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.     

Nonlinear Transport Phenomena in MX3

It was confirmed from experiments of the Hall effect that the non-linear conductivity of NbSe₃ results from depinning of CDW's. The non-linear conductivity was also observed in the monoclinic phase of TaS₃. The field dependence of the conductivity was very similar to that observed in the CDW state of NbSe₃.     

The internal friction and elastic modulus of amorphous Pd–Si and Fe–P–C alloys

The internal friction and the shear modulus of the amorphous Pd₈₀–Si₂₀–, Fe₈₀–P₁₃–C₇ and Fe₅₀–Ni₃₃–P₁₂–C₅ alloys have been measured at about 0.5 Hz over a temperature range from room temperature to about 550°C. The internal friction rises steeply with temperature and shows maxima around the crystallization temperature. The activation energy for the steeply rising internal friction has been estimated by the two different procedures; from the shift of the curve by the frequency change and from the slope of the In Q^?1 versus 1/T plot. The two values are very different, which indicates the existence of a wide distribution of activation energies. Crystallization of the amorphous alloys resulted in an increase of about 30% in the shear modulus.     

Anomalous High Pressure Properties in Fullerene Superconductors

Abstract

Correlations among lattice parameter, external pressure, and critical temperature T_c has been surveyed on fullerene superconductors with a wide range of lattice parameters and various valence states. The observed value of dT_c/dP for Na₂Rb_0.5Cs_0.5C₆₀ and Li₃CsC₆₀, having small interfullerene separations, is about a few times lager than that of K₃C₆₀. In contrast, small dT_c/dP values are found in fullerides with expanded unit cells, such as (NH₃)_xNaAA^?C₆₀ (A, A^? = K, Rb and Cs) and A₃Ba₃C₆₀. Interestingly, the chemical and physical pressure effects on T_c are considerably different in these fullerides with large interfullerene spacings. Our results suggest that the pressure dependence is scaled by an interfullerene distance when the interfullerene distances is small, but that this scaling fails when the unit cell is expanded.     

Thermoelastic and Piezoelastic Properties of Hexagonal RbMnCl3

Temperature and pressure derivatives of the elastic constants, indices of refraction, dielectric constants, Faraday effect and thermal expansion of RbMnCl₃ near T_c = 278 K confirm the existence of a second-order phase transition. A strongly anomalous behaviour is observed with the elastic shear stiffness c₄₄ which possesses a positive temperature derivative and a negative pressure derivative. These properties and also the pressure derivative of the transition temperature, dT_c/dp = 6.2 K kbar⁻¹ closely resemble the anomalous behaviour of two other ferroelectric species, betaine borate and betaine hydrogen maleinate.     

Synthesis,crystal growth and characterization of Tl3BX4-compounds (B = V,Nb, Ta; X = S,Se)

Single crystals of Tl₃BX₄-compounds (B = V, Nb, Ta; X = S, Se) were grown by the Bridgman-Stockbarger technique. The compounds were synthesized from the high-purity elements. The melting points were determined by differential thermal analysis. Tl₃VS₄, Tl₃TaS₄ and Tl₃TaSe₄ yielded single crystals up to 10 cm³. The other compounds show single crystalline pieces up to 1 cm³. Infrared photographs and electron microprobe analyses revealed inclusions of TlS in the sulfur compounds, in Tl₃VSe₄ and Tl₃NbSe₄ inclusions of TlSe and in Tl₃TaSe₄ small amounts of Ta.     

Synthesis and tribological properties of copper matrix solid self‐lubricant composites reinforced with NbSe2 nanoparticles

Copper matrix solid self‐lubricant composites were prepared by powder metallurgy route using copper powders and NbSe₂ nanofiber or sheet. The tribological properties of the as‐prepared materials were estimated by a ball‐on‐disc on UMT‐2 multi‐function friction and wear tester under dry conditions in a normal laboratory atmosphere (55‐70% relative humidity, 15‐25 °C). The NbSe₂ nanofiber or sheet was very effective in improving the tribological properties of the composites. It is found that the composites with 15 wt.% NbSe₂ nanofibers showed a very low coefficient of friction of 0.1487. Furthermore, the value levels of electrical resistivity for the composites with different contents of NbSe₂ nanofiber reached to 10^‐6 Ω.cm. When the NbSe₂ content was high enough, a continuous supply of NbSe₂ nanofiber to the tribo‐surface is an important precondition and the benefit of its anti‐friction properties for the copper matrix composites. This behavior is related that little NbSe₂ is reacted with the Cu‐base in the sintering process, although there needs enough NbSe₂ nanofibers as solid lubricants; too much NbSe₂ nanofibers will decrease the intensity of the composites, reducing the wear resistance. So adding the appropriate contents of NbSe₂ nanofiber to the composites play an important role in anti‐friction and wear resistance. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Kinetics of non-isothermal crystallization and glass transition phenomena in Ga10Se87Pb3 and Ga10Se84Pb6 chalcogenide glasses by DSC

The crystallization process affects solid properties through the crystal structure and morphology established during the transition process. An important aspect of the crystallization process is its kinetics, both from the fundamental point of view of amorphous material as well as the modeling and phase transition. In the present research work, non-isothermal crystallization data in the form of heat flow vs. temperature curves has been studied by using some well known models for amorphous Ga₁₀Se₈₇Pb₃ and Ga₁₀Se₈₄Pb₆ chalcogenide glasses, prepared by the melt quenching technique. The glass transition phenomena and crystallization of these glasses have been studied by using non-isothermal differential scanning calorimetery (DSC) measurements at constant heating rates of 5, 10, 15, 20, 25 and 30 K/min. The glass transition temperature (T_g), crystallization temperature (T_c), and melting temperature (T_m) were determined from DSC thermograms. The dependence of T_g and T_c on the heating rate was used to determine different crystallization parameters such as the order parameter (n), the glass transition energy (ΔE_g) and the crystallization activation energy (ΔE_c). The results of crystallization were discussed on the basis of different models such as Kissinger's approach and the modification for non-isothermal crystallization in addition to Johnson, Mehl, Ozawa and Avrami.     

Temperature dependence of the dielectric parameters of nonstoichiometric SrTiO3 single crystals

The results of experimental studies of the temperature dependence of the low-frequency permittivity ε ₀ and the loss tangent taδ of nominally undoped and doped single crystals of strontium titanate at T = 10–300 K are given. The samples were doped with ions of iron-group metals (V, Mn, Fe, Co) and/or ions of rare earth metals (Pr, Nd, Sm, Tm) with concentrations of 10^?3?5 × 10^?1 at %. Anomalous temperature dependences ε₀ T and tanδ(T) were found for a number of samples. The anomalies found were attributed to the violation of stoichiometry of the single crystals under study and the transition of some fraction of Ti⁴⁺ ions to the Ti³⁺ state.     

Synthesis and tribological properties of NbSe3 nanofibers and NbSe2 microsheets

NbSe₃ nanofibers and NbSe₂ sheets were prepared by solid state reaction. The as‐prepared products are characterized by powder X‐ray diffraction (XRD), and scanning electron microscopy (SEM). The results showed that the obtained NbSe₃ nanofibers have a diameter in the range of 100–300 nm and length about 10 μm, while the NbSe₂ sheets have a hexagon structure. The tribological properties of the as‐prepared NbSe_x powders as additives in HVI500 base oil were investigated on UMT‐2 multispecimen tribo‐tester. The wear scars were measured by VEECO WYKO NT1100 non‐contact optical profile testing instrument. It is found that the addition of both NbSe_x nanofibers/sheets improves the tribological properties of base oil. Furthermore, NbSe₂ sheets exhibit better friction reduction and wear resistance properties than NbSe₃ nanofibers in HVI500 base oil. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Anomalous Up-and-Down Variation of Positron Lifetime Near Tc in High-temperature Superconductors

The temperature dependence of the positron lifetimes in the polycrystalline high T_c superconducting Y Ba Cu O and Bi Sr Ca Cu O and the non-superconducting Y Ba Cu O have been investigated by using the position lifetime technique. It is found from the experiments that there is an evident temperature dependence for the two kinds of superconducting samples but no temperature dependence for non-superconducting sample. It is important that the anomalous up-and-down variation of positron lifetime near T_c has been observed in the measured superconducting samples. We suggest that this strange temperature dependence of the positron lifetimes near T_C may result from some changes induced by the phase transition.     

Temperature,frequency, and amplitude dependence of internal friction of metallic glass

The internal friction Q^?1 and Young’s modulus E of Zr–Ti–Cu–Ni–Be bulk metallic glass specimens were measured using a dynamical mechanical spectrometer. Temperature dependence measurements were carried out for differently heat-treated specimens from room temperature up to a temperature above the crystallization temperature using several vibration frequencies (0.2–25 Hz). Two types of Q^?1 peaks appeared: a high-temperature peak in the supercooled region, and a medium-temperature peak in the glassy state region. Both of them were shown to be Arrhenius-type anelastic peaks. From the obtained results, the kinetics of movable atoms in the material were discussed. The amplitude dependence of the internal friction was measured at a given frequency at various temperatures. Fluctuations and rapid changes were observed in Q^?1 when the amplitude was gradually increased. The fluctuations were especially large near the glass transition temperature. The results were considered on the basis of the dynamics of the atoms in glass-forming materials near the glass transition.     

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.     

In‐situ investigation of the temperature dependent structural phase transition in CuInSe2 by synchrotron radiation

The temperature dependent structural phase transition from the tetragonal chalcopyrite like structure to the cubic sphalerite like structure in CuInSe₂ was investigated by in‐situ high temperature synchrotron radiation X‐ray diffraction. The data were collected in 1K steps during heating and cooling cycles (rate 38 K/h). The Rietveld analysis of the diffractograms led us to determine the temperature dependence of the lattice parameters, including the tetragonal deformation, |1‐η|, and distortion |u‐¼| (η=c/2a, a and c are the tetragonal lattice constant; u is the anion x‐coordinate). The thermal expansion coefficients α_a and α_c of the tetragonal lattice constant which are related to the linear thermal expansion coefficient α_L were obtained, as were α_a of the cubic lattice constant, also α_u and α_η. The transition temperature is clearly identified via a strong anomaly in α_L. The temperature dependence of the anion position parameter was found to be rather weak, nearly α_u∼0, whereas α_η increases slightly. However, both increase strongly when approaching to within 10 K of the transition temperature (the critical region) and |1‐η| as well as |u‐¼| go to zero with |T‐T_trans|^0.2 approaching the phase transition. The cation occupancy values, derived from the Rietveld analysis, remain constant below the critical region. Close to the transition temperature, the number of electrons at the Cu site increases with a dercrease in the number of electrons at the In site with increasing temperature, indicating a Cu‐In anti site occupancy, which is assumed to be the driving force of the phase transition. At the transition temperature 67% of Cu⁺ were found to occupy the Me1 site with a corresponding 67% of In³⁺ at the Me2 site. Although full disorder is reached with 50%, this level seems to be high enough that the phase transition takes place. The order parameter of the phase transition, goes with |T‐T_trans|^β to zero with the critical exponent β=0.35(7) which is in good agreement to the critical exponent β=0.332 calculated for order‐disorder transitions according to the Ising model. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Tunneling Spectra in NbSe3./I/Pb junctions

NbSe₃, goes throught two phases transition inducing SDW states for T₁ = 149 K and T₂ = 59 K. These two transitions bring a partial condensation at Fermi level, the low temperature conductivity being always of metallic type. The evaluation of the width of the “gap” cannot be done simply. We employed this word “gap” althought it is incorrect here. Tunneling effect has permitted us to obtain an estimation of this “gap” for P=l bar and P=1 Kbar. We have kept tunneling spectra only for T < T₂. For T₁ > T > T₂, the width of the “gap” and thermal broadening are such that a measure is too imprecise. We have used the lead superconductivity as a test of good tunneling devices.     

Thermoelastische und elastische Eigenschaften des tetragonalen Quecksilberdijodids

All elastic constants c_ij and thermoelastic constants T_ij = d log c_ij/dT (T temperature) of tetragonal HgI₂ have been measured by ultrasonic methods on single crystals which had been grown from the vapour phase by the method of dynamical gradient reversal. HgI₂ exhibits strong anisotropic elastic properties which are qualitatively explained by structural details. The temperature dependence of the elastic constants is smaller than expected. Only the very small shear constant c₆₆ is strongly decreasing with increasing temperature. This behaviour might be seen in connection with the known phase transition at about 126°C.     

Crystal Growth and Elastic Properties of Hopeite,Zn3(PO4) · 4 H2O

Single crystals of orthorhombic Zn₃(PO₄)₃ · 4 H₂O of optical quality with dimensions up to 12 × 10 × 8 mm were grown from aqueous solutions of ZnSO₄ and NaH₂PO₄ by controlled diffusion of NH₃ into the solution. The elastic constants c_ij and the thermoelastic constants T_ij = dc_ij/dT, T temperature, were determined from ultrasonic resonance frequencies of thick plates in the range between 5 and 40 MHz and between 250 and 320 K. In respect to the longitudinal elastic stiffness hopeite behaves quasi isotropically, however, the elastic shear stiffness shows a large anisotropy of about a factor 2.5. Above 260 K the shear stiffness c₆₆ possesses a quite anomalous temperature dependence (T₆₆ > 0). Further, the linear thermal expansion reveals a strong anisotropy (α₁₁ = −3.3, α₂₂ = 3.4, α₃₃ = 33 · 10⁻⁶/K).     

Diffusion in bulk-metallic glass-forming Pd–Cu–Ni–P alloys: From the glass to the equilibrium melt

Since the discovery of bulk-metallic glasses there has been considerable research effort on these systems, in particular with respect to mass transport. Now the undercooled melt between the melting temperature and the caloric glass transition temperature, which has not been accessible before due to the rapid onset of crystallization, can be investigated and theories can be tested. Here we report on radiotracer diffusion measurements in metallic bulk-glass-forming Pd–Cu–Ni–P alloys. Serial sectioning was performed by grinding and ion-beam sputtering. The time, temperature as well as the mass dependence, expressed in terms of the isotope effect E, of Co-diffusion were investigated. In the glassy state as well as in the deeply supercooled state below the critical temperature T_c, where the mode coupling theory predicts a freezing-in of liquid-like motion, the experimentally determined very small isotope effects indicate a highly collective hopping mechanism involving some ten atoms. Below T_c the temperature dependence shows Arrhenius-type behavior with an effective activation enthalpy of 3.2 eV. Above T_c the onset of liquid-like motion is evidenced by a gradual drop of the effective activation energy and by the validity of the Stokes–Einstein equation, which is found to break down below T_c. This strongly supports the mode coupling scenario. The Stokes–Einstein equation is presently tested for other constituents of the alloy. The Co isotope effect measurements, which have never been carried out near T_c in any material, show atomic transport up to the equilibrium melt to be far away from the hydrodynamic regime of uncorrelated binary collisions.     

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.     

Ionic conduction, diffusion and glass transition in 0.2[XNa2O · (1−X)Rb2O] · 0.8B2O3

We have investigated the ionic transport in the 0.2[XNa₂O · (1−X)Rb₂O] · 0.8B₂O₃ mixed-alkali system with X=0.0, 0.2, 0.4, 0.6, 0.8, and 1.0 in the glassy and the undercooled-liquid state by means of impedance spectroscopy and tracer diffusion experiments. The calorimetric glass-transition temperature T_g obtained by differential scanning calorimetry shows a minimum with composition. The composition dependence of the electrical conductivity below T_g exhibits a minimum, as well. These deviations from an ‘ideal’ linear mixing rule are usually denoted as mixed-alkali effect. The dc conductivities times temperature σ_dc×T follow the Arrhenius law in the range below and above T_g, respectively. The glass transition appears as a kink in the Arrhenius presentation of σ_dc×T. Below the glass-transition temperature the onset frequency ν_on of the conductivity dispersion has an Arrhenius-like temperature dependence. According to ‘Summerfield scaling’ the activation enthalpies of σ_dc×T and ν_on are expected to be the same. This is indeed observed but only for the single-alkali compositions. The activation enthalpies of σ_dc×T as a function of composition show a classical mixed-alkali maximum, however the activation enthalpies of the onset frequencies as a function of composition exhibit a nearly constant behavior in contrast to the expectation from Summerfield scaling. The tracer diffusion measurements reveal a major difference in diffusion of ⁸⁶Rb and ²²Na in mixed-alkali glasses. A diffusivity crossover of tracer diffusion coefficients of ²²Na and ⁸⁶Rb occurs near X=0.2. By comparison of tracer and conductivity diffusivities the Haven ratio is deduced which shows a maximum near the conductivity minimum composition.