Computer simulation of ion transport in a new cathode material PrSrCuO4-δ

Oxygen diffusion in the new class of cuprates Pr_2-x Sr _x CuO_4-δ (x = 1) with perovskite structure has been simulated in the temperature range of 300–2100 K for the first time. A calculation has shown the presence of anisotropy of oxygen motion: the oxygen transport in PrSrCuO_3.7 in the temperature range of 300–2100 K is mainly two-dimensional, with an activation energy of no more than 0.40 eV. The coefficient of thermal expansion of PrSrCuO_3.7 (9.9 × 10^?6 K^?1 in the range 1300–2100 K) and the oxygen diffusivity in it, which exceed the corresponding values for La_2-x Sr _x CuO_4-δ, indicate that this compound is promising as an electrode material with a mixed ionic-electronic conductivity for various electrochemical devices. The results expand the previous concepts of the oxygen-ion transport in complex cuprates.     

Etude par Calorimetrie,Diffraction des Rayons X et Diffusion Raman d'une Transition de Phase dans (C4 H4P) Mn (CO)3

Abstract

Heat capacity measurements (95-300K), X-ray diffraction (78-300K) and low frequency Raman spectroscopy (10-350K) have evidenced an order-disorder phase transition in phosphacymantrene, (C₄ H₄P) Mn (CO)₃. This transition has been characterized by a monoclinic ←→ triclinic structural change at about 110 K and by a pretransitional phenomenon. The measured transition enthalpy and entropy are 480 ± 10J.mil^?1 and 4.17 ± 0.08J.K^?1 mol^?1, at 115 K, respectively.

A complete assignment of the observed Raman bands in h₄ and d₄ derivatives is proposed. From the temperature dependence of frequencies, intensities and half-widths of some Raman bands we have discussed the order, the nature and the mechanism of the phase transition: intermolecular interactions appear to be mainly involved in the mechanism and an activation energy roughly equal to 2100 ± 840 J. mol^?1 has been determined.     

Molecular and crystal structures of decamethylruthenocene (η5-C5Me5)2Ru in the temperature range 153–300 K: Thermal motion in a crystal according to X-ray diffraction data

The crystal structure of decamethylruthenocene (η⁵-C₅ Me ₅)₂Ru (I) is investigated by X-ray diffraction. It is demonstrated that the compound studied crystallizes in two polymorphic modifications, namely, modification Ia with space group P2₁/m (Z = 2) in the temperature range 153–300 K and modification Ibwith space group P2₁/n (Z = 4) at 203 K. No temperature phase transition between the modifications is found. In crystal Ia, the molecule occupies a special position in the mirror plane. In crystal Ib, the molecule is located in the general position. The cyclic ligands η⁵-C₅ Me ₅, (Cp*) are aligned parallel to each other and adopt an eclipsed conformation. The bond lengths in compounds Ia and Ib are identical. Analysis of the anisotropic displacement parameters of the atoms indicates that molecules Ia and Ib are not structurally rigid and that the Cp* rings involved in these molecules can execute independent librations. In the temperature range 153–300 K, the Cp*(1) ligand in molecule Ia is statically disordered over two positions. The barrier heights B ₅ for rotation of the Cp* ligands are estimated both from the root-mean-square amplitudes of librations 〈ϕ²〉 and with the use of the atom-atom potential method. __________ Translated from Kristallografiya, Vol. 48, No. 2, 2003, pp. 283–292. Original Russian Text Copyright ? 2003 by Zanin, Antipin.     

The Temperature Dependence of Electron Spin-Lattice Relaxation Data in Trans-Polyacetylene and the Evidence for a Soliton-Phonon Interaction

Abstract

The temperature and frequency dependence of the electron spin-lattice relaxation rate Rj for the paramagnetic defects in pristine transpolyacetylene is analyzed in terms of a model in which the electron-electron dipolar interaction between spins is modulated by one-dimensional diffusion induced by phonon scattering of the spins. According to this model, at room temperatures R₁ has a T¹/² temperature dependence; at low temperatures R₁ has a T² or T⁵/² temperature dependence. R₁ is described by a ω^?1/² frequency dependence. The model shows the symmetry between moving and stationary solitons scattered by phonons. The soliton wave function obtained from experimental ENDOR data is used to obtain analytical results. We discuss the possible temperature and frequency limitations of this model.     

Nuclear Spin-Lattice Relaxation in the Linear Chain Metal,NbSe3

Abstract

The nuclear spin-lattice relaxation time T ₁ of ⁹³Nb in NbSe₃ has been observed as a function of frequency v _N in the range 6-40 MHz at 4.2 K and of temperature T in the range 1.3-77 K at 10 MHz. [T ₁ T]^?1 is found to be proportional to 1/√ν corresponding to characteristic divergence of spin correlation function in 1-D system. Below ~10 MHz, a break down of the divergence is observed. The important features of the T dependence observed are a sharp minimum at ~45 K and an asymptotical approach to the metallic relation, T ₁ T= const, with decreasing T down to helium temperature. This [T ₁ T]^?1 minimum is explained by the destruction of the Fermi surface due to the CDW gap formation. The re-increase of [T ₁ T]^?1 below the CDW transition may suggest a partial recovery of the destructed Fermi surface.     

Raman Scattering From TMTSF-Salts

Abstract

Experimental results for Raman scattering from TSeF, TMTSF-D12, (TMTSF)₂PF₆ and (TMTSF)₂AsF₆ excited with blue-green laser light at liquid nitrogen temperature are reported. A strong resonance effect for excitation of TMTSF with 4579 Å was observed and an assignment for the a_g modes was obtained by comparison with results from TTF. Methyl group modes were identified at 328 cm^?1, 472 cm^?1 and 916 cm^?1, respectively. Charging the TMTSF molecule resulted in a strong frequency shift of -122 cm^?1 and -76 cm^?1 for the a_gu₂ mode, respectively. A study of the temperature dependence of the scattering from molecular modes between 2 K and 300 K did not reveal any correlation with the phase transitions in the conducting compounds.     

Triplet Excitions in Crystalline Trans-Stilbene I. Spectroscopy of the So → T1 Transition

Abstract

The first singlet → triplet absorption of trans-stilbene has been studied between 10 and 300 K. The triplet exciton energy (energy of the lowest 0–0 line) is 17380 cm^?1 at 10 K, and does not change significantly with temperature. Vibrational A_g modes of 206. 1250 and 1570 cm^?1 are active. The Franck-Condon factor of the origin region is small. The low temperature spectra of the 0–0 and 206 cm^?1 regions show a doublet structure with a splitting of 82 ± 1 cm^?1 which is attributed to the site splitting. Further structure shown by the lines is discussed. The lines are approximately lorentzian. From curve fitting, line-widths and exciton-phonon coupling constants, increasing linearly with T, are deduced. Exciton-phonon coupling appears to be different on the two sites. The product of absorption coefficient at 4880 Å, α, by the total triplettriplet interaction rate constant has been measured at 295 K: αγ=2.3 ± 0.3. 10^?15 cm² sec^?1, corresponding for γ_total to a value of a few 10^?12 cm³ sec^?1.     

Superconductivity in the Organic Charge Transfer Salts: (TMTSF)2X and (TMTTF)2X

Abstract

We report pressure dependent studies of the a-axis resistivity as a function of temperature for several members of the isostructural families of organic charge transfer salts, (TMTSF)₂X and (TMTTF) ₂X. For a typical (TMTSF)₂X material the low temperature metal-insulator transition seen at 1 bar is suppressed above some critical pressure, P_c, where a superconducting transition is observed near 1 K. We find a correlation between P_c and the ambient pressure c lattice parameter which reflects the anion size. The (TMTTF) _cX salts exhibit very different ambient pressure behaviour but we find that with the application of sufficiently high pressures (～30 kbar) their behaviour resembles that seen in the (TMTSF)₂X family but at lower pressures. In particular we find evidence of a possible superconducting transition near 4 K in (TMTTF)₂Br at 25 kbar. At this pressure the conductivity near 4 K is extremely high with a value approaching 10⁶ (Ωcm)^?1 and the resistivity ratio is about 400.     

Thermal expansion and structural properties of (CuAlTe2)1–x(CuAlSe2)x solid solutions

Investigations of the thermal expansion of (CuAlTe₂)_1–x(CuAlSe₂)_x solid solutions in the temperature range from 100 to 800 K have been carried out for the first time. It has been demonstrated that the thermal expansion coefficient α_L grows considerably in the temperature range from 100 to 300 K, whereas the temperature dependence above 300 K is rather weak. The isotherms of composition dependence of the thermal expansion coefficient α_L for 100, 293, 500 and 800 K were constructed, and it was found that linear relations could express them. The Debye temperatures θ_D , the average mean‐square dynamic displacements , the average root‐mean‐square amplitudes of thermal vibration RMS , the anion position parameter u using S. C. Abrahams & J. L. Bernstein (u_AB ) and J. E. Jaffe & A. Zunger (u_JZ ) models were calculated. The composition dependence of microhardness H using the phenomenological theory was also calculated, and it was discovered that this dependence has a non‐linear character with a maximum of 383 kg/mm² at x=0.67. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Time resolved thermal lens measurements of the thermo-optical properties of Nd2O3-doped low silica calcium aluminosilicate glasses down to 4.3 K

In this work, the thermal lens spectrometry was applied to measure the thermo-optical properties of Nd₂O₃-doped low silica calcium aluminosilicate glasses as a function of temperature, between 4.3 and 300 K. The thermal relaxation calorimetry was used to determine the specific heat, c_p. The results showed a decrease of the thermal diffusivity of about one order of magnitude from 4.3 K up to 300 K, with a T^?1 dependence in the interval between 20 and 70 K and a T^?0.35 between 4.3 and 20 K. The fluorescence quantum efficiencies of the doped samples were calculated down to 50 K, showing a variation of the order of 12% and 25% for the samples with 0.6 and 1.04 mol% of Nd₂O₃, respectively. In addition, the temperature corresponding to the maximum in c_p/T³, the so-called boson peak, was observed at about 17 K for the undoped sample and at lower temperatures for the doped glasses. In conclusion, our results showed the ability of the time resolved thermal lens to determine the thermo-optical properties of glasses at temperatures lower than 300 K, bringing new possibilities for experiments in a wide range of optical materials.     

Photoelectronic and electrical properties of CuIn5S8 single crystals

To identify the impurity levels in CuIn₅S₈ single crystals, the dark electrical conductivity and photoconductivity measurements were carried out in the temperature range of 50–460 K. The data reflect the intrinsic and extrinsic nature of the crystals above and below 300 K, respectively. Energy band gaps of 1.35 and 1.31 eV at 0 K and 300 K, were defined from the dark conductivity measurements and the photocurrent spectra, respectively. The dark and photoconductivity data in the extrinsic temperature region reflect the existence of two independent donor energy levels located at 130 and 16 meV. The photocurrent‐illumination intensity dependence (F) follows the law I_phαF^γ, with γ being 1.0, 0.5 and 1.0 at low, moderate and high intensities indicating the domination of monomolecular, bimolecular and strong recombination at the surface, respectively. In the intrinsic region and in the temperature region where the shallow donor energy level 16 meV is dominant, the free electron life time, τ_n, is found to be constant with increasing F. In the temperature region 140 K < T < 210 K, the free electron life time increases with increasing illumination intensity showing the supralinear character. Below 140 K, τ_n decrease with decreasing illumination intensity. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Observation of Strong Antiferromagnetic Exchange Interactions in [1-(4′-methobenzyl)isoquinolinium][Ni(mnt)<Subscript>2</Subscript>]: Crystal Structures,Magnetic Properties

Abstract  

An new ion-pair compound, [CH₃BzQl][Ni(mnt)₂] (1), (CH₃BzQl = 1-(4′-methobenzyl)isoquinolinium and mnt²⁻ = maleonitriledithiolate), has been synthesized, structurally and magnetically characterized. The [Ni(mnt)₂]⁻ anions (A) and [CH₃BzIQl]⁺ cations (C) of 1 alternate to form a mixed columnar stack in the manner of ···ACAC··· along the a-axis, and the neighboring columns connect together to generate a three-dimensional (3D) network structure via intermolecular H-bonding interactions and van der Waals forces. Variable-temperature magnetic susceptibility measurements in the temperature range 2–300 K show that 1 is a spin gap system with strong antiferromagnetic interaction.     

Mössbauer Spectra of FePS3-Cobaltocene Intercalation Compound

Abstract

Mössbauer spectra of the FePS₃-cobaltocene intercalation compound were measured in the temperature range of 300K to 10K. The spectra, distinct from those of pure FePS₃, suggest the charge transfer from cobaltocene to Fe-S antibonding orbitals of the FePS₃ host lattice.     

Temperature and Strain Rate Effects on Work-Hardening of KCl Single Crystals

The influence of temperature and strain rate on work hardening of a KCl crystal has been investigated. Using the experimental data on flow stress relaxation, the activation “volume” for dislocation motion and internal counteracting stresses have been calculated in the temperature range of 300 to 40 K. Estimation has been carried out for the variation of a number of mobile dislocations ϱ₂/ϱ₁, when the strain rate is changed by a factor of four.     

Pressure and Temperature Induced Frequency Shifts of Raman-Active Phonons in the MX Solid Pt2Br6(NH3)4

Abstract

We report the dependencies on hydrostatic pressure up to 100 kbar, and on temperature from 10 K to 300 K of the frequencies of Raman-active stretching modes in the single crystal MX chain solid Pt₂Br₆(NH₃)₄ (PtBrn). The data show that a predicted pressure-induced metallization does not occur, and suggests instead that either a 3-D structural distortion or a transition to a broken-symmetry ground state different from the charge-density wave (CDW) takes place.     

Single-crystal structure of Nd5Mo3O16 at T = 30 K

A precision X-ray diffraction study of Nd₅Mo₃O₁₆ single crystals is performed for the first time at 30 K. Measurements in the range from room temperature to 30K showed that the unit-cell parameters and volume change smoothly. The crystal structure at T = 30 K is similar to that at room temperature. The model of splitting of the atomic positions is confirmed.     

Simulation of ion transport in layered cuprate La2SrCu2O6

Oxygen diffusion in layered cuprate La₂SrCu₂O₆ has been simulated by the molecular dynamics method in the temperature range of 300–2500 K. The lattice is found to transform at temperatures above 1550 K; this transformation is accompanied by a change in the pair correlation functions. The abrupt change in the oxygen diffusion coefficient in the range of 1500–1550 K may indicate the presence of a phase transition to the superionic state. The motion of oxygen anions could be traced at the microscopic level. It has been proven for the first time that the La₂SrCu₂O₆ crystal lattice allows, along with displacements of O1 ions within the CuO₂ layer, their migration from the crystallographic positions to the intermediate unoccupied O3 positions. The motion of O2 anions is also fairly complicated: they move not only in their layer over the O2 positions but they also jump to the neighboring layer to occupy the O1 positions. The oxygen diffusion coefficient in layered cuprate La₂SrCu₂O₆ exceeds that in cuprates with perovskite structure and structure of the K₂NiF₄ type (at the same temperatures), which indicates that this material has good prospects for electrodes with mixed ionic-electronic conductivity.     

Transport Properties of (NH4)2 CuCl4.2H2O Low Dimensional Single Crystals

The X‐ray diffraction and Infrared (IR) spectral studies of (NH₄)₂ CuCl₄.2H₂O single crystals reveals that these crystals contains tetragonal crystal structure with the unit cell dimensions of a = 7.58Å, c = 7.95Å, z= 2, β =90° and two water molecules in the unit cell. The temperature dependence of thermally stimulated depolarization current (TSDC) and dc electrical conductivity (σ) studies of this two‐dimensional (NH₄)₂ CuCl₄.2H₂O single crystal have been carried out in 77K–300K temperature region. The TSDC thermograph shows only one sharp peak at 248K with a peak current of 130nA, which is attributed to the Maxwell‐Wagner peak. The activation energy (U), relaxation time (τ) are calculated as 0.78eV and 3.44×10^‐15 s respectively. Dc electrical conductivity studies of these crystals show a first order phase transition at about 248K.     

Effect of Pressure on Antiferromagnetic Transition in Alkali-Electro-Sodalite

Abstract

Pressure (0–19 kbar) and temperature (4–300 K) dependent EPR study of Sodium-Electro-Sodalite (SES) is presented. SES, which consists of a bcc sub-lattice of F-centers supported by a zeolite-like framework, is known to be a Mott insulator at room temperature. On cooling, SES undergoes an AF transition at 48±2 K providing the first example of an s-electron antiferromagnet. We find that the width of the EPR resonance above T _N is influenced not only by a strong exchange interaction, but also by a fast spin-lattice relaxation. Also, with increasing pressure, T _N decreases linearly and extrapolates to 0 K at about 65 kbar. The reason for this seemingly unexpected behavior is briefly discussed.