed. Impedance spectroscopy was used to study the electrical behavior in the frequency range 40 Hz–1 MHz and in the temperature range 80–350 K. Frequency-dependent conductivity spectra were found to obey the Jonscher's power law. Complex impedance plane plots have indicated that the dielectric response is mainly intrinsic. Materials bulk response was found to be dominated by non-localized or localized conduction, depending on temperature and frequency.     

Theoretical study of the aqueous medium effect on the reaction CO2+ OH− ⇄ HCO−3

A method for the theoretical study of chemical reaction mechanisms in aqueous solutions has been developed. Approximate reaction pathways are determined with the aid of a model Hamiltonian in which medium is described by a set of Langevin point dipoles. Solvation of stationary points on the potential energy surface so obtained is performed by a more exact calculation of the medium contribution using the method of minimization of the solvation shell potential energy. Usefulness of our approach is exemplified by the calculation of the reaction CO₂ + OH⁻ ⇄ HCO⁻₃. Method MIND0/13 has been employed in constructing the model Hamiltonian. A good agreement with experiment has been achieved.     

Influence of silver on the galvanomagnetic properties and energy spectrum of mixed (Bi1− x Sbx)2Te3 crystals

An investigation is made of the temperature dependences of the resistivity in the range 4.2–300 K, the Hall effect, and the Shubnikov-de Haas effect in magnetic fields up to 40 T in (Bi_{1? x} Sb_x)₂Te₃Ag_y single crystals (0 ≤ x ≤ 0.75). Doping (Bi_{1? x} Sb_x)₂Te₃ crystals with silver showed that in Sb₂Te₃ and (Bi_{1? x} Sb_x)₂Te₃ crystals unlike Bi₂Te₃ silver exhibits acceptor properties. The angular and concentration dependences of the Shubnikov-de Haas effect were studied in (Bi_{1? x} Sb_x)₂Te₃Ag_y. It was established that the anisotropy of the ellipsoids of the upper valence band in Bi_0.5Sb_1.5Te₃ remains unchanged as a result of silver doping.     

Influence of OH groups on 1.5-um emission of Yb_(3+)/Er_(3+) co-doped tungsten-tellurite glasses

The Yb3+/Er3+ co-doped TeO2-WO3-ZnO glasses bubbling with different times were prepared. The infrared (IR) transmission spectra, emission spectra and fluorescence lifetime of Er3+ at 1.5 um were measured. The quench effects of OH groups on emission intensity of Er3+ at 1.5 um as well as the relationships between fluorescence decay rate and OH group content were investigated. The constant kOH-Er which represents the strength of the interactions between Er3+ ions and OH groups, is approximately 17.0×10-20 cm4/s, and is comparable to that for Er3+ in Yb3+/Er3+ co-doped phosphate glasses.     

Influence of post-grown Li-rich and Li-poor vapor transport equilibration on composition, OH− absorption and optical-damage threshold of Mg (5 mol%) : LiNbO3 crystals

Li-rich (Li-poor) vapor transport equilibration (VTE) treatments on a number of Z-cut 0.47 mm thick congruent MgO (5 mol% in melt) : LiNbO₃ crystals were carried out at 1100°C over different durations ranging in 1–172 (40–395) h. Neutron activation analysis shows that neither Li-rich nor Li-poor VTE-induced Mg and Nb loss from the crystal occurred. The Li₂O content in the crystal was measured as a function of VTE duration by the gravimetric method. The Li-rich/Li-poor VTE effects on OH⁻ absorption were studied in comparison with the as-grown crystal. The study shows that the Li-rich VTE results in OH⁻ absorption band annihilation. After further oxidation treatment the band reemerges and peaks at the same wavenumber as that of the as-grown crystal (∼3535.6 cm⁻¹), showing that the MgO concentration in the Li-rich VTE crystal is still above the optical-damage threshold. The Li-poor VTE causes OH⁻ band shift to 3486.3–3491.6 cm⁻¹, indicating that the MgO concentration in all Li-poor VTE crystals is all below the optical-damage threshold. Further successive Li-rich VTE and oxidation treatments on the Li-poor VTE-treated crystal lead the band to shift back to 3535.6 cm⁻¹, showing that the post Li-rich VTE brought the Li-poor VTE-treated crystal above the optical-damage threshold again. It is found that the peaking position, band width, peaking absorption and band area of the absorption at ∼3486 cm⁻¹ all increase monotonously with the decrease of the Li₂O content arising from prolonged Li-poor VTE, and quantitative relationships to the Li₂O content are established for the latter two parameters. The VTE effects on the OH⁻ absorption are conducted with the VTE-induced OH⁻ content alteration and charge redistribution.     

Influence of iron ions on dielectric properties of the PbO–Bi2O3–B2O3 glass system

PbO–Bi₂O₃–B₂O₃ glasses containing small concentrations of Fe₂O₃ (0–1 mol%) were subject to dielectric studies (dielectric constant ε′; loss tan δ; and ac conductivity σ _ac) over a wide range of frequency and temperature. From spectroscopic (infrared, optical absorption and ESR spectra) and magnetic susceptibility studies, variations in these properties with dopant ion concentration were analyzed in terms of different oxidation states and iron ion environment in the glass network.     

Fluctuation-enhanced conductivity and magnetoconductivity of high-quality YBa2Cu3O7−δ crystals

We present measurements of the in-plane resistivity _ab of YBa₂Cu₃O_7– single crystals withT _c 92 K and _ab (100 K)50 cm. The temperature dependence of the fluctuation conductivity and of the magnetoconductivity aboveT _c is analyzed in terms of direct and indirect fluctuation contributions for layered superconductors. The combination of fluctuation conductivity and magnetoconductivity allows to determine both coherence lengths _ab (0) and _c (0) as well as the phase-relaxation time of the pairs in an unequivocal manner. Evidence for clean limit type-II superconductivity in our crystals is given by large values of the mean free pathl _{ab ab} (0).Dedicated to Prof. Dr. F. Hund on the occasion of his 95th birthday     

Influence of Zn concentration on electrical resistance of SmBa2(ZnxCu1−x 3O9−δ

It is known that perovskite type structures (ReBa₂Cu₃O_9–) exhibit superconductivity at temperatures below 90 K /1–3/. The substitution of other ions for Cu is interesting because the Cu-O interactions are probably responsible for the supercoductivity in those systems /4/. In the system Y-Ba-(Ag,Cu)-O Saito found that the gradual decrease of T_c with the increase of Ag concentration is due to the presence of the different phases /5/.In this letter we are reporting the experimental measurements of the electrical resistance of SmBa₂Cu₃O_9– oxygen defect perovskites with Zn substituted for Cu.     

Influence of space charges on the dielectric behaviour of (Li2B4O7)1−x(WO3)x-glasses

The dielectric behaviour of (Li₂B₄O₇)_1−x(WO₃)_x glass samples is to a large extent governed by space charges. A detailed analysis of the effective dielectric constant ϵ_eff and the loss tangent tan δ reveals the intrinsic dielectric properties of the materials. At large WO₃-contents (0.44<x<0.55) we find a characteristic temperature in ϵ(T) which coincides with a phase transition temperature in crystalline WO₃, suggesting a reconstruction of the local environment of many tungsten atoms near to their WO₃-crystalline state.     

Electrical properties and electronic structure of Cu1−xZnxInSe2 and Cu1−xZnxInS2 single crystals

Temperature dependence of the electrical conductivity of CuInS₂–ZnIn₂S₄ and CuInSe₂–ZnIn₂Se₄ solid solutions possessing n-type conductivity has been studied. It has been established that when the temperature decreases down to ~100 to 27 K, the hopping mechanism of electrical conductivity with a variable jumping length between localized states positioned in a narrow energy band near the Fermi level becomes dominant. The main parameters of the hopping conductivity have been determined. At higher temperatures (150–300 K), in the CuInSe₂–ZnIn₂Se₄ single crystals containing 15 and 20 mol% ZnIn₂Se₄ the thermally activated conductivity with activation energy of 0.018 and 0.04 eV, respectively, is detected. Among the CuInSe₂–ZnIn₂Se₄ single crystals, samples with 5 and 10 mol% ZnIn₂Se₄ were found to be close to degenerate semiconductors. Temperature dependences of the electrical conductivity of CuInS₂–ZnIn₂S₄ single crystals are described by a more complicated function that may indicate a competition of several conduction mechanisms in these compounds. For the CuInS₂–ZnIn₂S₄ solid solutions, X-ray photoelectron core-level and valence-band spectra have been measured for both pristine and Ar⁺ ion-bombarded surfaces. Our results indicate that the Cu_1−xZn_xInS₂ single-crystal surfaces are sensitive to Ar⁺ ion-bombardment. Additionally, for the Cu_1−xZn_xInS₂ crystal with the highest ZnIn₂S₄ content, namely 12 mol% ZnIn₂S₄, the X-ray emission bands representing the energy distribution of the Cu 3d, Zn 3d and S 3p states have been measured and compared on a common energy scale with the X-ray photoelectron valence-band spectrum.     

Lattice dynamics and superionic properties of Cd1−xPbxF2 crystals

Reflectivity in the infrared range, Raman scattering and ionic transport properties in Cd_1−xPb_xF₂ crystals have been investigated. A “one-mode” behaviour of these crystals has been found. The role of the PbF₂ cation sub-lattice in the anion disordering process has been discussed. A correlation has been found between the ionic transport properties and the fluorine-cation interaction coefficient for Cd_1−xPb_xF₂ crystals.     

Amplification and generation of surface plasmon polaritons in a dielectric−HTS film−dielectric structure

Propagation of surface polaritons in the dielectric?HTS?dielectric structure is investigated in the temperature region of T <T _c/2, where the structure can be considered nonabsorbing. It is shown that in the region where the polariton wave is appreciably slowed down it can effectively interact with a flux of charged particles and be amplified due to the energy transfer from the drift current wave. It is also shown that a periodic substrate can cause generation of the surface polariton wave in the structure.     

Influence of Sr substitution on thermoelectric properties of La1−xSrxFeO3 ceramics

Perovskite La_1−xSr_xFeO₃ (0.10 ⩽ x ⩽ 0.20) ceramics have been synthesized by the conventional solid-state reaction technique. Their electrical resistivity, Seebeck coefficient and thermal conductivity have been measured. It has been found that the increase of Sr content reduces significantly both the electrical resistivity and the Seebeck coefficient, but slightly increases the high-temperature thermal conductivity. An adiabatic hopping conduction mechanism of small polaron is suggested from the analysis of the temperature dependence of the electrical resistivity. Seebeck coefficients decrease with increasing temperature, and saturate at temperature above 573 K. The saturated value of Seebeck coefficient decreases with increasing of Sr contents, from 200 μV/K for x = 0.10 to 100 μV/K for x = 0.20. All samples exhibit lower thermal conductivity with values around 2.6 W/m K. The highest dimensionless figure of merit is 0.031 at temperature 973 K in La_0.88Sr_0.12FeO₃.     

Uncommon temperature dependence of the rotational motion of OH− centers in NaCl

Abstract

The correlation time τ_D of the rotational motion of OH^? centers in NaCl was studied for two different OH^?-concentrations (44 ppm and 220 ppm) between 0.5 K and 16 K by ²³Na nuclear spin relaxation measurements. The correlation time in the sample containing 44 ppm OH^? is given by phonon-assisted tunneling of isolated defects, whereas the OH^?-motion in the sample containing 220 ppm can be described by a coupled ensemble of quantum mechanical hindered rotators below a critical temperature which is determined by the strength of the interaction between the OH^? dipoles.     

Influence of frustration on a d = 3 diluted antiferromagnet: FexZn1−xF2

The influence of a frustrated bond on the magnetic properties of a d = 3 uniaxial (Ising) bcc diluted antiferromagnet, with emphasis in the compounds FexZn_1−xF₂, is investigated by a local mean-field numerical simulation. In particular we find that the initial drop of the saturation staggered magnetization (M_S) with concentration follows a percolation-like phenomenon characterized by an exponent β_P. For the frustrated samples, however, this regime is followed by a second one identified by a ‘long tail’ effect such that M_S is zero only at the percolation threshold. Our numerical data also confirms a spin-glass phase near this threshold.     

Influence of the synthesis method on the structural and electrochemical properties of LiCo1−yNiyO2 oxides

To optimize the LiCo_1−yNi_yO₂ cathode materials, we have systematically studied the influence of synthesis parameters on their electrochemical behavior using powders synthesized by different wet-chemistry methods (combustion and sol-gel). It is found that the thermal decomposition behavior plays an important role on the intrinsic properties of the powders. Products were studied in details. Characterizations include compositional analysis, morphology, crystallographic structure, and local structure for LiCo_1−yNi_yO₂ materials calcined at 800 °C. A comparison of the different methods is given for the synthesis of LiCo_1−yNi_yO₂ cathode materials. It is found that well-structured oxides with uniformly sized ultrafine particles are obtained using the sol-gel method assisted by succinic acid. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Electron spin resonance and luminescence in Ga2−xEuxS3 single crystals

In present work electron spin resonance (ESR) and luminescence have been studied in Ga_2−xEu_xS₃ single crystals. The ESR and photoluminescence (PL) spectra of Ga_2−xEu_xS₃ were observed and the intensity of these spectra has increased linearly with the Eu concentrations in the samples. At the low temperature of 3.8 K in Ga_2−xEu_xS₃ single crystals, when the magnetic field has been applied as the perpendicular to the direction of [110], the hyperfine structure of ESR spectra has been observed. The exchange interaction of Eu²⁺ atoms in Ga_2−xEu_xS₃ single crystals have been determined as, g=4.2. The PL spectrum of pure Ga₂S₃ single crystals was recorded at 4.2 K and consisted of a narrow high-energy band at 2.53–2.38 eV and a low-energy band at 2.14–1.59 eV. Ga_2−xEu_xS₃ resulted in complete quenching of the impurity luminescence bands gave rise to a green europium band (2.38–2.14 eV) in the luminescence spectrum. Crystals of Ga_2−xEu_xS₃ emitted bright green electroluminescence (EL) spectrum when they excited with static and alternating electric fields at 77 K. We have also determined the field and frequency dependences of the EL.