首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 31 毫秒
1.
The three thermo-optic coefficients of the biaxial laser host KLu(WO4)2 are measured at 633 nm by a deflection method. Their values at 300 K amount to n g / T=−7.4×10−6 K−1; n m / T=−1.6×10−6 K−1 and n p / T=−10.8×10−6 K−1. Nearly athermal propagation directions are found for polarizations along the N m and N p dielectric axes.  相似文献   

2.
New perovskite solid solution ceramics of (1−x)BaTiO3-xBi(Mg1/2Ti1/2)O3 ((1−x)BT-xBMT, x≤0.09) were synthesized by the solid-state reaction technique. X-ray diffraction studies have revealed a stable single perovskite structure for all samples. Dielectric measurements were carried out at different frequencies and temperatures. The polarization evolutions with temperatures were measured to investigate the ferroelectric properties. All the compositions show features of ferroelectrics with diffuse phase transition, though the temperature T m of their dielectric constant maximum ε m is frequency dependent. The dielectric constant peak ε(T) of (1−x)BT-xBMT ceramics become broad with increasing BMT content. During the temperature range of ε(T) peak summit, (1−x)BT-xBMT ceramics present quasi-linear dielectric phenomenon under high electric field with very high dielectric constant.  相似文献   

3.
The preparation of (La9.33−2x/3Sr x 0.67−x/3)Si6O24O2 (0 ≤ x ≤ 2) samples with different amounts of cation vacancies is reported. Structure and unit-cell parameters were deduced by Rietveld analysis of XRD patterns. Structural features that enhance oxygen conductivity in Sr-doped apatites are discussed. Up to three components were detected in 29Si MAS-NMR spectra which change with the amount and distribution of cation vacancies. In general, oxygen conductivity increases with the amount of vacancies at La1 (6h) sites, passing through a maximum for x = 0.4. In the case of activation energy, a minimum is detected near x = 1.2, indicating that entropic and enthalpic change in different ways. The presence of cation vacancies should enhance oxygen hopping along c-axis; however, the analysis of the frequency dependence of conductivity suggests that oxygen motions are produced along three axes.  相似文献   

4.
The absorption spectra, fluorescence spectrum and fluorescence decay curve of Nd3+ ions in CaNb2O6 crystal were measured at room temperature. The peak absorption cross section was calculated to be 6.202×10−20 cm2 with a broad FWHM of 7 nm at 808 nm for E//a light polarization. The spectroscopic parameters of Nd3+ ions in CaNb2O6 crystal have been investigated based on Judd-Ofelt theory. The parameters of the line strengths Ω t are Ω 2=5.321×10−20 cm2,Ω 4=1.734×10−20 cm2,Ω 6=2.889×10−20 cm2. The radiative lifetime, the fluorescence lifetime and the quantum efficiency are 167 μs, 152 μs and 91%, respectively. The fluorescence branch ratios are calculated to be β 1=36.03%,β 2=52.29%,β 3=11.15%,β 4=0.533%. The emission cross section at 1062 nm is 9.87×10−20 cm2.  相似文献   

5.
The results of investigations into spatial distribution of donors in p-HgCdTe graded band-gap layers with various composition profiles in the near-surface layer upon ion-beam etching are reported. It is found that the depth of the resulting n+-layer is weakly dependent on the conditions of ion-beam etching and composition of material of the surface and is about 0.5–1 μm. The electron density in the n+-layer on the surface is observed to increase with time of beam etching. It is shown that the conditions of ion-beam etching and the composition of the near-surface region significantly affect only the depth of the foregoing layer with low electron density. Analysis of experimental data shows that the process of ion-bean etching in p-HgCdTe heteroepitaxial structures with a composition gradient in the near-surface region is different from etching in HgCdTe homogeneous epitaxial structures and crystals. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 51–56, September, 2008.  相似文献   

6.
Solid polymer electrolytes (SPE) based on poly-(vinyl alcohol) (PVA)0.7 and sodium iodide (NaI)0.3 complexed with sulfuric acid (SA) at different concentrations were prepared using solution casting technique. The structural properties of these electrolyte films were examined by X-ray diffraction (XRD) studies. The XRD data revealed that sulfuric acid disrupt the semi-crystalline nature of (PVA)0.7(NaI)0.3 and convert it into an amorphous phase. The proton conductivity and impedance of the electrolyte were studied with changing sulfuric acid concentration from 0 to 5.1 mol/liter (M). The highest conductivity of (PVA)0.7(NaI)0.3 matrix at room temperature was 10−5 S cm−1 and this increased to 10−3 S cm−1 with doping by 5.1 M sulfuric acid. The electrical conductivity (σ) and dielectric permittivity (ε′) of the solid polymer electrolyte in frequency range (500 Hz–1 MHz) and temperature range (300–400) K were carried out. The electrolyte with the highest electrical conductivity was used in the fabrication of a sodium battery with the configuration Na/SPE/MnO2. The fabricated cells give open circuit voltage of 3.34 V and have an internal resistance of 4.5 kΩ.  相似文献   

7.
The effect of irradiation by ultraviolet light on the effective magnetic moment of a paramagnetic single crystal based on photochrome spiropyran (Sp) and chromium oxalates Sp3Cr(C2O4)3 molecules is detected. It is shown that the deviation of the temperature dependence of the magnetic moment from the Curie law is caused not by the exchange interaction, but by electron redistribution between Cr3+ and Cr4+ ions and spiropyran molecules Sp0 and Sp+. Analysis of the angular dependence of EPR spectra makes it possible to determine the contribution of Cr3+ ions to the magnetic properties of the crystals and to determine the crystal field parameters D = 0.619 cm−1 and E = 0.024 cm−1. Irradiation of hydrated samples by ultraviolet light leads to intensity redistribution of EPR lines attributed to Cr3+ and Sp0. Thermally stimulated paramagnetism of triplet states of spiropyran ions Sp+ and the SpI salt is observed.  相似文献   

8.
Several weak absorption bands have been observed in the optical absorption spectra of pure and rare-earth-doped YAl3(BO3)4 single crystals in the 3350– 3650 cm−1 wave number region. Two of them, peaking at about 3377 cm−1 and 3580 cm−1 in the 8 K spectra, appear in most of the samples. They are tentatively attributed to the stretching mode of OH ions incorporated in the crystal during the growth. An additional absorption band at about 5250 cm−1 at 8 K has also been detected in almost all samples. The temperature and polarization dependences of these bands, and their possible origin, are discussed.  相似文献   

9.
We hereby report a theoretical study on the equilibrium geometries, electronic structures and harmonic vibrational frequencies of Ga2Se3, Ga3Se2 and their anions. The ground and low-lying excited states of Ga2Se3, Ga2Se3, Ga3Se2 and Ga3Se2 are studied at the B3LYP and/or MP2 and CCSD(T) levels in conjunction with 6-311+G(d) and 6-311+G(2df) one particle basis sets. Ga2Se2 adopts the C2v kite geometry while Ga2Se3 has a ‘V’ geometry. Ga3Se2 has a three-dimensional ‘D3h ’ geometry and Ga3Se2 prefers the three-dimensional ‘C2v ’ structure. Electron detachment energies from the ground electronic states of the anions to several neutral states are reported and discussed. At CCSD(T)//MP2 level, the adiabatic electron affinity (AEA) of Ga2Se3 is calculated to be 3.23 eV when using the 6-311+G(2df) basis set and that of Ga3Se2 is 2.77 eV with the 6-311+G(d) basis set. The findings of this research are analyzed and compared with gallium oxide and sulfide analogues.  相似文献   

10.
The heat capacity of the manganite La0.87K0.13MnO3 has been measured in the temperature range 80–350 K. The nature of the ferromagnetic phase transition and the critical properties of heat capacity near the Curie temperature have been studied. The regularities of variations in the universal critical parameters near the phase transition point have been established. The calculated critical exponent and amplitudes of the heat capacity with allowance for corrections on the scaling (α = −0.13 and A +/A = 1.178) correspond to the critical behavior of the 3D Heizenberg model.  相似文献   

11.
The [N(CH3)4][N(C2H5)4]ZnCl4 compound has been synthesized by a solution-based chemical method. The X-ray diffraction study at room temperature revealed an orthorhombic system with P21212 space group. The complex impedance has been investigated in the temperature and frequency ranges 420–520 K and 200 Hz–5 MHz, respectively. The grain interior and grain boundary contribution to the electrical response in the material have been identified. Dielectric data were analyzed using the complex electrical modulus M * for the sample at various temperature. The modulus plots can be characterized by full width at half height or in terms of a non-exponential decay function ϕ(t) = exp[(−t/τ) β ]. The detailed conductivity study indicated that the electrical conduction in the material is a thermally activated process. The variation of the AC conductivity with frequency at different temperatures obeys the Almond and West universal law.  相似文献   

12.
Variable chain length di-urethane cross-linked poly(oxyethylene) (POE)/siloxane hybrid networks were prepared by application of a sol-gel strategy. These materials, designated as di-urethanesils (represented as d-Ut(Y′), where Y′ indicates the average molecular weight of the polymer segment), were doped with lithium triflate (LiCF3SO3). The two host hybrid matrices used, d-Ut(300) and d-Ut(600), incorporate POE chains with approximately 6 and 13 (OCH2CH2) repeat units, respectively. All the samples studied, with compositions ∞ > n ≥ 1 (where n is the molar ratio of (OCH2CH2) repeat units per Li+), are entirely amorphous. The di-urethanesils are thermally stable up to at least 200 °C. At room temperature the conductivity maxima of the d-Ut(300)- and d-Ut(600)-based di-urethanesil families are located at n = 1 (approximately 2.0 × 10−6 and 7.4 × 10−5 Scm−1, respectively). At about 100 °C, both these samples also exhibit the highest conductivity of the two electrolyte systems (approximately 1.6 × 10−4 and 1.0 × 10−3 Scm−1, respectively). The d-Ut(600)-based xerogel with n = 1 displays excellent redox stability.  相似文献   

13.
The atomic and magnetic structures of La0.5Ca0.5CoO3 cobaltite have been studied by the neutron diffraction technique at high pressures of up to 4 GPa in the 10- to 300-K temperature range. The pressure dependences of the structural parameters have been obtained. The Curie temperature increases with the pressure with the coefficient dT C/dP = 1 K/GPa, demonstrating the stability of the ground ferromagnetic (FM) state. The pressure dependence of the ground FM state in La0.5Ca0.5CoO3 is in drastic contrast with that for La1 − x Ca x CoO3 at a lower calcium content (x < 0.3). For the latter compound, the pressure suppressed the ground FM state and a large negative pressure coefficient of the Curie temperature (dT C/dP ∼ −10 K/GPa) was observed. The nature of such a phenomenon is analyzed in the framework of the double exchange model also taking into account the changes in the electron configuration of Co3+ ions.  相似文献   

14.
Ceramic samples of bismuth ferrite and solid solutions of Bi1 − x A x FeO3 type (where A = Lu, Yb, Tm, Er, Ho, Dy, Tb, Gb, Eu, Sm, Nd, Pr, La; 0.05 ≤ x ≤ 0.20; Δx = 0.05) were prepared. Spectra of the real part of electrical conductivity were studied within the range 10−4–10−6 Hz. The dependence of the samples’ thermal stability and electrical conductivity on the size of the substituting ions was established.  相似文献   

15.
The α-Zn2P2O7 compound was obtained by conventional solid-state reaction. The sample was characterized by X-ray powder diffraction, solid state 31P NMR MAS, and electrical impedance spectroscopy. The solid state 31P MAS NMR, performed at 121.49 MHz, shows three isotropic resonances at −21.1, −18.8, and −15.8 ppm, confirming the non-equivalency of the three PO4 groups in the α-Zn2P2O7 form. They are characterized by different chemical shift tensor parameters with the local geometrical features of the tetrahedra. Electrical impedance measurements of β-Zn2P2O7, form stable for temperature greater than 403 K, were performed as a function of both temperature and frequency. The electrical conduction and dielectric relaxation have been studied. The AC conductivity obeys the universal power law. The approximation type correlated barrier hopping model explains the universal behavior of the n exponent. The impedance plane plot shows semicircle arcs at different temperatures, and an electrical equivalent circuit has been proposed to explain the impedance results. The circuits consist of the parallel combination of bulk resistance R p and constant phase elements CPE. The simulated spectra show a good correlation with the experimental data.  相似文献   

16.
A pronounced step-like (kink) behavior in the temperature dependence of resistivity ρ(T) is observed in the optimally doped Sm1.85Ce0.15CuO4 thin films around T sf = 87 K and attributed to the manifestation of strong-spin fluctuations induced by Sm3+ moments with the energy ħωsf = k B T sf ≃ 7 meV. The experimental data are found to be well fitted by the residual (zero-temperature) ρres, electron-phonon ρe-ph(T) = AT, and electron-electron ρe-e(T) = BT 2 contributions in addition to the fluctuation-induced contribution ρsf(T) due to thermal broadening effects (of the width ωsf). According to the best fit, the plasmon frequency, impurity scattering rate, electron-phonon coupling constant, and Fermi energy are estimated as ωp = 2.1 meV, τ 0 −1 = 9.5 × 10−14 s−1, λ = 1.2, and E F = 0.2 eV, respectively. The text was submitted by the authors in English.  相似文献   

17.
Duan’s simple model is extended to analyze the mixing of the 4f N − 15d configuration with the 4f N states. The explicit static coupling and traditional dynamic coupling are considered, and the parameters are fitted according to the absorption spectrum in LiYF4: Nd3+. The parameter values obtained are as follows: T 32 = −28i × 10−7, T 52 = −1151i × 10−7, A 322 = 192i × 10−12 cm, A 524 = i × 10−12 cm, A 726 = 54i × 10−12 cm, and A 766 = −680i × 10−12 cm. Compared to the experimental measurements, the present model yields better results than those obtained from the Judd-Ofelt theory. The text was submitted by the authors in English.  相似文献   

18.
The mobility of electrons in vertical transport in GaAs/Ga1−y Al y As barrier structures was investigated using geometric magnetoresistance measurements in the dark. The samples studied had Ga1−y Al y As (0 ≤ y ≤ 0:26) linearly graded barriers between the n+-GaAs contacts and the Ga0:74Al0:26As central barrier, which contain N w (=0, 2, 4, 7 and 10) n-doped GaAs quantum wells. The mobility was determined as functions of (i) temperature (80–290 K) at low applied voltage (0.01–0.1 V) and (ii) applied voltage (0.005–1.6 V) at selected temperatures in the range 3.5–290 K. The experimental results for the temperature dependence of low-field mobility suggest that space-charge scattering is dominant in the samples with N w =0 and 2, whereas ionized impurity scattering is dominant in the samples with N w =4, 7 and 10. The effect of polar optical phonon scattering on the mobility becomes significant in all barrier structures at temperatures above about 200 K. The difference between the measured mobility and the calculated total mobility in the samples with N w =4, 7 and 10, observed above 200 K, is attributed to the reflection of electrons from well-barrier interfaces in the quantum wells and interface roughness scattering. The rapid decrease of mobility with applied voltage at high voltages is explained by intervalley scattering of hot electrons.   相似文献   

19.
We have studied the effect of cationic disorder on the spin polarization of the double perovskite system Sr2Fe1+x Mo1−x O6 with  −1 ≤ x ≤ 1/3. The composition x = 0 corresponds to the well-known double-perovskite Sr2FeMoO6, which is expected to have complete spin polarization, however all samples present some degree of Fe/Mo disorder which reduces the tunneling magnetoresistance in granular samples. We consider an electronic model within the renormalized perturbation expansion Green’s functions, consisting in a correlated electron picture with localized Fe-ions and itinerant electrons interacting with the local spins via a double-exchange type mechanism. Our results show the influence of disorder on the density of states and the ground-state properties, particularly on the spin polarization over the whole range of x.  相似文献   

20.
The dynamics of accumulation of electrically active radiation defects under ion doping of epitaxial Cd x Hg 1−x Te films is studied for various distributions of film composition in the implantation region. The epitaxial films were irradiated by boron ions at room temperature in the continuous regime, with the dose ranging within 1011−3·1015 cm−2, energy — 20–150 keV, and ion current density — j = 0.001–0.2 μA·cm−2. It is found that the natural logarithm of the introduction rate of electrically active radiation defects linearly depends on the epitaxial-film composition in the range of mean projected path of implanted ions. An analysis of the experimental data shows that the dynamics of accumulation of electrically active radiation defects is determined by the epitaxial-film composition in the implantation region. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 25–28, September, 2006.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号