E. M. Levin

Nonaromatic Core−Shell Structure of Nanodiamond from Solid-State NMR Spectroscopy

The structure of synthetic nanodiamond has been characterized by (13)C nuclear magnetic resonance (NMR) spectral editing combined with measurements of long-range (1)H-(13)C dipolar couplings and (13)C relaxation times. The surface layer of these approximately 4.8-nm diameter carbon particles consists mostly of sp(3)-hybridized C that is protonated or bonded to OH groups, while sp(2)-hybridized carbon makes up less than 1% of the material. The surface protons surprisingly resonate at 3.8 ppm, but their direct bonding to carbon is proved by fast dipolar dephasing under homonuclear decoupling. Long-range (1)H-(13)C distance measurements, based on (13)C{(1)H} dipolar dephasing by surface protons, show that seven carbon layers, in a shell of 0.63 nm thickness that contains approximately 60% of all carbons, predominantly resonate more than +8 ppm from the 37-ppm peak of bulk diamond (i.e., within the 45-80 ppm range). Nitrogen detected in (15)N NMR spectra is mostly not protonated and can account for some of the high-frequency shift of carbon. The location of unpaired electrons (approximately 40 unpaired electrons per particle) was studied in detail, based on their strongly distance-dependent effects on T(1,C) relaxation. The slower relaxation of the surface carbons, selected by spectral editing, showed that the unpaired electrons are not dangling bonds at the surface. This was confirmed by detailed simulations, which indicated that the unpaired electrons are mostly located in the disordered shell, at distances between 0.4 and 1 nm from the surface. On the basis of these results, a nonaromatic core-shell structural model of nanodiamond particles has been proposed.

Nature of the cubic to rhombohedral structural transformation in (AgSbTe2)15(GeTe)85 thermoelectric material

The existence of a large thermoelectric figure of merit in (AgSbTe2)15(GeTe)85 has been known for many years. However, the nature of the crystallographic transformation in these materials from a high-temperature cubic to a low-temperature rhombohedral polymorph and its effect on electrical transport has not been clearly established. Transmission electron microscopy studies were performed that show extensive twinning in the low-temperature structure, resulting from lattice strain during the dilation along the (111) crystallographic direction. Analysis of differential scanning calorimetric studies indicates that the transformation is of second order, so that the high-temperature cubic phase is nonquenchable. High-temperature x-ray diffraction was performed to establish the transformation temperature, which was found to be complete upon heating at a temperature of 510K. Results of electrical conductivity measurements as a function of temperature on as-cast samples are discussed in terms of the observed twinning.

The nonpareil R5(SixGe1−x)4 phases

Abstract The R5(SixGe1−x)4 materials have been known for over 30 years, but only in the last few years has their uniqueness become evident. Their exotic behaviors include a simultaneous first-order crystallographic–magnetic phase transition, the giant magnetocaloric effect, enhanced Curie temperature due to the existence of (Si,Ge)–(Si,Ge) bond pairs, and the absence of the magnetic contribution to the heat capacity below 10 K of ferrimagnetic Nd5Ge4, which orders at 55 K. These behaviors were observed in our studies of the phase relationships, crystallography, and the electrical, magnetic and thermal properties of the pseudo-binary 5:4 compounds with R=La, Nd, Gd, Dy and Lu.

Electronic and thermal transport in GeTe: A versatile base for thermoelectric materials

GeTe is a narrow-band gap semiconductor, where Ge vacancies generate free charge carriers, holes, forming a self-dopant degenerate system with p-type conductivity, and serves as a base for high-performance multicomponent thermoelectric materials. There is a significant discrepancy between the electronic and thermal transport data for GeTe-based materials reported in the literature, which obscures the baseline knowledge and prevents a clear understanding of the effect of alloying GeTe with various elements. A comprehensive study including XRD, SEM, EDS, Seebeck coefficient, electrical resistivity, thermal conductivity, and 125Te NMR of several GeTe samples was conducted. Similar Seebeck coefficient and electrical resistivity are observed for all GeTe samples used showing that the concentration of Ge vacancies generating charge carriers is constant along the ingot. Very short 125Te NMR spin-relaxation time agrees well with high carrier concentration obtained from the Hall effect measurements. Our data show ...

Real and imaginary components of the alternating current magnetic susceptibility of RAl2 (R=Gd, Dy, and Er) in the ferromagnetic region

The real (χac′) and imaginary (χac″) components of the ac magnetic susceptibility of the polycrystalline GdAl2, DyAl2, and ErAl2 have been measured as functions of temperature, ac and bias dc magnetic field amplitude, and ac magnetic field frequency. Both χac′ and χac″ of the ferromagnetic DyAl2 and ErAl2 are strongly dependent on the ac magnetic field parameters, especially when compared with those of the ferromagnetic GdAl2. The observed behavior of the ac magnetic susceptibility is determined mainly by the domain dynamics, rather than by a change of the exchange interactions below Curie temperature.

Magnetism and nuclear magnetic resonance of hectorite and montmorillonite layered silicates

The temperature and magnetic-field (H) dependencies of the bulk dc magnetization (M) and the M∕H ratio of montmorillonite (MMT), hectorite (HCT), and synthetic mica-montmorillonite (SMMT) clays have been measured and compared with the signal intensity of H1 and Si29 nuclear magnetic resonance (NMR) spectra. MMT exhibits Langevin paramagnetism with an effective magnetic moment of 5.5±0.1μB per Fe ion whereas SMMT has diamagnetic properties. At 300K, M∕H of HCT measured in a magnetic field of H⩽1kOe is larger than that of MMT, whereas in a field of 50kOe, the inverse situation is observed. The difference arises because the magnetization of HCT is dominated by a contribution from ferromagneticlike impurities. The H1 and Si29 NMR signals of MMT are broadened beyond detectability due to the paramagnetic effect. Although HCT contains ferromagneticlike components that result in a large M∕H in low field, it yields H1 and Si29 NMR spectra with signal intensities similar to those of diamagnetic SMMT. Our data highl...

Broadband "Infinite-Speed" Magic-Angle Spinning NMR Spectroscopy

High-resolution magic-angle spinning NMR of high-Z spin-1/2 nuclei such as (125)Te, (207)Pb, (119)Sn, (113)Cd, and (195)Pt is often hampered by large (>1000 ppm) chemical-shift anisotropies, which result in strong spinning sidebands that can obscure the centerbands of interest. In various tellurides with applications as thermoelectrics and as phase-change materials for data storage, even 22-kHz magic-angle spinning cannot resolve the center- and sidebands broadened by chemical-shift dispersion, which precludes peak identification or quantification. For sideband suppression over the necessary wide spectral range (up to 200 kHz), radio frequency pulse sequences with few, short pulses are required. We have identified Gans two-dimensional magic-angle-turning (MAT) experiment with five 90 degrees pulses as a promising broadband technique for obtaining spectra without sidebands. We have adapted it to broad spectra and fast magic-angle spinning by accounting for long pulses (comparable to the dwell time in t(1)) and short rotation periods. Spectral distortions are small and residual sidebands negligible even for spectra with signals covering a range of 1.5 gammaB(1), due to a favorable disposition of the narrow ranges containing the signals of interest in the spectral plane. The method is demonstrated on various technologically interesting tellurides with spectra spanning up to 170 kHz, at 22 kHz MAS.

Bulk magnetization and nuclear magnetic resonance of magnetically purified layered silicates and their polymer-based nanocomposites

The bulk magnetization and the H1 and Si29 nuclear-magnetic-resonance (NMR) spectra of two layered silicates, montmorillonite (MMT) and hectorite (HCT), purified by high-gradient magnetic separation, and of HCT-polymer nanocomposites have been measured. At 300K, the magnetization of MMT as received shows a behavior typical of paramagnets and does not change significantly even after ∼100h of magnetic separation. The magnetization of HCT as received is typical of a weak ferromagnet but it changes drastically after magnetic separation. The extracted particles have sizes varying from a few to about 150μm and show ferromagneticlike properties at 300K. While the magnetization/magnetic-field ratio, M∕H, of HCT is reduced 50-fold after ∼15min of magnetic separation and shows predominantly diamagnetic properties at 300K after ∼30h, we have also found that HCT contains ∼0.2mass% of paramagnetic Fe ions, with an effective magnetic moment of ∼5.2μB per Fe ion, as a regular element of its lattice. This is much smaller...

Magnetism of (Dy0.5Er0.5)Al2 Single Crystal in ac and dc Magnetic Fields

The temperature (4.2–90K), ac magnetic field (1.25–50Oe), frequency (5–125Hz), and bias dc magnetic field (0–10kOe) dependencies of the real and imaginary components of the ac magnetic susceptibility, and the temperature (4.2–250K) and dc magnetic field (0.1–50kOe) dependencies of the dc magnetic susceptibility and magnetization of a (Dy0.5Er0.5)Al2 single crystal have been studied. Isothermal magnetization measurement in a dc magnetic field indicates that (Dy0.5Er0.5)Al2 orders ferromagnetically at 37K. The ac and dc magnetic susceptibilities of (Dy0.5Er0.5)Al2 exhibit a similar behavior in the paramagnetic region but quite different behaviors in the ferromagnetic state. Both the real and imaginary components of the ac magnetic susceptibility are sensitive to the applied ac magnetic field, the crystallographic direction, and the bias magnetic field, showing that domain wall dynamics mainly account for the response to the ac magnetic field. The contributions to the magnetization process arise from the mag...