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Using a highly sensitive torsional vibration technique, the authors have determined the critical temperatures of the superconducting transition (Tc) of individual phases of multiphase cuprates [Bi1.7Pb0.3Sr2Ca(n-1)CunOy (n = 2–30)] in a constant external magnetic field (H) in the temperature range of 77 to 270 K. It has been found that oscillation damping peaks are more pronounced when a sample is rapidly cooled in an external magnetic field (FC) to a temperature of 77 K and then is slowly warmed to room temperature. An increase in the time of sample exposure to a magnetic field at 77 K leads to a rise in the temperature Tc of different phases, and to an increase in the intensity of a signal corresponding to high-temperature phases with Tc > 240 K. Structural studies have shown that with increasing sample exposure time at 77 K, as a result of sample compression an increase in stress concentration at grain boundaries appears to increase defect density and to enhance the pinning of Abrikosov vortices, i.e. creates new (additional) conditions for their “freezing” during the field cooling (FC) procedure. It has been established that in samples of the Bi1.7Pb0.3Sr2Ca(n-1)CunOy (n = 2–30) series, the critical temperature Tc increases from 107 K to ≥ 240 K with an increase in n.

In this study, the potential possibilities of the precision of the vibrating reed method for the evaluation of the . of superconducting precursors in an HTSC Bi-Pb-Sr-Cu-O system were investigated. A special technology for obtaining these samples by using solar energy for melting and following superfast melt quenching was applied to increase their internal inhomogeneity, allowing to receive high-quality textural ceramic samples of Bi1,7Pb0,3Sr2Can-1CunOy (n=2-30) showing critical temperatures of superconducting precursor transitions above bulk Tc. To determine critical temperatures of superconducting precursors above bulk Tc for the first time, the original vibrating reed method of studying these multiphase samples in an applied magnetic field was used. It was shown that this method has sensitivity to superconducting diamagnetism making possible to reveal new superconducting precursor phases above bulk Tc.

The Gibbs free enthalpy, chemical activity across the transition temperature to superconductivity, Tc in YBa2Cu3O7 − δ is obtained from reciprocally enhanced X-Ray absorbance, XAS and diffraction, XRD data near the Ba L3,2 edges' energy Ea, and orientations in the X-ray beam for preferred Miller indexed [HKL] planes' scattering, that are enhanced near Tc. The standard enthalpy and entropy for the formation of mixed normal metal/superconducting domains above Tc, determined individually across the two Ba L3,2 edges, to better than a percent accuracy: ΔH≠≥ = − 220 meV, and ΔS≠≥ = − 2 meV/K when 121 ≥ T ≥ 92 K ≈ Tc1, indicate that energy is gained, with an increased order by forming a mixture with reduced entropy. Below Tc the standard enthalpy and entropy to form at least two mixed superconducting phases are halved to ΔH≠≤ = − 86 meV, ΔS≠≤ = − 1 meV/K when 92 K ≥ T ≥ Tc2 ≈ 72 K. Thus reciprocal XAS/XRD enhancement at orientations in the X-ray field, of preferred 2D planes, induced by the transition to superconductivity in the layer solid, indicates one, the importance of the 2D-plane electron density scattering near Tc, and two, identifies the 2D-plane chemical activity, by the electron density leading to symmetry allowed excitations, a role similar to that of the electron density in linear bonds for molecular activity.

The work is related with the finding out of magnetic phases in strongly anisotropic high-temperature superconductor Bi1,7Pb0,3Sr2Ca2Cu3O10-d in the temperature region where the superconductor is in the normal state. It was studied the temperature dependence of the paramagnetic probe EPR line width. In the normal state at T>Tc near 175 K it was revealed a pick in the temperature dependence of line width. In this region it was observed the time increase of the line width with the characteristic time ~ 17 min. This shows the possibility of magnetic phase formation in this material., 4 pages, 0 figures

The investigation of temperature dependence of damping and period of vibrations of HTSC superconductive cylinder of Bi-Pb-Sr-Ca-Cu-O system suspended by a thin elastic thread and performing axial-torsional vibrations in a magnetic field at temperatures above the critical one for the main phase Tc=107 K were carried out. It was observed some "chaos" temperature region in the temperature interval 107-138 K, where it is seen separate ripples of dissipation and oscillation frequency. It is assumed that the "chaos" region could point to a possibility of existence of other magnetic and more high-temperature phases as single islands in a normal materials matrix., Comment: 10 pages, 0 figures

A simple mechanical method for the investigation of Abrikosov vortex lattice stimulated dynamics in superconductors has been used. By this method we studied the action of pulsed magnetic fields on the vortex lattice and established the resulting change of the course of relaxation processes in the vortex matter in high-temperature superconductors. This method can be used for investigation of phase transitions in vortex matter both high-temperature and exotic superconductors., Comment: 5 pages, 6 figures

The onset of electro-magnetic optic effects, observed at the Ba L2,3 edges synchrotron X-ray absorption by a YBa2Cu3O7 single crystal, 20 K above the transition temperature to superconductivity, Tc ∼ 92 K is used to identify the role played by the Ba donor layer in the transition to superconductivity in the CuO2 layers. Negative permeability leads to Faraday rotation of the transmitted beam below T = 112 to 56 K for the 22 μm thick single crystal (c-axis orientation of 8π/18 relative to eX-rays) and sharp changes in the density of empty final states lead to zero transmitted radiation in an interval ΔE at the given orientation. The temperature dependence: ΔE(L2) = 1.4, 3.5 and 3.9 eV, while ΔE(L3) = 5.3, 6 and 7 eV at T = 92, 74 and 63 K, respectively, indicates that the width of the empty final states bands increases as T decreases. ΔE(L3)/ΔE(L2) = 3.8 at 92 K to 1.8 at 63 K also indicates that the d5/2 symmetry bands fill faster than those of d3/2 symmetry below Tc, providing the first experimental evidence of unpaired spin-orbit states in the Ba donor layer of a superconductor. These effects, characteristic of ferromagnetic and anti-ferromagnetic materials near a resonance absorption, signal the onset of a Mott transition. The interaction between the layer states is described using 1D conjugate molecular orbitals.

The techniques described in this work illustrate the type of changes which may be observed, by precise X-ray absorption spectroscopy, XAS measurements of absorbance A vs. temperature T , across a phase transition in the solid state: (i) the changes in the final states relaxation time τ, which are due to the onset of fluctuations near a phase transition at T c ; (ii) the XAS enhancement which is due to the anomalous Bragg diffraction at an absorption edge, coupled to phonon modes in the temperature interval where these are active; (iii) new allowed transitions to finite states near T c which are due to symmetry changes; (iv) changes in the bond order detected by the measurement of X-ray temperature difference absorption spectra near a phase transition: XTDAS To =XAS( T )-XAS( T 0 ) which identifies the bonds responsible for the phase transition by the progressive changes in the pair correlation term affecting the Debye–Waller factor vs. T , when no other structural changes occur, except in the vibrational modes of a specific bond. The methods have been applied to identify the phenomena that occur near the superconducting transition temperature in layer cuprates and near the metal to insulator transition in NiS 2− x Se x .

Measurements of the relaxation time, τ of electron systems to a disturbance, by two different spectroscopic methods are examined in detail, with the purpose to establish how the presence of fluctuations near a solid state phase transition are made evident in insulators, conductors and superconductors. The absolute temperature and the relaxation time determine the thermodynamic stability of the electronic system near a phase transition by the Uncertainty Principle. At a given temperature T, Landau and Lifshitz obtain the stability from the lower limit of the uncertainty in entropy in units of the Boltzmann constant, ΔS/kB >3.82 K ps. Magnetic resonance can measure τ>>10−10 s, when v=9 GHz. X-ray spectroscopy can measure τ 5 keV. The results extract information about phenomena that occur at the phase transition by following the evolution of spectral features versus T and crystal orientation. Electron spin resonance identifies the phase transition by the evolution of doublet, triplet and antiferromagnetic resonance, and energy loss. Analysis of the X-ray absorption near an element edge determines one, the relative valence: V(Cu in chains)−V(Cu in planes) ≈1 in YBa2Cu3O7−δ, two, the appearance of allowed Cu K pre-edge quadrupole transitions at Tc, three, the enhancement of Ba L3,2 edge transitions by an order of magnitude, just above Tc, at a crystal orientation of the c-axis to the X-ray polarization vector of 8 π/18, and four, difference X-ray absorption spectra, relative to the transition temperature, identify the bonds as well as the atoms involved in the transition. The figure abstract shows the changes in electron density obtained by temperature difference X-ray absorption near the Y K-edge in YBa2Cu3O7−δ below Tc.

Phenomenological equations of state (EOS) for fluids near their critical point have been obtained using literature compression factor data, Z c = P c V c /( nRT c )=0.40 to 0.10 in ( P c , V c , and T c are the pressure, volume per n mole, and the absolute temperature of the fluid at the critical point). The objective is to explain the deviations from the van der Waals value, Z c (vdW)=3/8 (−70% for molten Se and alkali metals up to 6% for molten Pb, Hg, and In) by including in the commonly used phenomenological thermodynamic relations a term which explicitly describes the Heisenberg spin-exchange interactions, in order to understand electron-transfer reactions in solvents near their critical point. Literature data near the critical point indicate that the 199,201 Hg( Z c ≅0.4) Knight shift pluments to zero while the alkali metals and Se ( Z c =0.2 to 0.1) are paramagnetic fluids, and that the enhanced rates for free radical electron-exchange reactions in CO 2 , n -C 2 H 6 , and CHF 3 with intermediate values of Z c , are correlated to Z c . The difference in the solvent behavior for electron-exchange reactions near its critical point is ascribed to spin-exchange interactions. The analysis shows that the solvated electron osmotic pressure in metal–ammonia solutions goes through a maximum where enhanced rates of electron exchange also attain a maximum versus the solvent density ρ r = V c / V ≅0.5. The results can be applied to choose the best solvents, near their critical point, for the syntheses of new materials and heavy metal oxide extraction.

Dynamic electron spin resonance, ESR measurements suggest that this paramagnetic, layered organic metal behaves similarly to cuprate superconductors. There is a dynamic equilibrium between doublet, D and triplet, T* states near the transition temperature to superconductivity Tc= 10±3 K; antiferromagnetic, AF resonance is observed from room temperature to ≈ 150 K, in this temperature region the temperature dependence of the D Lorentzian ESR line widths obtains a first order rate constant for the disappearance of the state (k1 = 3.5 ×107 e−111/T/s) and T* ESR is not observed, it is detected only below 44 K when the lifetime of AF domains, τAF decreases below 10-10s; the onset of superconductivity is ascertained by the appearance of an energy loss at exactly H=0 near 10 K and by the magnetization oscillations observed versus H below Tc when the samples are cooled in a non-zero field H; the fourfold increase in the relaxation time τ1 for the half field T* ESR absorption near 10 ±3 K is interpreted us...

Dynamic electron spin resonance (ESR) measurements compare the paramagnetic and antiferromagnetic (AF) properties of superconducting oxides in the range 4 K to room temperature, at 8 MHz and 9.36 GHz. Two are derivatives of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$: I: Nd(${\mathrm{Nd}}_{0.05}$${\mathrm{Ba}}_{0.95}$${)}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$, ${\mathit{T}}_{\mathit{c}0}$=72 K and II: ${\mathrm{Y}}_{0.2}$${\mathrm{Ca}}_{0.8}$${\mathrm{Sr}}_{2}$[${\mathrm{Cu}}_{2}$(${\mathrm{Tl}}_{0.5}$${\mathrm{Pb}}_{0.5}$)]${\mathrm{O}}_{7}$, ${\mathit{T}}_{\mathit{c}0}$=108 K and two are cases where AF ordering dominates the weak superconductivity: III: ${\mathrm{NbO}}_{1.1}$, 1.25\ensuremath{\le}${\mathit{T}}_{\mathit{c}0}$\ensuremath{\le}10 K and IV: ${\mathrm{La}}_{2}$${\mathrm{NiO}}_{4.00}$, 70 K \ensuremath{\ge}${\mathit{T}}_{\mathit{c}0}$\ensuremath{\ge}40 K. At temperatures 298\ensuremath{\ge}T\ensuremath{\ge}64 K, the ESR absorption by I indicates orthorhombic symmetry. The peaks at ${\mathit{g}}_{\mathit{c}}$=2.06, ${\mathit{g}}_{\mathit{b}}$=2.13, and ${\mathit{g}}_{\mathit{a}}$=2.24 are identified with the presence of 5% ${\mathrm{Nd}}^{3+}$${(}^{4}$${\mathit{I}}_{9/2}$) in the Ba layer because the characteristic ${\mathrm{Cu}}^{2+}$ impurity hyperfine structure is absent and the ESR signal disappears several degrees below ${\mathit{T}}_{\mathit{c}}$. Near ${\mathit{T}}_{\mathit{c}}$ the ESR absorption is reduced by two orders of magnitude.Proximity effects give rise to interference fringes with period ${\mathrm{\ensuremath{\tau}}}_{\mathit{f}}$(T) independent of the field B and the rate of sweep ${\mathit{dB}}_{\mathit{z}}$/dt. ESR is observed below ${\mathit{T}}_{\mathit{c}}$ because flux penetrates the superconductor. The temperature dependence of ${\mathrm{\ensuremath{\tau}}}_{\mathit{f}}$ leads to an activation energy for the flux motion ${\mathit{E}}_{\mathit{a}}$(I)/R\ensuremath{\le}16 K and ${\mathit{E}}_{\mathit{a}}$ (III)/R\ensuremath{\simeq}3 K \ensuremath{\simeq}${\mathit{T}}_{\mathit{c}}$/4. In the superconducting state a coherent flux expulsion response to a change in ${\mathit{B}}_{\mathit{z}}$ from 500 mT to zero is observed in times ${\mathrm{\ensuremath{\tau}}}_{\mathit{r}}$=8 to 10 s. The inverse rate of noise spikes due to flux expulsion, when the samples are cooled through ${\mathit{T}}_{\mathit{c}}$ in a magnetic field, varies from ${\mathrm{\ensuremath{\tau}}}_{\mathrm{noise}}$=3.5 s for III to 21 s for IV. The microwave absorption spectra identify three temperature regimes: (i) For 3.5 K T${\mathit{T}}_{\mathit{m}}$\ensuremath{\simeq}${\mathit{T}}^{\mathrm{*}}$${\mathit{T}}_{\mathit{c}}$, superconducting behavior was confirmed by the energy loss near zero magnetic field and the kinetics of high-field noise due to flux expulsion. Near g=2.00 ESR absorption is observed for all materials. A broad absorption near 50 to 100 mT at 9.36 GHz has been attributed to AF resonance. (ii) ${\mathit{T}}_{\mathit{m}}$\ensuremath{\simeq}${\mathit{T}}^{\mathrm{*}}$\ensuremath{\le}T\ensuremath{\le}${\mathit{T}}_{\mathit{c}}$ identifies the range where flux motion gives rise to interference fringes in the ESR absorption. (iii) ESR and AF resonance are observed immediately after warming above ${\mathit{T}}_{\mathit{c}}$.

ESR and energy loss measurements at 9 GHz have been carried out on granular samples of La2NiO4.00 in the range from 3.8 K to room temperature. The observed phenomena are dependent on the temperature and O2 sample treatment. For exactly 4.00 oxygen stoichiometry, the ESR spectra indicate that the material undergoes a series of phase transitions which are of some importance for understanding the antiferromagnetic and/or superconducting behavior observed in La2NiO4. A low temperature regime (3.8 < T < 70 K) is characterized by a zero field microwave absorption signal and by the noise generated in fields above 0.3 T; this is conventionally interpreted as a very sensitive signature for the presence of superconducting domains. There also is a broad, temperature-independent ESR absorption arising from antiferromagnetic domains. Phase transitions are identified by changes in the ESR spectra above 70 and 160 K. Above 160 K (162-298 K), the ESR spectra remain unchanged, as would be expected for the ESR absorption from antiferromagnetic domains (the Neel temperature for La2NiO4 has been reported as high as 650 K). The transition near 160 K is interpreted as being due to the onset of motional narrowing of the spin-spin interactions in the crystal.

An SCF analysis has been carried for ensembles that simulate the CuO2 conduction layer in the tetragonal layer crystal T′—Nd2CuO4 (Fig. 1). In this work, the CuO2 layer is described by a planar macromolecule, (CuO2)n, subject to the crystal field produced by the point charges in the ionic layers of D4h symmetry The computations were carried out using the KGNMOL, HONDO, and KGNGRAF codes in MOTECC -91. The computations were carried out with different oxygen and copper basis sets and energy convergence to less than 10−8 Hartrees. The purpose of the SCF computation was to estimate the cohesive energy of the ensembles, the electron density for the individual molecular orbitals, and the excess correlation energy, to ascertain the nature of the CuO bond in the conduction layer. The results indicate the following: (1) The cohesive energy of the ensembles (measured by the SCF energy plus the correlation energy, above the atomic values): ΔUc ≡ ΔESCF + ΔEC = −4.35 to −4.17 Hartrees per CuO bond as n increases from 4 to 9. Further insight was obtained by considering the electrostatic energy contributions to ΔUc; Eelectrostatic (ensemble) → EModeling (infinite lattice) were evaluated by replacing the oxygen and copper atoms by point charges determined by a Mullion population analysis. The larger oxygen basis set (13, 8/5,3) gave consistent results for the different ensembles of ΔEcovalent ≡ ΔUc − Eelectrostatic = −1.1 Hartrees per CuO bond. (ii) The electron density indicates that covalent bonds are formed and that the oxygen atoms play an important role in the structure stability. The covalent bonds formed indicate that nominal ionic valences do not apply. Mulliken population analyses gave valences of the order of one at the copper and oxygen atoms. The CuO bond orders are 0.47 between neighboring atoms and 0.048 for those separated by two atoms. (iv) The covalent nature of the CuO bonds in (CuO2)n was compared to that for the H2 molecule using as a measure the electron density and the excess correlation energy. The excess correlation energy per CuO bond above the atomic values is one order of magnitude lower that that for the H2 molecule and that for the CC bond in alternant hydrocarbons. © 1993 John Wiley & Sons, Inc.

The techniques described in this work illustrate the type of changes which may be observed, by precise X-ray absorption spectroscopy, XAS measurements of absorbance A vs. temperature T , across a phase transition in the solid state: (i) the changes in the final states relaxation time τ, which are due to the onset of fluctuations near a phase transition at T c ; (ii) the XAS enhancement which is due to the anomalous Bragg diffraction at an absorption edge, coupled to phonon modes in the temperature interval where these are active; (iii) new allowed transitions to finite states near T c which are due to symmetry changes; (iv) changes in the bond order detected by the measurement of X-ray temperature difference absorption spectra near a phase transition: XTDAS To =XAS( T )-XAS( T 0 ) which identifies the bonds responsible for the phase transition by the progressive changes in the pair correlation term affecting the Debye–Waller factor vs. T , when no other structural changes occur, except in the vibrational modes of a specific bond. The methods have been applied to identify the phenomena that occur near the superconducting transition temperature in layer cuprates and near the metal to insulator transition in NiS 2− x Se x .

The recent structural and electrochemical measurements of the alkali metal-ammonia systems are reconciled in a model where the correlation energy leads to the formation of bound states. This is evident in the X-ray absorption of Rbx(NH3)1−x near the Rb K-edge as a white peak which appears only when the metal mole fraction is reduced below xc = 0.04. These transitions must be associated with the correlation effects which lead to the formation of sodium anions predicted by electrochemical measurements. If the anions act as impurity centers with a minus two valence change from the uniformly charged positive background, Friedel oscillations in the electron density must be taken into account in a statistical description of the system. The deviations of the activity coefficients from the usual Debye-Huckel expression are discussed.

New experiments that characterize high Tc superconducting materials from the classical electrochemistry to the bulk quantized states are described.

It is used the mechanical method of Abrikosov vortex stimulated dynamics investigation in superconductors. With its help it was studied relaxation phenomena in vortex matter of high-temperature superconductors. It established that pulsed magnetic fields change the course of relaxation processes taking place in vortex matter. The study of the influence of magnetic pulses differing by their durations and amplitudes on vortex system of isotropic high-temperature superconductors system HoBa2Cu3O7-d showed the presence of threshold phenomena. The small duration pulses does not change the course of relaxation processes taking place in vortex matter. When the duration of pulses exceeds some critical value (threshold), then their influence change the course of relaxation process which is revealed by stepwise change of relaxing mechanical moment . These investigations showed that the time for formatting of Abrikosov vortex lattice in HoBa2Cu3O7-d is of the order of 20 microsec. which on the order of value exceeds the time necessary for formation of a single vortex observed in type II superconductors., 6 pages, 0 figures

We present a comparative study in terms of structural properties deduced from X - ray diffraction diagrams between YBa2Cu3O7 (YBCO) thin films fabricated on SrTiO3 (STO) single crystal substrates and bicrystallines substrates with a symmetrical tilt angle of 24 degrees. Periodic Lattice Distortions (PLD) have been observed around different Bragg peaks in YBCO thin films deposited on STO bicrystals while only diffraction peaks have been measured in the diagrams corresponding to the YBCO thin films deposited on STO single crystal substrates. Only in regions situated a 3.5 mm at both sides of the grain boundary the PDL have been investigated. Scans along different (h, k, l) directions allow us to conclude that the qPLD vector associated to the distortion is along the (h,-k,0) direction. However, we found that the amplitude of the components of the qPLD vector depend on the Bragg peak chosen. We believe such a result indicates that we have not a PDL with a simple qPLD vector. In this line, other (h, k, l) directions must be investigated to deduce the exact origin of the qPLD vector. Nevertheless, independently on the qPLD vector associated to the PLD, we believe that such a distortion in the lattice is a consequence of the stress field induced by the grain boundary in the YBCO thin film deposited on the bricrystal.

Dynamic esr measurements compare I :Nd(Nd .05 Ba .95 ) 2 Cu 3 O 7 ,T c0 =72K; II :(Y .2 Ca .8 )Sr 2 [Cu 2 Tl 1 2 P b 1 2 ]O 7 , T c 0 = 108 K ; III:NbO 1.1 , T c- 2 NiO 4.00 , T c- I by the 5% dilute Nd 3+ , 4 I 9 2 in the Ba layer, following the Curie-Weiss law above T C0 , g a,b,c =2.06, 2.13, 2.24; below T c proximity effects give rise to temperature dependent interference fringes with period τ f (T)=1 to 10s, giving an activation energy for the flux motion E a ( I )/ R =16 K and E a (III)/R=3K⋍T c /4. In the superconducting state a coherent flux expulsion response to a change in B Z from 1 2 tesla to 0 is observed in times τ r =8 to 10 s; the inverse rate flux expulsion, when the samples are cooled through T c in B>0, varies from τ n =1 to 20s. Spectra identify 3 temperature regimes: i) T c /4 obtains superconducting behavior in the energy loss near zero magnetic field and kinetics of flux expulsion, near g=2 esr absorption is observed for all materials; ii) T c >T>T c /4 obtains a broad absorption near 0.1T at 9.36 GHz (attributed to antiferromagnetic AF resonance), flux motion produces interference fringes in spectra; iii) T>T c obtains AF resonance on warming.

Dynamic electron spin resonance (ESR) and extended x-ray absorption edge fine structure (XAFS) measurements suggest that layered organic metals and cuprate superconductors behave similarly. The response to microwave radiation in a modulated external magnetic field indicates that: (i) triplet state, T * ESR is observed below Tc for both; (ii) the condensation of free spin doublet D to T* occurs above the transition temperature to superconductivity Tc (10 ± 1 K for the organic metal (BEDT-TTF)3Ta2F11 and 92 to 12 K for YBa2Cu3O7-δ and its rare earth derivatives); (iii) antiferomagnetic (AF) resonance is detected above Tc for the organic metal. Here the exchange field between the aligned AF domains: JAF(150 K) = 130.7 mT (153 mK) is greater than the exchange term J(150 K) ≈ 15 mT (20 mK) between free spins (S = 1/2) leading to T* states; the lifetime of AF domains τAF decreases below 150 K and resonance is not detected below 44 K (i.e. τAF

The layered compounds 2H‐NbSe2, 4Hb‐TaS2, and 1T‐TaS2 have been intercalated with organic molecules; and the resulting crystal structure, heat capacity, conductivity, and superconductivity have been studied. The coordination in the disulfide layers was found to be unchanged in the product phase. Resistance minima appear and the superconducting transition temperature is reduced in the NbSe2 complex. Conversely, superconductivity is induced in the 4Hb‐TaS2 complex. Corresponding evidence of a large change of the density of states, negative for 2H‐NbSe2 and positive for 4Hb‐TaS2, was also observed upon intercalation. The transport properties of all the intercalation complexes show a pronounced dependence upon the coordination of the transition metal.

The van der Waals equation of state for a conduction-electron gas at concentrations near the Mott metal-to-insulator transition is examined. The Mott concentration is found to be universal for conduction-electron gas condensation.

The reactions of the lamellar metal TaS, with adduct gases have been investigated by measuring the variations in the X-ray diffraction spectra of the reactant and of the products versus time and experimental conditions. For NH3 and N2H4 adducts the results suggest that : (1) the intercalation starts at the exposed layers and proceeds sequentially into the bulk at a rate much faster than that measured for the increase of mole ratio of adduct to disulfide; (2) below 100°C more than on stage I intermediate intercalated phases are present before the end of reaction; and (3) the product phase reacts with pure O2 (25°C, 1 atm) to regenerate the original 2H phase.

We have measured XANES (X-ray Absorption Near Edge Spectra) near the Re Li (i=l,II,III) edges in (TMTTF)2Re04 and (TMTSeF) 2Re04 and the Se K edge in the latter com­ pound. An important dependence of the XANES at the Se K edge on the polarisation of the incident beam with respect to the crystal axes was observed, whose interpretation can give information on the symmetry of the unoccupied conduction band states. The positions of the Re L edges were compared with those in several inorganic compounds containing Re in various oxidation states, and were found to be, within experimental error, the same as those in KRe04, suggesting the molecular environment in the organic materials affects little the chemical state of the perrhenate anion.

Electron diffraction has been used to study the formation of a variety of complex superlattices in the IT, 2H and 4Hb polytypes of TaS2 during intercalation with hydrazine gas (N2H4), in situ in the electron microscope. Results are presented over the temperature range 15 K-360 K and are interpreted in terms of scattering from periodic lattice distortions linked to charge density waves. Comparisons are also made with similar effects in both the as-grown crystals and TaS2 samples containing other adducts.

Proton nuclear magnetic resonance was used to probe the environment of the intercalant molecule in low dimensional (aeolotropic) solids 3RI-TaS2(NH3)2/3 and 3RII-TaS2(N2H4)4/3, and, specifically to look for phase transitions associated with changes in superlattice geometry. The proton resonance frequency for the intercalated molecules in the temperature interval 200 to 300K indicates that there are different magnetic environments in three temperature domains. (i) There is only one resonance field for molecules in isotropic magnetic environments. (ii) The high- and low-temperature domains (I and III) have molecules in only two different environments, whereas in the middle temperature domain (II) there are at least four different magnetic environments. (iii) The different magnetic environments measured by the ratio of anisotropic to isotropic resonance absorption intensity (rai) indicate that the ratios rai are constants of the sample in domains I and II. (iv) At low temperatures (domain III), rai depends on magnetic field polarisation effects and the temperature treatment of the sample. The authors conclude that the different magnetic environments observed for ammonia and hydrazine appear to be determined by the host, octahedrally coordinate TaS2.

We have identified an optically enhanced magnetic phase transition in the newly synthesized organic molecular charge transfer salt, (BEDT–TTF)3Ta2F11 (BEDT–TTF 3/4 bis‐ethylenedithiolotetrathiafulvalene) by ESR absorption measurements in the X band microwave region. At room temperature, only a doublet state ESR absorption is observed, but below 30 K several triplet ESR absorptions appear. The orientation dependence of the ESR absorption under illumination at energies near the band gaps in the material (640 nm, T=12 to 5 K, H0

The preparation of HxTaS2 (0 < x < 0.87) is described. The compounds are only marginally stable at room temperature, slowly evolving H2S and H2 (and possibly H2O in air). Magnetic susceptibility data show that a low temperature transformation in 2H⋅⋅⋅TaS2 (at 80°K) is suppressed with the addition of hydrogen, and at the same time the superconducting transition temperature Tc rises from 0.8 to ∼4.2°K at x = 0.11. Heat capacity measurements near this concentration show the superconductivity to be a bulk effect. Finally, by correlation of this data with susceptibility and Tc measurements in other intercalation compounds, we suggest that the rise of Tc (at low electron transfer) is due to suppression of the low temperature transformation and not due to an excitonic mechanism of superconductivity.

The concept of bond order, based on an idea of Pauling's, was introduced into molecular orbital calculations by Coulson and by Penney. Layered solids can be described as stacks of macromolecules held together by van der Waals forces, and the concept of bond order is useful for the study of the reactions between these solids and adduct gases adsorbed on the surface layers at sites of inhomogeneous mobile electron density such as those produced by periodic lattice distortions and charge-density waves. In this paper the mobile electron bond order for intercalated layered materials is evaluated with the purpose of describing the structural changes produced by the intercalation reaction. The mobile electron bond order in intercalated 1T-TaS2 was obtained using the experimental data obtained in optical transmission and electron diffraction studies.

The authors report measurements of the X-ray absorption spectra near the Ta L edges of different polytypes of TaS2 and TaSe2 and TaSe3. Analysis of structure seen near the absorption edges and associated with transitions from well defined core levels to conduction band states has enabled the extent of the p character of the predominantly d character conduction band to be probed. The results show that conduction band in the Ta chalcogenides studies has a significant amount of p character, and a lower limit of between 6 and 10% has been determined. The amount of p character varies between the various compounds in the following way: 1T-TaS2

The properties of intercalated transition element dichalcogenides are examined using the Mulliken concept of charge transfer complexes. Different behaviour is found for compounds of Groups IVB, VB and VIB of the Periodic Table. The observed shifts are bathochromic for the IVB and VB but hypsochromic for 2H-MoS2 intercalated compounds. Only one or two optical transitions are affected and these depend on the intercalated molecule. Some evidence is given for the presence of charge transfer bands in intercalated 2H-MoS2 and 1T-HfS2. The importance of the symmetry and ionization potentials of the energy levels in the interacting compounds is discussed.

The X-ray absorption spectra (XAS) of Rb metal, Rbx(NH3)1-x, 2H-NbSe2Rb0.28 and RbBr near the Rb K-edge have been used to ascertain that the oxidation state V of rubidium dissolved in ammonia and intercalated in the layer compound is in the range 0

We have intercalated TaSe3 with N2H4. The room temperature resistance Rb(RT) along the b-axis decreases by 30% during the reaction. The ratio of R b (RT) R b (4.2 K) varies from 60 in TaSe3 to 5 in the intercalated samples. An anomaly in Rb is observed near 100 K. It is suggested that this anomaly is due to the formation of charge-density-wave (CDW). The superconducting transition temperature, Tc, is suppressed from 2.2 to 1.5 K. The correlation between the suppression of superconductivity and the formation of CDW is discussed as compared with the result obtained in the layered dichalcogenides.

Kinetic transmittance measurements at a fixed wavelength of 1600 nm have been made during the intercalation of thin single crystals of 2H-NbSe2 with gaseous hydrazine. Measurements have been made as a function of pressure at room temperature. The results indicate that adsorption of the adduct on the exposed crystalline surface is the first step of the reaction. A delay time measured before the changes in transmittance occur has been associated with the weakening of the van der Waals forces that hold together the planar macromolecules in the crystal. Subsequent changes in transmittance are explained by the insertion of adduct between these planar macromolecules. A self-consistent mechanism for the intercalation reaction has been postulated to explain the following observations: (i) for adduct pressures p⩾l torr the initial rate of reaction is proportional to p; and (ii) for p < 1 torr this rate varies as p 6. The latter observation indicates that cooperative phenomena involving six adduct molecule...

The nuclear spin functions for the A3(X3)3 system have been developed and used to analyse the NMR spectra of (CH3)3C6H3 and (CF3)3C6H3. The π-electron coupled hyperfine splitting constants, obtained from the Ramsey theory with simple Huckel M.O.'s for benzene and without the assumption of an average energy of excitation for the virtual processes which give rise to the coupling, are in agreement with the hyperfine constants observed between the methyl groups and the ring protons in the above compounds.

The resonance absorption of p‐benzosemiquinone in sodium ethylate—ethanol solution has been determined from t=−50°C to room temperature in the Zeeman region at frequencies of the order of the zero field splitting produced by the isotropic hyperfine coupling. At t=−50°C, the resonance absorption indicated free‐radical interactions with two sodium nuclei. The isotropic coupling constants measured for the free electron spin—1H and 23Na interactions were, respectively, | A | = 6.63±0.01 MHz and | A′| = 0.34±0.05 MHz. Due to the finite widths, resonance absorptions were observed near Hz = 0 for rf frequencies of ν/ | A | = 2.48±0.01 and 1.49±0.01. The observed linewidth inverse temperature dependence for −20°≤t≤0°C was explained on the basis of the over‐all chemical exchange process [Complex chemical formula], whence the first‐order rate of disappearance chemical exchange time τe,k=|A′|−1×exp[Ea(T−1−T0−1)/R] was obtained. The over‐all energy of activation was determined to be Ea = 10±1 kcal mole−1 and T0 = 253...

An analysis of the Knight shift, magnetic susceptibility and conductivity of ‘metallic’ solutions of sodium in ammonia shows that the metal-non-metal (MNM) transition takes place when the density of states N(E F) at the Fermi level is about one-third of the free-electron value, and that it drops as the transition is approached. This supports a model put forward by Mott and Davis (1968) and Mott (1971) for the MNM transition in disordered systems; fixed magnetic moments, varying only with the frequency of ionic motion, are supposed to remain on the metal atoms, or on donors in a semi-conductor, on the metallic side of the transition, so that a splitting of the conduction band into two overlapping Hubbard bands occurs. For a non-crystalline array of centres, this leads to a minimum (a pseudogap) in the density of states, and the MNM transition takes place when states at the Fermi energy become localized in the Anderson sense, which occurs when the ratio g defined by eqn. (2) has a value of about ⅓.

Spectra of the alkali metal intercalation products of MoS2 and NbSe2 are interpreted in terms of a previously published band model. (see abstr. A17683 of 1970).

Mott ratios of the density of states have been obtained for Na-NH3 using the theory recently developed by Mott (1971) and by Friedman (1971), and the conductivity and Hall constant data of Kyser and Thompson (1965), for concentrations greater than ∼2 mole % metal. These are given as evidence of the self-consistency of g derived from different transport measurements and of the physical significance of g in describing the MNM transition that the system undergoes.

The insertion of adducts (I) in between the layers of pseudo-two-dimensional (2D) solids is called intercalation and the purpose of this chapter is to review recent work which has led to a better understanding of the following selected topics: 1.1.1 the chemical process of intercalation in transition element dichalcogenides TX2, i.e. the reaction: $$T{X_2}(c) + xI = {I_x}.T{X_2}(c)$$ (1) and 1.1.2 the physicochemical properties of the complex formed in (1). The work will be concerned mostly with cases where both TX2 and the intercalated product in (1) are solids and the changes in the direct and reciprocal lattice will form the basis for the discussion of experimental observations. Typical changes in TaS2 phases are shown in Figure 1 and the notation used normally is described in Figure 2.

A new method of intercalating metals into layer compounds has been developed using electrolytic generation from the salt solution in ammonia. The results suggest that metals that are soluble in ammonia will form a homogeneous metal-ammonia intercalate layer, NH 3 ·M x , when x is less than the limiting solubility of M in NH 3 . The superconducting transition temperature ( T c ) was found to increase as the c -axis expansion [2δ = c (TaS 2 ·NH 3 ·M x ) - c (2H-TaS 2 )] decreased when M = lithium, sodium, and potassium. Of all the alkali metals, potassium gave the most stable compounds and the highest T c , 4.7°K.

We have prepared the sulfide analogues of the new warm (≈ 90 K) superconducting oxides (M1M′2Cu3X8-δ, M = Y, Eu, Gd, M′=Ba, X=S and δ= 0 to 1.5). These are referred to in the literature by the stiochiometry e.g., (1, 2, 3, 7.1) is one of the superconducting phases reported in the literature. The purpose of this work is to find out if the sulfide analogues of the above oxides can be prepared by high temperature reactions in a sulphur atmosphere, the composition of the metallic (superconducting?) phases if present and the structure of the unit cell.

Materials formed by the intercalation of adducts (NH3, N2H4, MoF6, Rb-NH3, SbF5 and AsF5) in between the one- and two dimensional networks provided by solids called by Lord Kelvin aeolotropic (TaS2, MoS2, NbSe2, TaSe3, and graphite) have applications in connection to organic conductors, high duty batteries, catalysts and solid lubricants. The intercalation of low dimensional solids (LDS) from the gas phase is a complex process which involves at least three steps. First Langmuir adsorption of the adduct must occur, this is followed by chemisorption and finally insertion of the adduct in between the layers produces a phase transition. The rate determining steps will vary depending on the LDS and on the adduct gas. The process of intercalation of layer dichalcogenides from the gas phase is reproducible and fairly well understood. However the intercalation of graphite is less well understood though it has received considerably more attention than the former. Electrolytic intercalation is the least understood to date because the laws of gas phase intercalation do not apply.

The system M1 - NH3 (M1 = metallic dichalcogenide) has been studied. Based on the theory of electron donor-acceptor complexes it is assumed that the intermolecular forces between the crystalline layer material and small intercalate molecules are due to dispersion (or van der Waals type interactions) and to charge transfer interactions. It is postulated that the factors which favor intercalation are low ionization energy of the highest filled spin orbital of the intercalate donor, high work function or electron affinity for the lowest empty spin orbital of the metallic acceptor, and that symmetry and directional and size effects are of utmost importance. The results are applied to interpret the superconducting properties of intercalated 2H-TaS2 and 2H-NbSe2.

23Na and 14N Knight shift and e.s.r. measurements have been carried out for lead-sodium in ammonia solutions. The local field at 23Na for lead saturated solutions was found to be independent of the concentration for mole ratios R — [NH3]/[Na] of 50 to 400 and temperatures t = −40° to 21°, k(23Na) =0.33±0.04 × 10-4 relative to 23Na+(aq.) at room temperature. The 14N Knight shift relative to pure NK3 was negligible. For a non-saturated solution of r = [Na]/[Pb] = 2 and R= [NH3]/[Na] = 200, k(23Na) = 0.78±0.01 × 10-4 was found to be almost the same as for a reference sample containing no lead with k (23Na) = 0.81±0.01 × 10−4 at room temperature. The e.s.r. absorptions of a lead saturated solution and for r — 2 are comparable at room temperature.

New experiments, that characterize thr disorder present in Type II superconducting ceramics, are reported: Electrochemical measurements at 298> T>150 K > Tc ≈ 92 K, in the cell: Cu¦CuBr2 .05 M in CH3OH¦1:2:3¦Pt and /orCu, obtain important thermochemical information, ΔHcell = 30kJ/mole e- and ΔScell = 212J/K/mole e-± 15%, which suggests that the 1:2:3 phase is disordered. The rf dissipation measurements of superconducting lamellae dispersed in a matrix give information on the effects of the static and rf fields on the mixed state.