Li Plating on Carbon Electrode Surface Probed by Low-Field Dynamic Nuclear Polarization 7Li NMR

Lithium deposition on graphite electrode not only reduces fast-charging capability of lithium ion batteries but also causes safety trouble. Here, a low-field 7Li dynamic nuclear polarization (DNP) is used to probe Li plating on the surfaces of three types of carbon electrodes: hard carbon, soft carbon and graphite. Owing to the strong Fermi contact interaction between 7Li and conduction electrons, the 7Li nuclear-magnetic-resonance (NMR) signal of Li metal deposited on electrode surface could be selectively enhanced by DNP. It is suggested that low-field 7Li DNP spectroscopy is a sensitive tool for investigating Li deposition on electrodes during charging/discharging processes.

Hyperfine Interaction of Electron and Nuclear Spins

This chapter discusses the key interaction–hyperfine coupling–which underlies most of phenomena in the field of electron and nuclear spin dynamics. This interaction originates from magnetic interaction between the nuclear and electron spins. For conduction band electrons in III–V or II–VI semiconductors, it is reduced to a Fermi contact interaction whose strength is proportional to the probability of finding an electron at the nucleus. A more complex situation is realized for valence band holes where hole Bloch functions vanish at the nuclei. Here the hyperfine interaction is of the dipole–dipole type. The modification of the hyperfine coupling Hamiltonian in nanosystems is also analyzed. The chapter contains also an overview of experimental data aimed at determination of the hyperfine interaction parameters in semiconductors and semiconductor nanostructures.

STUDY OF ELECTRON PARAMAGNETIC RESONANCE SPECTRA OF LEAD BOROVANDATE GLASSES CONTAINING MoO3 CONTENT

The oxide glasses MoO3-PbO-B2O3:V2O5 have been investigated by the standard melt-quenching technique. The amorphous nature of studied glasses is ascertained by X- ray diffraction patterns. Electron paramagnetic resonance (EPR) of these glasses has been studied. The hyperfine coupling parameter (P) decreases with an increase in MoO3 content. The increase in the value of the Fermi contact interaction term (K) with an increase in MoO3 content is due to an increase in the tetragonality of the V4+O6 complex.

EPR AND OPTICAL STUDIES OF VANADYL IONS IN ALKALI LEAD BOROTELLURITE GLASSES

Electron Paramagnetic Resonance (EPR) and optical absorption spectra of VO 2+ ions in different alkali lead borotellurite glasses have been studied. The spin-Hamiltonian parameters (g and A), bonding parameter [Formula: see text] and Fermi contact interaction parameter k have been calculated. The values of spin-Hamiltonian parameters confirm that the vanadyl ions are present in the glasses as VO 2+ molecular ions in an octahedral site with a tetragonal compression. The number of spins (N) participating in resonance is calculated as a function of temperature (123–393 K) for 9 mol% of VO 2+ ions in lithium lead borotellurite glass sample. It is observed that N obeys the Boltzmann law. From EPR data, the paramagnetic susceptibility (χ) is calculated at various temperatures and the Curie constant has been evaluated from 1/χ–T graph. The optical absorption spectrum exhibits two bands characteristic of VO 2+ ions in tetragonal symmetry. The band gap (E opt ) and the Urbach energies (ΔE) have been determined from the ultraviolet absorption edges and are found to be dependant on the size of the alkali ion. The theoretical values of optical basicity (Λ th ) of these glasses have also been evaluated.

Magnetism of F centers; Indication of an antiferromagnetic phase transition in potassium-electro-sodalite

Temperature-dependent EPR data of potassium-electro-sodalite (PES), K8[Al6Si6O24](e-)2, are consistent with the occurrence of an antiferromagnetic phase transition at 71?2 K. PES is a Mott insulator which contains an unpaired electron in every sodalite cage. The same transition in sodium-electro-sodalite occurs at a considerably lower temperature (42 K), indicating that the exchange interaction among localized electrons is stronger in PES. PES is obtained by the inclusion of one potassium atom in every cage of potassium sodalite. The 27 Al MAS NMR resonance of PES is shifted downfield in respect to diamagnetic potassium-sodalite, K6[Al6Si6O24]. The NMR shift is due to unpaired electrons and is caused by hyperfine Fermi contact interaction.