Spin-Exchange Term in the Solvent Equation of State Near the Critical Point for Electron Transfer Reactions

Phenomenological Equations of State (EOS) for fluids near their critical point have been obtained using literature compression factor data, Zc = Pc Vc/(n R Tc) = 0.40 to 0.10 in Table I (Pc, Vc/n, Tc 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, Zc(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 199,201Hg (Zc 0.4) Knight shift plummets to zero while the alkali metals and Se (Zc = 0.2 to 0.1) are paramagnetic fluids, and that the enhanced rates for free radical electron exchange reactions (in CO2, n-C2H6 and CHF3 with intermediate Zc) are correlated to Zc. The difference between the solvent behavior for electron spin exchange reactions near its critical point is ascribed to spin interactions. The analysis shows that the solvated electron osmotic pressure in metal ammonia solutions versus the solvent density r,NH3 = Vc/V goes through a maximum where enhanced rates of electron exchange also attain a maximum. This can be applied to choose the best solvents, near their critical point, for the syntheses of new materials and metal oxide extraction

YBa2Cu3O7 electro-magneto-optic effects in Ba L2,3 X-ray absorption near Tc

The onset of electro magneto-optic effects, observed at the Ba L2,3 edges synchrotron X-ray absorption by a YBa2Cu3O7 single crystal, 10 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 εX-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, 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

Solid state phase transitions characterized by esr and xas

Measurements of the relaxation time, 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, S/kB \u3c\u3c 1 when T \u3e\u3e 3.82 K ps. Magnetic resonance can measure \u3e\u3e 10-10 s, when v = 9 GHz. X-ray spectroscopy can measure \u3c 10-16s for hv \u3e 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-, 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 of 8 /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- below Tc

Chemical activity in YBa2Cu3O7 − δ across the normal to superconducting phase transition

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\u27 energy Ea, and orientations in the X-ray beam for preferred Miller indexed [HKL] planes\u27 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

XANES in (TMTSeF)2Re04: Polarization Dependence of the Se K-Edge

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 compound. 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

Optical Studies of Metal- Semiconductor Transmutations Produced by Intercalation

Spectra of the alkali metal intercalation products of MoS2 and NbSc2 arc interpreted in terms of a previously published band model

ESR Study of Optically Induced Phase Transitions

We have identified an optically enhanced magnetic phase transition in the newly synthesized organic molecular charge transfer salt, (BEDT-TTF)3Ta2F11 (BEDT-TTF bisethylenedithiolotetrathiafulvalene) 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 severa~ tripl~t E.SR absorpti_ons appear. The orientation dependence of the ESR absorption under lllummat10n at energies near the band gaps in the material ( 640 nm, T = 12 to 5 K H0 \u3c 0.34 T) indicates that there are rapid spin exchange processes with times r \u3c w-s; near 7 ~o 5 K.along cert~in ~rystallographic directions with a temperature dependen~e suggesting spm-lattlce relaxation times which proceed via Van Vleck direct processes. This, to our knowledge, is the first case where the magnetic properties of a charge transfer salt are altered by the interaction with photons of energy equal to the band gaps in a low dimensional solid providing a new, interesting way to investigate these materials