Abstract NH 4 TaWO 6 , ND 4 TaWO 6, and NH 4 NbWO 6 have been studied by NMR at various frequencies and over a large range of temperatures. At low temperatures, ammonium ions exist as an entity tunnelling in the center of the cavities of the pyrochlore framework. From relaxation data an activation energy of this process of about 6.5 kJ/mol is obtained for all three materials. At temperatures above about 250 K translational motion narrows the spectra and produces relaxation. The experiments suggest a Grotthus type of proton transfer, where both local back ‐ and forth‐hopping (13–20 kJ/mol) and long range migration (31–39 kJ/mol) plays a role. At higher temperatures NH 4 + and NH 3 may diffuse as a whole. This is shown in particular for NH 4 NbWO 6 which served as diaphragm in a water vapour electrolysis cell, and where ammonia becomes exchanged by water.
Abstract Uncovering brain-tissue microstructure including axonal characteristics is a major neuroimaging research focus. Within this scope, anisotropic properties of magnetic susceptibility in white matter have been successfully employed to estimate primary axonal trajectories using mono-tensorial models. However, anisotropic susceptibility has not yet been considered for modeling more complex fiber structures within a voxel, such as intersecting bundles, or an estimation of orientation distribution functions (ODFs). This information is routinely obtained by high angular resolution diffusion imaging (HARDI) techniques. In applications to fixed tissue, however, diffusion-weighted imaging suffers from an inherently low signal-to-noise ratio and limited spatial resolution, leading to high demands on the performance of the gradient system in order to mitigate these limitations. In the current work, high angular resolution susceptibility imaging (HARSI) is proposed as a novel, phase-based methodology to estimate ODFs. A multiple gradient-echo dataset was acquired in an entire fixed chimpanzee brain at 61 orientations by reorienting the specimen in the magnetic field. The constant solid angle method was adapted for estimating phase-based ODFs. HARDI data were also acquired for comparison. HARSI yielded information on whole-brain fiber architecture, including identification of peaks of multiple bundles that resembled features of the HARDI results. Distinct differences between both methods suggest that susceptibility properties may offer complementary microstructural information. These proof-of-concept results indicate a potential to study the axonal organization in post-mortem primate and human brain at high resolution. Highlights Introduction of High Angular Resolution Susceptibility Imaging (HARSI) for advancing Quantitative Susceptibility Mapping (QSM). HARSI-derived fiber orientation distributions in fixed chimpanzee brain. HARSI-based visualization of complex fiber configurations. Comparisons between HARSI and High Angular Resolution Diffusion Imaging. Potential for high-resolution post-mortem imaging of fiber architecture.
