The velocityv of the propagation of discharge along the anode of a self-quenchingG—M-counter is a function of total pressureP, pressure of the quenching gasPD, radius of the cathodera and of the anoderi andVü the difference between working- and starting-potential. For the mixtures argon-methylal, argon-alcohol and helium-alcohol isv=v0·exp[k·(Vü/Ve)1/2] withv0 the velocity at the starting potentialVev0=(a+b·PD/P)·Vn1/2·exp [(c?d·PD/P·Vn?1/2] andVn=Ve·(lnra/ri)?1.k, a, b, c andd are characteristical constants of the filling gas. 相似文献
For a two-dimensional Schrödinger operator HαV = ?Δ ?αV with the radial potential V(x) = F(|x|), F(r) ≥ 0, we study the behavior of the number N?(HαV) of its negative eigenvalues, as the coupling parameter α tends to infinity. We obtain the necessary and sufficient conditions for the semi-classical growth N?(HαV) = O(α) and for the validity of the Weyl asymptotic law. 相似文献
The interpretation of diffraction spectra of ordered high-temperature phases of solid solutions and strongly nonstoichiometric compounds is discussed. It has been shown that variations of the intensities of superstructure reflections, which cannot be explained within simple ordering models, can be due to the superposition of superstructures with different symmetries in the matrix of the basis crystal structure. Using an example of atom–vacancy ordering in TiO1.0 titanium monoxide, a model of the order–order transition state formed by the superposition of low-temperature monoclinic (space group A2/m (C2/m)) and high-temperature cubic (space group Pm3?m) M5X5 superstructures has been proposed. It has been shown that the transition state is thermodynamically equilibrium and should be implemented instead of the M5X5 cubic superstructure. The transition state model can be considered as an M(5–i)X(5–i) superstructure (i = 1, 14/18, 11/18) with the monoclinic symmetry (space group P1m1). 相似文献
The spinel structure of lithium titanate Li4Ti5O12 is refined by the Rietveld full-profile analysis with the use of x-ray and neutron powder diffraction data. The distribution and coordinates of atoms are determined. The Li4Ti5O12 compound is studied at high temperatures by differential scanning calorimetry and Raman spectroscopy. The electrical conductivity is measured in the high-temperature range. It is shown that the Li4Ti5O12 compound with a spinel structure undergoes two successive order-disorder phase transitions due to different distributions of lithium atoms and cation vacancies (□, V) in a defect structure of the NaCl type: (Li)8a[Li0.33Ti1.67]16dO4 → [Li□]16c[Li1.33Ti1.67]16dO4 → [Li1.33□0.67]16c[Ti1.67□0.33]16dO4. The low-temperature diffusion of lithium predominantly occurs either through the mechanism ... → Li(8a) → V(16c) → V(8a) → ... in the spinel phase or through the mechanism ... → Li(16c) → V(8a) → V(16c) → ... in an intermediate phase. In the high-temperature phase, the lithium cations also migrate over 48f vacancies: ... Li(16c) → V(8a, 48f) → V(16c) → .... 相似文献
The effect of uniaxial mechanical pressure σm ≤ 150 bar on the spectral (300–800 nm) dependence of the birefringerence Δni and refractive indices ni of (NH4)2SO4 crystals has been investigated. It is shown that the dispersion of ni(λ) and Δni(λ) is normal and sharply increases with approach to the absorption edge. It is established that uniaxial pressure does not change the character of the dispersions dni/dλ and dΔni/dλ and only affects their magnitudes. It is shown that the increase in the refractive indices under uniaxial stress is mainly due to the increase in the refraction caused by the increase in the band gap and long-wavelength shift of the UV absorption band maximum. 相似文献
The temperature-dependent field cooling (FC) and zero-field cooling (ZFC) magnetizations, i.e., MFC and MZFC, measured under different magnetic fields from 500 Oe to 20 kOe have been investigated on two exchange–spring CoFe2O4/CoFe2 composites with different relative content of CoFe2. Two samples exhibit different magnetization reversal behaviors. With decreasing temperature, a progressive freezing of the moments in two composites occurs at a field-dependent irreversible temperature Tirr. For the sample with less CoFe2, the curves of ?d(MFC ? MZFC)/dT versus temperature T exhibit a broad peak at an intermediate temperature T2 below Tirr, and the moments are suggested not to fully freeze till the lowest measuring temperature 10 K. However, for the ?d(MFC ? MZFC)/dT curves of the sample with more CoFe2, besides a broad peat at an intermediate temperature T2, a rapid rise around the low temperature T1~15 K is observed, below which the moments are suggested to fully freeze. Increase of magnetic field from 2 kOe leads to the shift of T2 and Tirr towards a lower temperature, and the shift of T2 is attributable to the moment reversal of CoFe2O4.
Graphical abstract CoFe2O4/CoFe2 composites with different relative content of CoFe2 were prepared by reducing CoFe2O4 in H2 for 4 h (S4H) and 8 h (S8H). The temperature-dependent FC and ZFC magnetizations, i.e., MFC and MZFC, under different magnetic fields from 500 Oe to 20 kOe have been investigated. Two samples exhibit different magnetization reversal behaviors. With decreasing temperature, a progressive freezing of the moments in two composites occurs at field-dependent irreversible temperature Tirr. For the S4H sample, the curves of ?d(MFC ? MZFC)/dT versus temperature T exhibit a broad and field-dependent relaxing peak at T2 below Tirr (figure a), and the moments were suggested not to fully freeze till the lowest measuring temperature 10 K. However, for the S8H sample, it exhibits the reentrant spin-glass state around 50 K, as evidenced by a peak in the MFC curve (inset in figure b) and as a result of the cooperative effects of the random anisotropy of CoFe2O4, exchange–spring occurring at the interface of CoFe2O4 and CoFe2 together with the inter-particle dipolar interaction (figure c); in ?d(MFC ? MZFC)/dT curves, besides a broad relaxing peat at T2, a rapid rise around the low-temperature T1~15 K is observed, below which the moments are suggested to fully freeze. Increase of magnetic field from 2 kOe leads to the shift of T2 and Tirr towards a lower temperature, and the shift of T2 is attributable to the moment reversal of CoFe2O4.
The sample of Mg0. 5+y (Zr1-y Fey) 2 (PO4) 3 (0.0 ≤y ≤0.5) was synthesized using the sol-gel method. The structures of the samples were investigated using X-ray diffraction and Fourier transform infrared spectroscopy measurement. XRD studies showed that samples had a monoclinic structure which was iso-structured with the parent compound, Mg0.5Zr (PO4) 3. The complex impedance spectroscopy was carried out in the frequency range 1–6 MHz and temperature range 303 to 773 K to study the electrical properties of the electrolytes. The substitutions of Fe3+ with Zr4+ in the Mg0.5Zr (PO4) 3 structure was introduced as an extrainterstitial Mg2+ ion in the modified structured. The compound of Mg0.5+y (Zr1-y Fey)2(PO4)3 with y?=?0.4 gives a maximum conductivity value of 1.25?×?10?5 S cm?1 at room temperature and 7.18?×?10?5 S cm?1 at 773 K. Charge carrier concentration, mobile ion concentration, and ion hopping rate are calculated by fitting the conductance spectra to power law variation, σac(ω)?=?σo? +?Aωα. The charge carrier concentration and mobile ion concentration increases with increase of Fe3+ inclusion. This implies the increase in conductivity of the compounds was due to extra interstitial Mg2+ ions. 相似文献
It is demonstrated that 50% substitution of vanadium for molybdenum in the pyrochlore lattice of the complex oxide Y2(VxMo1 ? 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, TC = 55 K). The Gd2V0.67Mo1.33O7 compound (a = 10.2862(3) Å) is a ferromagnet with TC (84 K) exceeding that of undoped Gd2MnO2O7. 相似文献
The magnetic superconductorRu0.9Sr2YCu2.1O7.9 (Ru-1212Y) has beeninvestigated using neutron diffraction under variable temperature and magnetic field. Withthe complementary information from magnetization measurements, we propose a magnetic phasediagram T-H for the Ru-1212 system. Uniaxialantiferromagnetic (AFM) order of 1.2μB/Ruatoms with moments parallel to the c-axis is found below the magnetictransition temperature at ~140 K in the absence of magnetic field. In addition,ferromagnetism (FM) in the ab-plane develops below ~120 K, butis suppressed at lower temperature by superconducting correlations. Externally appliedmagnetic fields cause Ru-moments to realign from the c-axis to theab-plane, i.e. along the ?1,1,0? direction, and induce ferromagnetismin the plane with ~1μB at 60 kOe.These observations of the weak ferromagnetism suppressed by superconductivity and thefield-induced metamagnetic transition between AFM and FM demonstrate not only competingorders of superconductivity and magnetism, but also suggest a certain vortex dynamicscontributing to these magnetic transitions. 相似文献
Indium oxide-multi-walled carbon nanotubes (In2O3-MWCNTs) were prepared by sol-gel method for DSSCs. The synthesis of indium oxide (In2O3) was carried out by dissolving indium chloride (InCl3) in a solvent of 2-methoxyethanol. Different annealing temperatures of 400, 450, 500, 550, and 600 °C were proposed in this study. The changes in the structural properties were analyzed by means of X-ray diffraction (XRD) and atomic force microscopy (AFM) analysis. The XRD spectrum estimated the average crystallite sizes of 3 nm for each sample. AFM results indicated very rough surface area of the films where it increased linearly from 1.8 to 11 nm as the annealing temperature increases. The In2O3-MWCNTs-based DSSC exhibited good photovoltaic performance with power conversion efficiency (η), photocurrent density (Jsc), open circuit voltage (Voc), and fill factor (FF) of 1.13 %, 5.5 mA/cm2, 0.53 V, and 0.42, respectively. Even though the film annealed at 450 °C exhibited low τeff, it achieved the greatest Deff of 29.67 cm2 s?1 which provides an efficient pathway for the photogenerated electrons with minimum electron recombination loss that increased the Jsc and Voc in the DSSC. The obtained structural and electron transport analysis was proposed as a suitable benchmark for In2O3-MWCNTs-based dye-sensitized solar cell (DSSCs) application. Hence, this study suggests that the optimum temperature for In2O3-MWCNTs is at annealing temperature of 450 °C prepared via sol-gel method. 相似文献
Nuclear magnetic resonance (NMR) experiments recording the recovery of the magnetization of the nuclei in one phase, following the excitation of the nuclei in the other phase, is a classical way of studying blends inhomogeneous at the nanometer scale. Interpretation of the time recovery in terms of the spatial dimension requires knowledge of the two-phase spin diffusion coefficientsD0. A new method of measurement ofD0 is proposed on the basis of variable angle-tilted rotatory frame relaxation in homogeneous samples doped with paramagnetic centers. The choice of the tilt angle allows one to finely balance the direct relaxation by the paramagnetic center and the spin diffusion. The shape of the relaxation is analyzed with the solution for the diffusion-limited regimeM(t)/M(0)=exp[?(r2t)1/2?r1t] andD0 then calculated fromr1,r2 and the concentration of paramagnetic centers. Conditions where reliable results can be obtained both theoretically and numerically are explored. The method has been implemented and applied to polycrystalline Mn-doped CaF2 leading toD0=540±60 nm2/s, in agreement with existing values on this model compound. 相似文献
The unit cell parameters a, b, and c of [N(CH3)4]2ZnCl4 have been measured by x-ray diffraction in the temperature range 80–293 K. Temperature dependences of the thermal expansion coefficients αa, αb, and αc along the principal crystallographic axes and of the unit cell thermal expansion coefficient αV were determined. It is shown that the a=f(T), b=f(T), and c=f(T) curves exhibit anomalies in the form of jumps at phase transition temperatures T1=161 K and T2=181 K and that the phase transition occurring at T3=276 K manifests itself in the a=f(T) and b=f(T) curves as a break. A slight anisotropy in the coefficient of thermal expansion of the crystal was revealed. The phase transitions occurring at T1=161 K and T2=181 K in [N(CH3)4]2ZnCl4 were established to be first-order. 相似文献
This mini-review is dedicated to the 85th birthday of Prof. L.V. Keldysh, from whom we have learned so much. In this paper, we study the potential and electron density depth profiles in surface accumulation layers in crystals with a large and nonlinear dielectric response such as SrTiO3 (STO) in the cases of planar, spherical, and cylindrical geometries. The electron gas can be created by applying an induction D0 to the STO surface. We describe the lattice dielectric response of STO using the Landau–Ginzburg free energy expansion and employ the Thomas–Fermi (TF) approximation for the electron gas. For the planar geometry, we arrive at the electron density profile n(x) ∝ (x + d)–12/7, where d ∝ D0–12/7. We extend our results to overlapping electron gases in GTO/STO/GTO heterojunctions and electron gases created by spill-out from NSTO (heavily n-type doped STO) layers into STO. Generalization of our approach to a spherical donor cluster creating a big TF atom with electrons in STO brings us to the problem of supercharged nuclei. It is known that for an atom with a nuclear charge Ze where Z > 170, electrons collapse onto the nucleus, resulting in a net charge Zn < Z. Here, instead of relativistic physics, the collapse is caused by the nonlinear dielectric response. Electrons collapse into the charged spherical donor cluster with radius R when its total charge number Z exceeds the critical value Zc ≈ R/a, where a is the lattice constant. The net charge eZn grows with Z until Z exceeds Z* ≈ (R/a)9/7. After this point, the charge number of the compact core Zn remains ≈ Z*, with the rest Z* electrons forming a sparse TF atom with it. We extend our studies of collapse to the case of long cylindrical clusters as well. 相似文献
The effect of a uniaxial mechanical compression (σm ≤ 100 bar) on the spectral dependences (300–800 nm) of the birefringence Δni and refractive indices ni of K2SO4 crystals is studied. The electronic polarizabilities, refractions, and parameters (λ0i, B1i) of ultraviolet oscillators of mechanically compressed crystals are calculated. It is shown that the dispersions of Δni(λ) and ni(λ) are normal and sharply increase near the absorption edge. It is found that the uniaxial compression changes the value of the dispersions dΔni/dλ and dni/dλ rather than their character. It is ascertained that the simultaneous action of the compressions σx and σz, as well as of σy and σz, leads to the appearance of new isotropic states in the K2SO4 crystal, which manifests itself in the equality of corresponding birefringences. It is shown that the baric dependences ni(σ) are determined by the change in the density of oscillators (~30%), by the shift of the absorption edge and effective band maximum and by the change in the oscillator strength (~70%). 相似文献
We employed density-functional theory (DFT) within the generalized gradient approximation(GGA) to investigate the ZrTi2 alloy, and obtained its structural phase transition,mechanical behavior, Gibbs free energy as a function of pressure, P-V equation of state,electronic and Mulliken population analysis results. The lattice parameters andP-V EOS for α, β and ω phases revealed by ourcalculations are consistent with other experimental and computational values. The elasticconstants obtained suggest that ω-ZrTi2 and α-ZrTi2 are mechanically stable, and that β-ZrTi2 is mechanically unstableat 0 GPa, but becomes more stable with increasing pressure. Our calculated resultsindicate a phase transition sequence of α → ω → β forZrTi2. Both thebulk modulus B and shear modulus G increase linearly withincreasing pressure for three phases. The G/B values illustrated goodductility of ZrTi2alloy for three phases, with ω<α<β at0 GPa. The Mulliken population analysis showed that the increment of d electron occupancystabilized the β phase. A low value for B '0 is the feature of EOS for ZrTi2 and this softness in the EOS isrepresentative of pressure induced s-d electron transfer. 相似文献
The structure factor and correlation energy of a quantum wire of thickness b ? aB are studied in random phase approximation (RPA) and for the less investigated region rs < 1. Using the single-loop approximation, analytical expressions of the structure factor are obtained. The exact expressions for the exchange energy are also derived for a cylindrical and harmonic wire. The correlation energy in RPA is found to be represented by ?c(b, rs) = α(rs)/b + β(rs) ln(b) + η(rs), for small b and high densities. For a pragmatic width of the wire, the correlation energy is in agreement with the quantum Monte Carlo simulation data. 相似文献
The present paper reports the synthesis, crystal structure, 13C and 111Cd cross-polarization magic-angle spinning nuclear magnetic resonance(CP-MAS-NMR) analysis and ac conductivity for a new organic–inorganic hybrid salt, [C7H12N2][CdCl4]. The compound crystallizes in the triclinic system, space group P\( \overline 1 \), with unit cell dimensions: a?=?7.1050(3) Å, b?=?8.9579(3) Å, c?=?9.4482(3) Å, α?=?81.415(1)°, β?=?89.710(2)°, γ?=?85.765(1)°, V?=?592.97(4) Å3, and Z?=?2. The asymmetric unit is composed of one-2,4-diammonium toluene cation and one [CdCl4]2? anion. The Cd atom is in a slightly distorted octahedra coordination environment. Its structure can be described by infinite chains of CdCl6 octahedron linked to organic cations by a strong charge-assisted N–H???Cl interactions in order to build organic–inorganic layers staked along \( \left[ {0\overline 1 1} \right] \) direction. The solid state 13C CP-MAS-NMR spectra has shown seven isotropic resonances, confirming the existence of seven non-equivalent carbon atoms, which is consistent with crystal structure determined by X-ray diffraction. As for 111Cd MAS-NMR, it has shown one cadmium site with isotropic chemical shift observed at 167.2 ppm. The complex impedance of the compound has been investigated in the temperature range of 403–460 K and in the frequency range of 200 Hz–5 MHz. The impedance plots have shown 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 Rp and constant phase elements. 相似文献
Temperature dependences of specific heat Cp(T) and coefficient of thermal expansion ;(T) for Na0.95Li0.05NbO3 sodium-lithium niobate ceramic samples are investigated in the temperature range of 100–800 K. The Cp(T) and α(T) anomalies at T3 = 310 ± 3 K, T2 = 630 ± 8 K, and T1 = 710 ± 10 K are observed, which correspond to the sequence of phase transitions N ? Q ? S(R) ? T2(S). The effect of heat treatment of the samples on the sequence of structural distortions was established. It is demonstrated that annealing of the samples at 603 K leads to splitting of the anomaly corresponding to the phase transition Q → R/S in two anomalies. After sample heating to 800 K, the only anomaly is observed in both the Cp(T) and ;(T) dependence. Possible mechanisms of the observed phenomena are discussed. 相似文献
Epitaxial BaFe1.8Cr0.2As2 thin films with the tetragonal c-axis perpendicular to the thin film surface were grown on (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) single crystalline substrates using pulsed laser deposition (PLD). Resistive measurements indicate the existence of two transitions at temperatures of about 80 K and 40 K. The transition at 80 K is attributed to the structural transition from the high temperature tetragonal phase to the low temperature orthorhombic phase accompanied with the magnetic transition from a paramagnetic to an antiferromagnetic state as known for doped bulk systems. Below T ≈ 40 K the magnetization curves measured perpendicularly to the orthorhombic c-axis in fields up to 9 Tesla show two inflexion points indicating metamagnetic transitions. 相似文献
The electro-optical coefficients rij and half-wave voltage Vλ/2 of strontium-barium niobate crystals poled in the ferroelectric phase are shown to vary along the polar axis. The rij(z) and Vλ/2(z) dependences indicate the presence of a residual domain density D(z) and clearly depend on the sign of the polarizing field, with rij being minimum (D being maximum) near the negative electrode. This character of the D(z) distribution and, hence, the rij(z) and Vλ/2(z) coordinate dependences can be explained by predominant domain nucleation near the negative electrode, which is revealed when the switching processes are studied using 90° (Rayleigh) light scattering from domain walls. 相似文献