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1.
New triethylammonium salts: [(C2H5)3NH]SbCl6 (TCA) and [(C2H5)3NH]SbCl6·1/2[(C2H5)3NH]Cl (TCAT) have been synthesized. The compounds crystallise in monoclinic symmetry: space groups P21/n and P21/c, for TCA at 293 K and TCAT at 100 K, respectively. The crystal structure of [(C2H5)3NH]SbCl6 consists of discrete ionic pairs—triethylammonium cations and hexachloroantimonate anions—linked via the bifurcated N-H?Cl hydrogen bonds. The crystal structure of [(C2H5)3NH]SbCl6·1/2[(C2H5)3NH]Cl is composed of three symmetrically independent triethylammonium cations, chlorine anion and two symmetrically independent hexachloroantimonate anions. TCA undergoes a structural phase transition at 336 K (on heating) into the orthorhombic C222 space group, whereas TCAT reveals a structural phase transition at 332 K. The phase transitions are of the first order type. TCA shows a ferroelastic domain structure below 336 K. Differential scanning calorimetry, dilatometric, dielectric dispersion and Raman scattering measurements have been used to study the phase transition mechanisms in these triethylammonium salts.  相似文献   

2.
Two methods—the solid-phase high-temperature (1300 °C) and the liquid-phase low-temperature (750 °C) routes—were used to synthesize the complex oxide La1.25Sr0.75MnCoO6, which has the structure of rhombohedral perovskite and is characterized by a disordered distribution of Mn and Co in structural sites. It was found by means of X-ray absorption near edge spectroscopy (XANES) at the K-edge that mixed valence states of Co2+/Co3+ and Mn3+/Mn4+, exist in both phases. Measurements of dc magnetization and real (χ′) and imaginary (χ″) parts of the ac susceptibility showed that the magnetic properties of these oxides are determined by a ferromagnetic transition at TC=217 K and a frequency-dependent transition at Tg<100 K. The high frequency dependence of Tg is indicative of the cluster-glass behavior of La1.25Sr0.75MnCoO6 (7 5 0) at T<TC within the ferromagnetic state.  相似文献   

3.
The complex conductivity in polycrystalline C60 and C70 has been investigated for frequencies 20 Hz≤ν≤106 Hz and temperatures 10 K≤T≤750 K. The high-frequency dielectric constants εα= 2.6±0.1(C60) and ε= 4.6±0.1 (C70) were deduced from these experiments. The observed low temperature relaxation process in C60 fits well into the relaxation dynamics of the C60 molecules as determined by many other experimental techniques operating on very different time scales. In addition to the study of the dipolar relaxation process, the dc and ac conductivities were determined. From the temperature dependence of the dc conductivities energy barriers of EG=1.75±0.1 eV (C60) and EG=1.7±0.1 eV (C70) were estimated. In C70 we found indications for small polaron tunneling.  相似文献   

4.
The two-channel thermal decomposition of toluene, C6H5CH3 → C6H5CH2 + H (1) and C6H5CH3 → C6H5 + CH3 (2), was investigated in shock tube experiments over the temperature range of 1400-1780 K at a pressure of 1.5 (±0.1) bar. Rate coefficients for reactions (1) and (2) were determined by monitoring benzyl radical (C6H5CH2) absorption at 266 nm during the decomposition of toluene diluted in argon and modeling the temporal behavior of the benzyl concentration with a kinetic model. The first-order rate coefficients determined at a pressure of 1.5 bar are expressed by k1(T) = 2.09 × 1015 exp (−87510 [cal/mol]/RT) [s−1] and k2(T) = 2.66 × 1016 exp (−97880 [cal/mol]/RT) [s−1]. The resulting branching ratio, k1/(k1 + k2), ranges from 0.8 at 1350 K to 0.6 at 1800 K.  相似文献   

5.
A new program is described for fitting rotation-torsion energy levels in molecules like toluene, in which the frame (C6H5) has C2v symmetry and the methyl top has C3v symmetry, i.e., for molecules where the internal rotation barrier is expanded in cos6, where α is the internal rotation angle and n = 1,2,…. The program is based on the theoretical framework developed by Sørensen and Pedersen in their application of the Longuet-Higgins permutation-inversion group G12 to the microwave spectrum of CH3NO2. It is specifically designed for sixfold barrier molecules, and allows the user to select almost any symmetry-allowed torsion-rotation term for inclusion in the fitting Hamiltonian. This program leads to a very successful fit of transitions in the microwave spectrum of toluene characterized by J ? 30, Ka ? 12, and by the free-rotor quantum number ∣m∣ ? 3. In these fits we included both published and rather extensive unpublished new measurements, for which fits using other torsion-rotation programs have not been very successful. The fit presented here uses 28 parameters to give an overall standard deviation of 7.4 kHz for 372 line frequencies, and results in a much improved value for the sixfold barrier for toluene, V6 = 13.832068(3) cal mol−1.  相似文献   

6.
Double-layered manganite La1.4Ca1.6Mn2O7 has been synthesized using the solid-state reaction method. It had a metal-to-insulator transition at temperature TM1≈127 K. The temperature dependence of ac susceptibility showed a broad ferromagnetic transition. The two-dimensional (2D)-ferromagnetic ordering temperature (TC2) was observed as ≈245 K. The temperature dependence of its low-field magnetoresistance has been studied. The low-field magnetoresistance of double-layered manganite, in the temperature regions between TM1 and TC2, has been found to follow 1/T5. The observed behaviour of temperature dependence of resistivity and low-field magnetoresistance has been explained in terms of two-phase model where ferromagnetic domains exist in the matrix of paramagnetic regions in which spin-dependent tunneling of charge carriers occurs between the ferromagnetic correlated regions. Based on the two-phase model, the dimension of these ferromagnetic domains inside the paramagnetic matrix has been estimated as ∼12 Å.  相似文献   

7.
The radiative quantum efficiencies η of the CdSe/ZnS core-shell nanoparticles embedded into polymethyl methacrylate (PMMA) and suspended in three different solvents: chloroform (CHCl3), toluene (C6H5CH3) and tetrahydrofuran (C4H8O) were measured using thermal lens (TL) technique. The mode-mismatched pump-probe TL measurements were accomplished in function of the CdSe/ZnS quantum-dot concentration (12-60 mg/ml) at λe = 594 nm (pump) and λp = 632.8 nm (probe). The values obtained for η were higher for CdSe/ZnS nanoparticles suspended in tetrahydrofuran and chloroform, as compared to the values for toluene. Thermal diffusivity (D) and the absolute nonradiative quantum efficiency (φ) were determined.  相似文献   

8.
The auto-ignition of toluene/air mixtures was studied in a shock tube at temperatures of 1021-1400 K, pressures of 10-61 atm, and equivalence ratios of Φ = 1.0, 0.5, and 0.25. Ignition times were measured using endwall OH∗ emission and sidewall piezoelectric pressure measurements. The measured pressure time-histories do not show significant pre-ignition energy release, in agreement with the rapid compression machine study of Mittal and Sung [G. Mittal, C.-J. Sung, Combust. Flame 150 (2007) 355-368] and disagreement with the shock tube study of Davidson et al. [D.F. Davidson, B.M. Gauthier, R.K. Hanson, Proc. Combust. Inst. 30 (2005) 1175-1182]. Kinetic modeling predictions from three detailed mechanisms are compared. Sensitivity analysis indicates that the reaction of toluene (C6H5CH3) and the benzyl radical (C6H5CH2) with molecular oxygen are important and examination of the rate coefficients for these reactions suggests that improved rate parameters for the multi-channel C6H5CH2 + O2 reaction may improve model predictions.  相似文献   

9.
The paper contains a time-dependent investigation of the tunneling effect observed in the photoassociation spectrum of Cs2 and attributed to the 0g -(6s, 6p 3/2) double well. When by photoassociation of two cold cesium atoms a vibrational level of the outer well is populated, tunneling is an efficient mechanism for transferring the population to the inner well (R < 15a 0), where spontaneous emission may lead to formation of cold molecules in low vibrational levels of the a 3Σ+ u(6s, 6s) electronic state. This tunneling effect is analyzed by wavepackets propagation, first considering the double well potential alone, and following a packet made by a superposition of states initially located at large distances. Characteristic times for the vibration dynamics, corresponding to a beating phenomenon between the two wells, to partial “revival” at large distances, and to maxima in the population localized in the inner well are reported and discussed. Second, we simulate the two-channels a 3Σ+ u(6s, 6s)↦0g -(6s, 6p 3/2) photoassociation at detunings around 2.9 cm-1: the inner well can be populated either by the excitation of a vibrational level of the external well (resonant excitation), or by tuning the photoassociation laser at the energy of the inner well level which displays tunneling (“off-resonance excitation”). In the first case the photoassociation is efficient, while the tunneling probability is small; in the second, the tunneling probability is large, so that despite the poor efficiency of the photoassociation process, more population can be transferred to the inner well. This second choice is shown to be very sensitive to the laser intensity, which could be used to control the population of the inner well and hence the formation of ultracold molecules in low vibrational levels. Received 19 April 2002 Published online 1st October 2002 RID="a" ID="a"e-mail: francoise.masnou@lac.u-psud.fr  相似文献   

10.
MgO-based magnetic tunnel junctions (MTJs) with a layer sequence Ir22Mn78 or Fe50Mn50 (10 nm)/CoFe (2 nm)/Ru (0.85 nm)/CoFeB (0.5?t<2 nm)/MgO (2.5 nm)/CoFeB (3 nm) have been fabricated. The bias voltage dependence of tunneling magnetoresistance (TMR) is given as a function of the annealing temperature for these MTJs, which shows the TMR ratio changes its sign from inverted to normal at a critical bias voltage (VC) when an unbalanced synthetic antiferromagnetic stack CoFe/Ru/CoFeB is used. VCs change with the thickness of the pinned CoFeB and annealing temperature, which implies one can achieve different VCs by artificial control. The asymmetric VC values suggest that a strong density-of-states modification occurs at bottom oxide/ferromagnet interface.  相似文献   

11.
The barium phenylarsonate compound, Ba(HO3AsC6H5)2·2H2O, has the ability to intercalate n-alkyldiamine molecules, H2N(CH2)nNH2 (n=2-5). The amount intercalated (nf) from a batchwise procedure and the variation of the original basal distance (d) of 1245 ppm determined through X-ray diffractions, gave linear correlations as a function of the number of carbon atoms in the aliphatic chain (nc): nf=(2.66±0.06)−(0.13±0.02)nc and d=(2168±65)+(114±14)nc. The intercalation process was calorimetrically followed to give exothermic enthalpy and negative Gibbs energy, reflecting spontaneous intercalation reactions at the solid/liquid interface. The displacement of solvent molecules bonded to amine and of those on the matrix during the intercalation increases the disorder to result in positive entropy, giving a favorable set of thermodynamic data for this system.  相似文献   

12.
We report the synthesis, structure and low-field magnetotransport properties of Mischmetal (Mm)-doped La0.7−xMmxCa0.3MnO3 (0?x?0.45) manganite. Mischmetal—Mm—is a natural mixture of rare earth elements La, Ce, Pr and Nd with ∼28%, 50%, 6% and 16% composition, respectively. All the samples crystallize in orthorhombic structure. Increasing x (Mm), corresponding to decreasing the La-site average ionic radii (〈rA〉) hence increasing the size mismatch (i.e. variance σ2), results in strong suppression of ferromagnetism (TC) and the associated metallicity (TIM). It may be pointed out that Mm (La, Ce, Pr and Nd) substitution has been done to create two effects. First, creation of multivalence of Mn (2+, 3+ and 4+) via Ce substitution and second to create higher degree of disorder due to size difference brought in not only by Ce but also by Pr and Nd. Evidences and arguments based on XPS analysis suggest that multivalent ions La, Mm and Ca, and the resulting presence of Mn2+, Mn3+ and Mn4+, causes the simultaneous operation of ferromagnetism-double exchange (Mn2+/Mn3+ and Mn3+/Mn4+) and antiferromagnetic-superexchange (Mn3+/Mn3+ and Mn2+/Mn2+) interaction. In addition, Mm doping also creates inhomogenities at La—as well as Mn—site due to size and valency difference. A curiously huge magnetoresistance as high as ∼63% for x=0.35, under a moderate magnetic field of ∼10 kOe has been observed and even at low magnetic field of ∼3 kOe MR is ∼30%. The competing double exchange and superexchange coupled with inhomogenities are the most likely cause for the occurrence of large ∼63% CMR in the Mm-doped LCMO.  相似文献   

13.
Deep level transient spectroscopy (DLTS) and Laplace-DLTS (L-DLTS) have been used to investigate defects in an n-type GaAs before and after exposure to a dc hydrogen plasma (hydrogenation). DLTS revealed the presence of three prominent electron traps in the material in the temperature range 20-300 K. However, L-DLTS with its higher resolution enabled the splitting of two narrowly spaced emission rates. Consequently four electron traps at, EC—0.33 eV, EC—0.36 eV, EC—0.38 eV and EC—0.56 eV were observed in the control sample. Following hydrogenation, all these traps were passivated with a new complex (presumably the M3), emerging at EC—0.58 eV. Isochronal annealing of the passivated material between 50 and 300 °C, revealed the emergence of a secondary defect, not previously observed, at EC—0.37 eV. Finally, the effect of hydrogen passivation is completely reversed upon annealing at 300 °C, as all the defects originally observed in the reference sample were recovered.  相似文献   

14.
Adsorption of acetylene and ethane molecules by water clusters has been investigated by the molecular dynamics method at T=233 K. With the help of determination of statistical weights the cluster systems are created. In the frequency range of 0?ω?1000 cm−1 the integral absorption coefficient of IR-radiation increases after the adsorption of acetylene or ethane molecules by the ultra disperse water system. The dissipation power of IR-radiation by cluster systems increases if C2H2 molecules are adsorbed, and it reduces in the case of C2H6 molecules' adsorption.  相似文献   

15.
Thermal (specific heat) and optical (linear birefringence) studies were performed for a new ferroelectric crystal (C3N2H5)5Bi2Cl11. Two phase transitions were confirmed and described. The first-order paraelastic-ferroelastic phase transition at 360 K was studied with a polarizing microscope. The continuous second-order phase transition at 165 K to the ferroelectric phase is described by the Landau model using specific heat and linear birefringence data. The Landau expansion coefficients B and C are of an order of magnitude higher than the closely related ferroelectric crystal; MAPCB — (CH3NH3)5Bi2Cl11. Thermal parameters (such as the excess enthalpy and the excess entropy ) of the continuous transition were estimated and discussed. The ‘two-site’ model describing the motion of three of the five imidazolium cations, which is proposed from the structural studies, is fully confirmed by the data from the ac-calorimetric measurements.  相似文献   

16.
Electrical conductivity and magnetoresistance of a series of monovalent (K) doped La1−xKxMnO3 polycrystalline pellets prepared by pyrophoric method have been reported. K doping increases the conductivity as well as the Curie temperature (TC) of the system. Curie temperature increases from 260 to 309 K with increasing K content. Above the metal-insulator transition temperature (T>TMI), the electrical resistivity is dominated by adiabatic polaronic model, while in the ferromagnetic region (50<T<TMI), the resistivity is governed by several electron scattering processes. Based on a scenario that the doped manganites consist of phase separated ferromagnetic metallic and paramagnetic insulating regions, all the features of the temperature variation of the resistivity between ∼50 and 300 K are described very well by a single expression. All the K doped samples clearly display the existence of strongly field dependent resistivity minimum close to ∼30 K. Charge carrier tunneling between antiferromagnetically coupled grains explains fairly well the resistivity minimum in monovalent (K) doped lanthanum manganites. Field dependence of magnetoresistance at various temperatures below TC is accounted fairly well by a phenomenological model based on spin polarized tunneling at the grain boundaries. The contributions from the intrinsic part arising from DE mechanism, as well as, the part originating from intergrannular spin polarized tunneling are also estimated.  相似文献   

17.
The dependence of basal spacing and water content of BaC6H5PO3·xH2O on the relative humidity was studied. Intercalates of 1-alkylamines (C2-C10) and 1-alkanols (C3-C10) were prepared from barium phenylphosphonate dihydrate and also from anhydrous host and characterized by powder X-ray diffraction and thermogravimetric analysis. The intercalates of alkanols and alkylamines prepared from dihydrate are quite stable at ambient conditions and contain one guest molecule per formula unit. The guest molecules are probably arranged in monomolecular way and are perpendicular to the host layers in the case of amines or tilted to the host layers at an angle of about 80° in the case of alkanols. The intercalates prepared from anhydrous host are unstable and their basal spacings indicate parallel arrangement of the guests chains. Formation of mixed intercalates was not observed when barium phenylphosphonate dihydrate was contacted with a mixture of alkanols or amines.  相似文献   

18.
Linear optical absorption and emission spectra of C70 fullerene molecules in single-crystal toluene are investigated. It is established that the lines of purely electronic S 0-S 1 transitions are significantly polarized. The degree of linear polarization of the spectral lines depends on the position of the fullerene molecule in the toluene matrix and can be as high as 100%. The polarization characteristics of the lines can be understood in the context of a model in which the S 0S 1 electronic transition is represented by the excitation of a planar oscillator whose axis is oriented along the principal axis of the C70 molecule. The relationship between the polarization of the spectral lines and the position of the fullerene molecule in the matrix is consistent with the conclusions drawn from a theoretical analysis of different configurations possible upon the embedding of C70 molecules into crystalline toluene.  相似文献   

19.
The decomposition of malic acid (C4H6O5) in the presence of Mg and B was studied using Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) which revealed that malic acid reacted with Mg but not B. Also, the addition of toluene (C7H8) to dissolve malic acid followed by subsequent drying resulted in no reaction with Mg, indicating that the malic acid had decomposed during the dissolution/drying stage. The total carbon contributed by toluene versus a toluene/5 wt.% malic acid mixture was measured using a LECO CS600 carbon analyzer. The toluene sample contained ~0.4 wt.% C while the toluene/malic acid mixture had ~1.5 wt.% C, demonstrating that the toluene contributed a significant amount of carbon to the final product. Resistivity measurements on powder-in-tube MgB2 monofilamentary wires established that the toluene/malic acid doped sample had the highest Bc2. However, the toluene-only sample had the highest transport Jc over most of the magnetic field range (0–9 T), equaled only by that of toluene/malic-acid sample in fields above 9 T.  相似文献   

20.
This paper describes the synthesis and characterization of self-assembled organic-inorganic layered perovskite compounds, (C6H5-CnH2n-NH3)2PbBr4 (n=1-4). the effect of the number of carbon atoms of the alkyl chain length (n) on optical properties has been studied. (C6H5-CnH2n-NH3)2PbBr4 films fabricated by spin-coating are microcrystalline form, single phase and oriented with the c-axis. Crystallinity, the maximum PL intensity and the lifetime of exciton emissions varied with the number of carbon atoms. the lowest-energy exciton splits into a few fine-structure levels at low temperatures. Time-resolved photoluminescence spectra reveal that (C6H5-CnH2n-NH3)2PbBr4 shows both singlet and triplet excitons. with decreasing temperature, triplet exciton emissions become dominant for (C6H5-CnH2n-NH3)2PbBr4 (n=1-3), while (C6H5-C4H8-NH3)2PbBr4 shows mainly singlet exciton emissions. The intersystem crossing from excited singlet state to triplet state plays an important role in the relaxation process of excitons.  相似文献   

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