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1.
A new model is proposed for a local transition in a Jahn-Teller impurity center in a crystal with a ferroelastic (ferroelectric) phase transition. This model is based on direct interaction of the order parameter of the phase transition in the matrix with the Jahn-Teller impurity degrees of freedom. It is shown that, under these conditions, the order parameter field can induce lifting of degeneracy of the electronic states active in the Jahn-Teller effect, which is accompanied by a transition from the Jahn-Teller effect to the pseudo-Jahn-Teller effect with its subsequent suppression. As a result, a decrease in temperature gives rise to a structural local transition in the region of the low-symmetry ferroelastic (ferroelectric) matrix phase from the many-well local adiabatic to a single-well potential. The model proposed allows interpretation of experimental data obtained in an EPR study of the molecular impurity ion MnO 4 2? in the K3Na(CrO4)2 ferroelastic.  相似文献   

2.
Using time-resolved photoelectron spectroscopy, the decay channels of AuO2 and Au2O2 following photoexcitation with 3.1-eV photons have been studied. For AuO2, a state with a rather long lifetime of 30 ps has been identified. Its decay path could not be determined but photodesorption can be excluded. For Au2O2, the spectra indicate O2 desorption after 3.1-eV photoexcitation on a time scale of 1 ps. While comparing these results on Au n O2 with analogous data on Ag n O2 clusters, a discernible pattern emerges: for dissociatively bound O2(AuO2, Ag3O2), there are long-living excited states which do not decay by oxygen desorption, while for molecular chemisorption (Au2O2, Ag2O2, Ag4O2, Ag8O2), the 3.1-eV photoexcitation triggers fast O2 desorption with a high quantum yield.  相似文献   

3.
The effect of the photon energy of the exciting laser radiation on the Raman spectra of Ba1?xKxBiO3 with x=0.25, 0.40, and 0.50 is studied. An increase in the laser wavelength from 488 to 750 nm scarcely affects the amplitudes and frequencies of the spectral lines in the Raman spectra of the nonsuperconducting compound with x=0.25. For the optimally doped (x=0.40) and overdoped (x=0.50) superconducting compounds, a substantial increase in the line intensity and a considerable shift of the characteristic frequencies are observed. This result suggests that, in the whole range of superconducting compositions 0.37≤x≤0.50, the local symmetry of the Ba1?xKxBiO3 crystal lattice differs from the perfect cubic symmetry, which should take place according to the literature data. The fact that resonance phenomena are observed when the laser photon energy is shifted toward the optical gap testifies to the presence of local electron pairs in the whole range of superconducting compositions 0.37≤x≤0.50 and is evidence in favor of the superconductivity mechanism proposed for Ba1?xKxBiO3 on the basis of the X-ray absorption studies in our previous paper.  相似文献   

4.
Composite cathode material LiFePO4–Li3V2(PO4)3 is synthesized through a chemical reduction and lithiation using FeVO4·xH2O as both iron and vanadium sources. The structural properties of LiFePO4–Li3V2(PO4)3 are investigated. X-ray diffraction results show the composite material containing olivine type LiFePO4 and monoclinic Li3V2(PO4)3 phases. High-resolution transmission electron microscopy and energy-dispersive X-ray spectrometry results indicate that mutual doping effects take place between the LiFePO4 and Li3V2(PO4)3 particles with V3+ doping the LiFePO4 while Fe2+ dopes the Li3V2(PO4)3. LiFePO4–Li3V2(PO4)3 nanocomposites are formed in the carbon webs. There is no structural compatibility between monoclinic (Li3V2(PO4)3) and olivine (LiFePO4) domains in composite material LiFePO4–Li3V2(PO4)3.  相似文献   

5.
The effect of electron instability of heterojunctions based on Ba0.6K0.4BiO3?x single crystals has been found experimentally. This effect is shown to have the opposite sign along the electric field of the current compared to a similar effect observed in structures based on hole-doped systems.  相似文献   

6.
The effect of simultaneous Sr substitution at the Ba and Y sites has been studied in the Sr0.75Y0.75Ba1.5Cu3O7–dsystem. Attempts to replace 25% Y and 25% Ba have been successful and superconductivity was observed above 78 K for Sr0.75Y0.75Ba1.5Cu3O7–d compound with high oxygen content, i.e., O6.76, and having orthorhombic crystal symmetry. The compound was treated in argon gas at 800°C to reduce the oxygen content and to induce some structural changes. The Sr0.75Y0.75Ba1.5Cu3O6.1 compound thus obtained has tetragonal symmetry and low oxygen content, O6.1. It has also shown superconductivity at 28 K. The paper presents a careful comparison of the structural and electrical properties and infrared absorbance spectra of the two compounds with the same metallic composition, Sr0.75Y0.75Ba1.5Cu3, but with different oxygen content and crystal symmetry. The study clearly establishes the occurrence of superconductivity in tetragonal Sr-substituted (both at Y and Ba sites in) YBCO.  相似文献   

7.
The unit-cell parameters of the nonlinear crystal K3Li2?xNb5+xO15±δ were measured with x-ray diffraction in the temperature region 80–300 K. The room-temperature parameters of the crystal of this composition are a=12.599±0.001 and c=4.018±0.001 Å. The temperature dependences of the thermal expansion coefficients (TECs) along the [100] and [001] crystallographic directions were determined. A small TEC anisotropy was established. The specific heat of this crystal was measured using adiabatic calorimetry in the temperature interval 80–300 K. The experimental data on the specific heat were used to calculate the changes in the thermodynamic functions.  相似文献   

8.
The specific heat of [NH2(CH3)2]2ZnCl4 was measured calorimetrically in the temperature region 80–300 K. As the temperature T decreases, the C p (T) dependence indicates a phase transition sequence, with the phase transition at T6=151 K observed for the first time. The thermodynamic characteristics of the crystal were refined. The transformation occurring at T2=298.3 K is shown to be an incommensurate-commensurate phase transition.  相似文献   

9.
The electronic spectra of the valence band and core levels of the surface of polygrain alloys with the icosahedral structure and the β-(CsCl)-type solid solution of Al50Cu33Fe17 were investigated by X-ray photoelectron spectroscopy (XPS). The obtained XPS spectra of the Al62.5Cu25Fe12.5 alloy, in comparison with those of the crystalline Al50Cu33Fe17 alloy demonstrate narrowing and a decrease in asymmetry of the Fe2p core level and a decrease in the electron state density N(E F ) near the Fermi level, features expected for the poorly conducting icosahedral phase. The XPS data are compared with the estimates of N(E F ) based on the low-temperature specific heat measurements.  相似文献   

10.
The reflectivity spectra and the magnetorefractive effect (MRE) of (Co50Fe50)x(Al2O3)1?x metal-dielectric granular films (0.07<x<0.52) are analyzed in the IR spectral range λ=2.5–25 µm. It is revealed that the specific features observed in the spectra at λ≈8.5 and 20 µm are associated with the excitation of phonon modes in the dielectric matrix. The magnetorefractive effect in the films is observed below the percolation thresh-old only in p-polarized light and above the percolation threshold for both the p and s polarizations. It is demonstrated that the optical properties of (Co50Fe50)x(Al2O3)1?x films in the IR spectral range, to a first approximation, can be interpreted in the framework of the effective-medium theory and the magnetorefractive effect can be explained in terms of the modified Hagen-Rubens relation.  相似文献   

11.
K-shell ionization cross section measurements are reported for35Br,37Rb and39Y targets caused by protons over 300–400 keV energy range in 20 keV increment. The K-shell ionization cross sections (σ k l ) at different energies were deduced from the Kα and Kβ X-ray production cross sections which were obtained from X-ray yields of the Kα and Kβ transitions. The experimental values are compared with the calculated values of ECPSSR theory and empirical reference cross sections. The resultant K-shell ionization cross sections are found to be in reasonable agreement with the ECPSSR theory. The Kα/Kβ intensity ratios are also presented and compared with other experimental values and also with the theoretical one-hole values given by Scofield.  相似文献   

12.
The effect of external magnetic field on the Kα and Kβ X-ray production cross-sections and K shell fluorescence yields for ferromagnetic elements Nd, Gd and Dy and paramagnetic elements Eu and Ho have been measured at the excitation energy of 59.5 keV γ -rays from 241Am radioactive source of strength 100 mCi in the external magnetic field of intensities ±0.75 T. Furthermore, I /I intensity ratios for these elements have been measured in the external magnetic field. The K X-rays from different targets were detected using a high resolution Si(Li) semiconductor detector. For B = 0, the measured K X-ray production cross-sections, K shell fluorescence yields and the I /I intensity ratios were compared with the experimental and theoretical data in literature. The results have shown that the fluorescence parameters as photoionization cross section, fluorescence yield, radiation rates and spectral linewidth can change when the irradiation is conducted in a magnetic field.  相似文献   

13.
K to L shell vacancy transfer probabilities (ηKL) for 26 elements in the atomic region 23≤Z≤57 were determined by measuring the I/I intensity ratios. The targets were irradiated with γ-photons at 59.543 keV from 241Am annular source. The K X-rays from different targets were detected with a high resolution Si(Li) detector. Theoretical values were calculated using the radiative and radiationless transition rates of these elements. The measured values of ηKL are compared with the theoretical values and data of others. The measurement vacancy transfer probabilities are least-square fitted to third-order polynomials to obtain analytical relations that represent these probabilities as a function of atomic number. The measured values of ηKL for V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se and Br are being reported here for the first time.  相似文献   

14.
A method is suggested to effect the self absorption correction in a different way to estimate the K X-ray intensity ratios particularly when heavy ions are used as projectiles. Employing this method, the Kβ/Kα intensity ratios are measured in some 3d shell elements by using Carbon and Nitrogen ions as exciting agents. The Kβ/Kα intensity ratios thus obtained in the present work are compared with the intensity ratios due to some previous authors and also with Scofield theoretical values. Received 14 August 2000 and Received in final form 3 January 2001  相似文献   

15.
The measurements of the K X-ray intensity ratio I(Kβ)/I(Kα) for the 17 elements Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Br, Rb, Sr, Y, Zr, Nb and Mo have been done following ionization by 59.5 keV γ-rays from a 241Am point source. Ratios of emission probabilities of Auger electrons and the vacancy transfer coefficients have been extracted in terms of the intensity ratios. It is found that the present results agree well with earlier fitted values and the semi-empirical values.  相似文献   

16.
Summary Chemical effects on theKβ/Kα X-ray intensity ratios were investigated for different constitutions of Ti, V, Fe, Se, Br, Zr and Ce by a Ge(Li) X-ray detector. The vacancies were produced by heavily filtered241Am gamma-rays. It is found that theKβ/Kα X-ray intensity ratios measured with compounds deviated up to 12% from the corresponding values of the pure elements (Ti, V, Fe) are larger than for the others (Se, Br, Zr and Ce).  相似文献   

17.
The Kα to Lα and Kβ to Lβ intensity ratios of seven elements have been measured following photoionization at 59.5 keV by using a Si (Li) detector (FWHM=155 eV at 5.96 keV). The intensity ratios were determined by measuring K and L x-rays emitted from a standard target of a given element. The theoretical values of the Kα to Lα and Kβ to Lβ intensity ratios were calculated using theoretically tabulated values of shell/subshell photoionization cross sections, fluorescence yields, Coster-Kronig transition probabilities, and radiative decay rates for ηKLi≠ and ηKLi=0. The measured values are in good agreement with theoretical results. From Optika i Spektroskopiya, Vol. 97, No. 2, 2004, pp. 186–189. Original English Text Copyright ? 2004 by A. Kü?ük?nder, S?ğüt, E. Kü?ük?nder, Büyükkasap. This article was submitted by the authors in English.  相似文献   

18.
DK Basa  S Raj  HC Padhi  M Polasik  F Pawlowski 《Pramana》2002,58(5-6):783-786
K β-to-K α X-ray intensity ratios of Fe and Ni in pure metals and in Fe x Ni1−x alloys (x=0.20, 0.50, 0.58) exhibiting similar crystalline structure have been measured following excitation by 59.54 keV γ-rays from a 241Am point source, to understand as to why the properties of permalloy Fe0.2Ni0.8 is distinct from other alloy compositions. It is observed that the valence electronic structure of Fe0.2Ni0.8 alloy is totally different from other alloys which may be attributed to its special magnetic properties.  相似文献   

19.
K X-ray emission spectra of Ti, V, Cr and Mn generated by photon excitation have been studied with a crystal spectrometer. The measured energy shifts of Kα satellite relative to the diagram line are compared with values obtained by electron excitation and with different theoretical estimates. The present experimental values of KαL1/KαL0 relative intensities are compared with values obtained by electron excitation.   相似文献   

20.
The Measurement of K β K /α intensity ratios are measured in some 3d shell elements by using a 2 MeV proton beam along with a high resolution Si(Li) detector. The present Measurement of K β K /α intensity ratios are in good agreement with Scofield modified theoretical values, thus supporting the basic assumptions in that theory. From the present Measurement of K β K /α intensity ratios, it is evident that due to chemical effects, the experimental Measurement of K β K /α intensity ratios will be increased while they will be decreased due to the presence of simultaneous M-shell vacancies which are produced due to proton excitation.  相似文献   

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