EPR and pulsed NMR study of (NH4)2ZnBr4:Mn2+

The EPR and pulsed NMR measurements of (NH₄)₂ZnBr₄:Mn²⁺ have been carried out in the temperature range 110–473 K and 83–284 K, respectively. EPR spectra from three different paramagnetic entities have been observed. Intensity changes of certain lines in the EPR spectrum observed at ≈ 430 K indicate the occurrence of a phase transition. Another phase transition at ≈ 235 K is indicated as a change of slope in the T₁ vs 1/T curve.     

Pressure and temperature dependence of the electrical resistivity of bulk Al23 Te77 glass

Electrical resistivity of bulk amorphous Al₂₃T₇₇ samples has been studied as a function of pressure (up to 80 kbar) and temperature (down to 77 K). At atmospheric pressure the temperature dependence of resistivity obeys the relation ? = π₀ exp(δE/RT) with two activation energies. In the temperature range 300 K ? T > 234 K the activation energy is 0.58 eV and for 234 >T ? 185 K the value is δE = 0.30 ev. The activation energy has been measured as a function of pressure. The electrical resistivity decreases exponentially with the increase of pressure and at 70 kbar pressure the electrical behaviour of the sample shows a metallic nature with a positive temperature coefficient. The high pressure phase of the sample is found to be a crystalline hexagonal phase.     

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.     

Preparation and magnetic properties of CoWO4 nanocrystals

Cobalt tungstate (CoWO₄) nanocrystals with an average size of 20–50 nm were synthesized via a template‐ or surfactant‐free hydrothermal route. The crystal structure and morphology of the as‐synthesized CoWO₄ sample were characterized by X‐ray diffraction, scanning electron microscopy and transmission electron microscopy. Magnetic measurements on the as‐synthesized CoWO₄ nanocrystals indicate a Néel temperature (T_N) of ∼40 K. This lower T_N may be a result of the nanostructured particles that reduce the exchange coupling. The new synthetic route presented in this paper has potential applications to fabricate other metal tungstates (MWO₄) materials.     

Effect of the magnetic flux trapping upon the temperature dependence of the flow stress change at the superconducting transition

The effect of the superconducting transition on the flow stress in single and polycrystals of Pb-In and Pb-Sn alloys has been studied in the temperature range 1.8 K to T_e. Unlike earlier efforts, these experiments involved investigation of the magnetic flux trapping. Two types of temperature dependences of the magnetic flux trapping, B₀(T), were revealed. In Pb-1.9 at.% In and Pb-6 at.% Sn alloys (subjected to various thermal treatment), B₀ monotonically rises with lowering temperature. In Pb-1.8, 3 at.% Sn single crystals the flux trapping arises at high strains (> 50%) and low temperatures (<2.5 K). In this case B₀ magnitude depends on the strain rate. The difference in the B₀(T) behaviour is associated with dissimilar flux pinnig by pinning centres. The date on B₀(T) obtained give an insight into the two types of the Δσ(T) dependences; taking into account of the flux trapping allowed the Δσ values to be corrected, so that as a result, a single Δσ_SN(T) curve was obtained for all the alloys and structures studied.     

Magnetic Order in Organic Superconductors

We have undertaken an electron spin resonance study at both low and high magnetic field on the superconducting phase of the ambient pressure superconductor (BEDT-TTF>₂I₃, (ET)₂I₃, over the temperature range 1-300 K and hydrostatic pressure range from 0 to 2 kbar. At ambient pressure the ESR results are consistent with the picture of (ET)₂I₃ as a metal. Superconductivity is observed at 1.6 K via low field ESR. Application of modest pressures strongly suppresses the superconducting transition temperature. At pressures above about 0.3 kbar the superconductivity is suppressed in favor of an as-yet-unidentified magnetic state whose onset is at 7K. Through an analysis of the microwave ESR lineshape we find that the microwave conductivity over the temperature range 5-50 K is in agreement with dc measurements.     

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.     

A new equation for the temperature dependence of density of saturated aqueous solutions of electrolytes

A new empirical three-parameter equation d = d₀ exp[β′(T – T₀)²] (where β′, d₀, and T₀ are constants for a salt) has been proposed to describe the dependence of density d (g/ml³) of saturated aqueous solutions of electrolytes on temperature T (K) in the whole range of temperature in which solubility increases or decreases with an increase in temperature.     

Phase Transition and Phase Diagram of Anion Radical Salts of [(C6H5)3PCH3]+ [(C6H5)3AsCH3]+ ×(TCNO)− 2 (0 ≤×≤ 1)

Abstract

Much attention has been paid to the solid anion radical salts of 7,7,8,8-tetracyanoquinodimethane (TCNQ), because of their prominent electronic properties.^1–4 In particular, the salts containing mixed cations represented by [(C₆H₅)₃PCH₃]⁺ _1–x [(C₆H₅)₃AsCH₃]⁺ _× (TCNQ)^? ₂, (0 ≤×≤ 1), are known to undergo phase transitions at 1 atm pressure in the solid state.^1–4 The phase transition of pure methyltriphenylphosphonium salt, (x = 0.00), takes place at 315.7 K. Heat-capacity measurements of this phase transition have been made by Kosaki et al. ³ The transition has thus been found to be of the first order. The enthalpy and the total entropy change associated with the phase transition were experimentally determined to be 485.18 cal/mol and 1.7206 cal/deg.mol, respectively. For the solid solutions, it was found that the transition temperature (T_c ) is increased, while the magnitude of the heat of transition (δH) is decreased, progressively with an increase in the composition parameter (x) and that pure methyltriphenylarsonium salt, (x = 1.00), has no such phase transition up to the decomposition temperature of about 480 K at 1 atm pressure.^1–3 Figure 1 shows the experimental relation between T _c and x, together with the relation between δH and x.⁴ In the present paper, we attempted to explain thermodynamically the phase diagram of Figure 1 for the solid solutions of those TCNQ anion radical salts.     

Growth,structure, and magnetic properties of CuFeTe<Subscript>2</Subscript> single crystals

CuFeTe₂ single crystals were grown and the temperature dependence of their magnetic susceptibility in the temperature range 1.8–400 K was investigated. It is found that the magnetic susceptibility shows anomalies at temperatures T _s = 65 and T _N = 125 K. At T > 125 K, the crystal is in the paramagnetic state controlled by Fe²⁺ and Cu²⁺ ions with an effective magnetic moment of 1.44 μB.     

Magnetic interactions in frustrated Mn3Fe4(VO4)6

Magnetic interactions in a site-disordered multicomponent vanadate Mn₃Fe₄(VO₄)₆ are studied using DC magnetization and multifrequency Electron Paramagnetic Resonance (EPR). The static magnetic susceptibility χ shows antiferromagnetic interactions between Fe³⁺ and Mn²⁺ spins with a Curie–Weiss temperature Θ = ?165(5) K. EPR measurements indicate a strong dependence of χ on the frequency and temperature. The EPR spectra due to iron and manganese ions are observed in the X-band. It is mostly manganese ions that are observed at 80 GHz while two kinds of magnetic centers are identified at frequencies above 160 GHz. The observed shifts of the resonance lines for Fe³⁺ ions at low frequencies differ from those at high frequencies. The observed features may be due to different magnetic sublattices which modify the magnetic ground state, while competing magnetic interactions may lead to magnetic frustration. It appears that the very high magnetic fields employed in our high-frequency EPR measurements may affect the spin-flop transitions anticipated below Neel temperature T_N.     

Pyroelectric Properties of Potassium and Rubidium Titanyl—Arsenate Single Crystals in the Temperature Range of 4.2–300 K

The temperature dependences of the pyroelectric coefficients of KTiOAsO₄ and RbTiOAsO₄ single crystals grown by flux crystallization have been investigated in the temperature range of 4.2–300 K. With an increase in temperature, superionic conductivity first arises in KTiOAsO4 (at T > 200 K) and then (at T > 270 K) in RbTiOAsO₄. This conductivity is much higher in the samples polarized at T = 4.2 K. An exponential change in the crystal resistivity along the polar direction is simultaneously observed. The results of measurements in the range of 4.2–200 K indicate larger values of pyroelectric coefficients when compared with potassium and rubidium titanyl-phosphate crystals. A correlation between the pyroelectric coefficients and a change in the lattice constants at isomorphic substitutions of K atoms for Rb and P atoms for As has been revealed within the symmetry approach.     

A novel self‐consistent Nývlt‐like equation for metastable zone width determined by the polythermal method

Using a power‐law relation between three‐dimensional nucleation rate J and dimensionless supersaturation ratio S, and the theory of regular solutions to describe the temperature dependence of solubility, a novel Nývlt‐like equation of metastable zone width of solution relating maximum supercooling ΔT_max with cooling rate R is proposed in the form: ln(ΔT_max/T₀) = Φ + β lnR, with intercept Φ = {(1–m)/m }ln(ΔH_s/R_GT_lim) + (1/m)ln(f/KT₀) and slope β = 1/m. Here T₀ is the initial saturation temperature of solution in a cooling experiment, ΔH_s is the heat of dissolution, R_G is the gas constant, T_lim is the temperature of appearance of first nuclei, m is the nucleation order, and K is a new nucleation constant connected with the factor f defined as the number of particles per unit volume. It was found that the value of the term Φ for a system at saturation temperature T₀ is essentially determined by the constant m and the factor f. The value of the factor f for a solute–solvent system at initial saturation temperature T₀ is determined by solute concentration c₀. Analysis of the experiment data for four different solute‐water systems according to the above equation revealed that: (1) the values of Φ and m for a system at a given temperature depend on the method of detection of metstable zone width, and (2) the value of slope β = 1/m for a system is practically a temperature‐independent constant characteristic of the system, but the value of Φ increases with an increase in saturation temperature T₀, following an Arrhenius‐type equation with an activation energy E_sat. The results showed, among others, that solubility of a solute is an important factor that determines the value of the nucleation order m and the activation energy E_sat for diffusion. In general, the lower the solubility of a solute in a given solvent, the higher is the value of m and lower is the value of E_sat. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and electrical properties of epitaxial CuInS2 thin films on GaAs substrates

CuInS₂ thin films with thicknesses in the range of 500 Å were deposited onto semi-insulating (111) A-oriented GaAs substrates by flash evaporation in the substrate temperature range T_sub = 570 … 870 K. Epitaxial growth begins at T_sub = 645 K. The films had always the chalcopyrite structure. Indications to a transition from the chalcopyrite phase to the sphalerite phase were observed at T_sub = 870 K. Films grown at T_sub ≦ 800 K showed n-type conductivity whereas at growth temperatures T_sub ≧ 850 K the films were always p-type conducting. A donor level and an acceptor level with ionization energies of 0.24 eV and 0.22 eV, respectively, were found from an analysis of the electrical measurements.     

Epitaxial layers of CuGaTe2 on GaAs

CuGaTe₂ thin films with thicknesses in the range from 0.1 to 0.2 μm were deposited onto semi-insulating (111) A-oriented GaAs substrates by flash evaporation under controlled growth conditions. Epitaxial growth begins at a substrate temperature of T_sub = 695 K. In the range T_sub = 695 … 820 K the films have the chalcopyrite structure, at T_sub = 820 to 840 K a second phase was observed besides the chalcopyrite phase. At T_sub > 840 K the films become polycrystalline. All epitaxial films were p-type conducting and showed an increase of the hole concentration with rising substrate temperature. The measured mobilities indicate to the presence of a high concentration of neutral impurities or defects.     

Synthesis and Nonlinear Optical Properties of Novel Polyester Containing Nitrophenylazonitroresorcinoxy Group

A new X-type polyester (4) containing nitrophenylazonitroresorcinoxy groups as NLO chromophores, which are components of the polymer backbone was prepared. Polyester 4 is soluble in common organic solvents such as N,N-dimethylformamide and dimethylsulfoxide. It shows a thermal stability up to 280°C from thermogravimetric analysis with glass-transition temperature (T_g) obtained from differential scanning calorimetry near 120°C. The second harmonic generation (SHG) coefficient (d₃₃) of poled polymer films at the 1064 nm fundamental wavelength is around 5.08 × 10^?9 esu. The dipole alignment exhibits a thermal stability even at 5°C higher than T_g, and no SHG decay was observed below 125°C due to the partial main-chain character of polymer structure, which is acceptable for NLO device applications.     

Structural phase transition in crystals of the molecular conductor α′-(BDH-TTP)<Subscript>6</Subscript>[Hg(SCN)<Subscript>3</Subscript>][Hg(SCN)<Subscript>4</Subscript>]

Single crystals of the organic conductor α′-(BDH-TTP)₆[Hg(SCN)₃][Hg(SCN)₄] are studied by low-temperature X-ray diffraction at 100–300 K. It is shown that, in the temperature range 250–200 K, the compound undergoes a phase transition, which is accompanied by the appearance of superstructure reflections and a rapid increase in their intensity due to the partial anion and cation ordering. The superstructure is observed down to a temperature of 100 K, at which the set of diffraction data is collected. The temperature dependence of the electrical resistance exhibits a minimum in the range of the structural transition temperature. Under a pressure of up to 10 kbar, the general behavior of the dependence remains unchanged and only the minimum is shifted toward the low-temperature range. The phase transition is completely reversible.     

EPR study of crystalline and glassy ethanol

Pulsed X-band electron paramagnetic resonance (EPR) spectroscopy was applied in studying molecular dynamics in two different solid ethanol matrices. Nitroxyl radicals as paramagnetic reporter groups were embedded in crystalline and glassy ethanol and the phase memory time, T_m, was investigated at 5–80 K. Temperature variation revealed a maximum in 1/T_m centered around 50 K and a small linear decrease with temperature, below ca. 25 K. Faster phase memory time relaxation in crystalline ethanol than in ethanol glass was observed throughout the temperature range studied. This can be attributed to differences in spectral diffusion due to distinct molecular packing densities.     

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.     

Thermodynamics of the Al–Ga–N2 system

The thermodynamic properties of the Al–Ga–N₂ system under high N₂ pressure up to 10 kbar and 1800 °C are investigated. On the basis of the experimental p–T growth conditions for (Al,Ga)N crystals, the standard Gibbs free energy as well as the standard enthalpy and entropy of formation of the Al_xGa_1−xN crystals as a function of composition x were calculated. The a_N2–T and x–T phase diagrams for (Al,Ga)N are presented.