Li Magic Angle Spinning Research Articles

Reversible Migration of Lithium in Montmorillonites

The migration of lithium cations into the lattice of dioctahedral clays upon heating, considered up to now as irreversible, has been investigated. The samples examination are two lithium-saturated montmorillonites, selected with different charge-deficit distributions between the tetrahedral and octahedral sheets, to obtain direct information on the structural changes occurring in both layers. Reexpansion of these samples previously collapsed at 300[degree]C has been attained, for the first time, under high vapor water pressures. A structural characterization of the smectities under the different treatments has been monitored by means of X-ray diffraction, XRD, Fourier transform infrared spectroscopy, FTIR, and [sup 29]Si, [sup 27]Al, and [sup 7]Li magic angle spinning nuclear magnetic resonance, MAS-NMR. The effect that Li[sup +] ions exert on the quadrupolar coupling constant of tetrahedral Al ions has been shown. Direct experimental evidence on the location of lithium ions in the hexagonal holes of the collapsed structure is provided. Additionally, generation of protons during the rehydration process is concluded. 35 refs., 7 figs.

The thermal reactions of synthetic hectorite studied by 29Si, 25Mg and 7Li magic angle spinning nuclear magnetic resonance

The thermal decomposition sequence of synthetic hectorite (Laponite CP) is shown by a combination of thermal analysis, X-ray powder diffraction and solid-state MAS-NMR to be very similar to that of the structurally related mineral talc, in transforming smoothly into the high-temperature product without the intervention of any crystallographically distinguishable intermediates. Loss of interlayer water at about 200°C causes little change in the hectorite basal spacing or in the 29Si, 25Mg or 7Li MAS-NMR spectra; small but significant changes in the latter after heating to 600–650°C may however be related to the movement of interlayer Na + closer to the tetrahedral sheets, influencing the octahedral Li (but not the octahedral Mg). Dehydroxylation above 650°C disrupts the phyllosilicate structure into pyroxene units (MgSiC 3) and amorphous silica with distortion of the octahedral Mg sites, as evidenced by the broadening and loss of intensity of the 25Mg resonance. The 7Li MAS-NMR spectra suggest that the Li becomes mobile just prior to dehydroxylation, and may eventually be incorporated, together with the interlayer Na, in the siliceous phase.

A Study of the System Li-P-Se

Compound formation in the system Li-P-Se is investigated by differential thermal analysis, differential scanning calorimetry (DSC), X-ray powder diffraction, 7Li, 77Se, and 31P magic-angle spinning NMR, and high-temperature 31P static NMR. No glasses can be formed by melt-quenching techniques. The two new crystalline phases found possess stoichiometries Li 4P 2Se 6 and Li 7PSe 6 and are not isostructural with their known sulfide analogs. Li 4P 2Se 6 crystallizes in the orthorhombic system, a = 11.239(5) Å, b = 11.811(5) Å, c = 13.528(5) Å, V = 1796 Å 3, d (calc) = 4.151 g · cm -3 ( Z = 8). It contains dimeric [P 2Se 6] 4- units, the two P atoms of which are structurally inequivalent. Li 7PSe 6 is indexed in the cubic system, a = 20.73 Å, and has two crystallographically inequivalent [PSe 4] 3- groups. In addition, the two types of selenium atoms, belonging to PSe 3- 4 and Se 2- units, respectively, are distinguished by a large chemical shift difference. The thermal behavior of Li 7PSe 6 is investigated by multiple DSC studies and in situ high-temperature NMR, suggesting that this phase is metastable at room temperature.

A carbon-13 and lithium-6 nuclear magnetic resonance study of lithium perchlorate/poly (ethylene oxide) electrolytes

13C and 6Li high-resolution solid-state NMR techniques are used to characterize the morphology and dynamics of LiClO 4/ poly(ethylene oxide) )(PEO) electrolytes. 13C spectra and relaxation are sensitive to the polymer morphology and the local chain motions. The 6Li magic-angle spinning (MAS) NMR technique offers a detailed quantitative structural picture of the lithium atoms. At low concentrations of LiClO 4 in PEO, where the conductivity rises with increasing concentration, the number of dissociated lithium ions is apparently the determining factor. At higher concentrations the observed drop in conductivity is associated with an increase in ion-ion interactions as well as decreased ion mobilities due to a lack of available sites on the polymer and due to decreasing polymer chain mobility. The number of dissociated lithium ions therefore is less important at these concentration.

6Li magic angle spinning nuclear magnetic resonance spectroscopy: a powerful probe for the study of lithium-containing materials

A narrow chemical-shift range has been established for a variety of lithium compounds via study of their 6Li magic angle spinning nuclear magnetic resonance (MAS NMR) spectra. Comparative studies of 6Li and 7Li spectra (MAS, and ‘off-angle’ spinning) establish that, for solid-state (and even solution) analytical purposes, 6Li is the preferred nucles, since the advantage of narrow absorption lines outweighs the poorer sensitivity of 6Li relative to 7Li.

Effect of precursors on lithium containing silicate gels studied by 7Li nuclear magnetic resonance

Lithium containing silica gels were prepared by hydrolysis of tetraethylorthosilicate (TEOS) with a methanol-water-LiNO 3 or LiOH solution and by infiltration of silica gel with LiOH or LiNO 3 solution. Gels prepared in this way, dried and heat treated between 200 and 850°C were studied by solid state 7Li static and magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. There are dramatic differences in the 7Li NMR spectra of the various gels. The lineshape of the NMR spectra for gels prepared from LiOH (both via hydrolysis of TEOS and infiltration of prepared silica gel) is similar to that of crystalline Li 2SiO 3 where the lithium ions are associated with the silica network. The spectra do not change significantly with increasing temperature up to 850°C. For gels prepared from LiNO 3, the 7Li NMR spectra indicate the presence of two different Li species; one of the Li species is attributed to hydrated lithium ions trapped in the pores of the gels, the other Li species is due to crystalline LiNO 3. When the LiNO 3 gels are heat treated above 650°C, the lineshape of the NMR spectra becomes similar to that of Li 2SiO 3.