RESUMEN
The extraordinary sensitivity of 129Xe, hyperpolarized by spin-exchange optical pumping, is essential for magnetic resonance imaging and spectroscopy in life and materials sciences. However, fluctuations of the polarization over time still limit the reproducibility and quantification with which the interconnectivity of pore spaces can be analyzed. Here, we present a polarizer that not only produces a continuous stream of hyperpolarized 129Xe but also maintains stable polarization levels on the order of hours, independent of gas flow rates. The polarizer features excellent magnetization production rates of about 70 mL/h and 129Xe polarization values on the order of 40% at moderate system pressures. Key design features include a vertically oriented, large-capacity two-bodied pumping cell and a separate Rb presaturation chamber having its own temperature control, independent of the main pumping cell oven. The separate presaturation chamber allows for precise control of the Rb vapor density by restricting the Rb load and varying the temperature. The polarizer is both compact and transportableâmaking it easily storableâand adaptable for use in various sample environments. Time-evolved two-dimensional (2D) exchange spectra of 129Xe absorbed in the microporous metal-organic framework CAU-1-AmMe are presented to highlight the quantitative nature of the device.
RESUMEN
La3 B6 O13 (OH) was obtained by a high-pressure/high-temperature experiment at 6â GPa and 1673â K. The compound crystallizes in the space group P21 (no. 4) with the lattice parameters a=4.785(2), b=12.880(4), c=7.433(3)â Å, and ß=90.36(10)°, and is built up of corner- as well as edge-sharing BO4 tetrahedra. It represents the first acentric high-pressure borate containing these B2 O6 entities. The compound develops borate layers of "sechser"-rings with the La3+ cations positioned between the layers. Single-crystal and powder X-ray diffraction, vibrational and MAS NMR spectroscopy, second-harmonic generation (SHG) and thermoanalytical measurements, as well as computational methods were used to affirm the proposed structure and the B2 O6 entities.
RESUMEN
Proton-containing point defects in solid materials are important for a variety of properties ranging from ionic transport over thermal conductivity up to compressibility. Ultrafast magic-angle spinning techniques nowadays offer high-resolution solid-state NMR spectra, even for 1H, and thus open up possibilities to study the underlying defect chemistry. Nevertheless, disorder within such defects again leads to heavy spectral overlap of 1H resonances, which prevents quantitative analysis of defect concentrations, if several defect types are present. Here, we present a strategy to overcome this limitation by simulating the 1H lineshape as well as 1H-1H double-quantum buildup curves, which we then validate against the experimental data in a joint cost function. To mimic the local structural disorder, we use molecular dynamics simulations at the DFT level. It turned out to be advantageous for the joint refinement to put the computational effort into the structural optimisation to derive accurate proton positions and to use empirical correlations for the relation between isotropic and anisotropic 1H chemical shifts and structural elements. The expressiveness of this approach is demonstrated on ringwoodite's (γ-Mg2SiO4) OH defect chemistry containing four different defect types in octahedral and tetrahedral voids with both pure Mg and mixed Si and Mg cation environments. Still, we determine the ratio for each defect type with an accuracy of about 5% as a result of the minimization of the joint cost function. We expect that our approach is generally applicable for local proton disorder and might prove to be a valuable alternative to the established AIRSS and Monte Carlo methods, respectively.
RESUMEN
Spin-ices are frustrated magnets that support a particularly rich variety of emergent physics. Typically, it is the interplay of magnetic dipole interactions, spin anisotropy, and geometric frustration on the pyrochlore lattice that drives spin-ice formation. The relevant physics occurs at temperatures commensurate with the magnetic interaction strength, which for most systems is 1-5 K. Here, we show that non-magnetic cadmium cyanide, Cd(CN)2, exhibits analogous behaviour to magnetic spin-ices, but does so on a temperature scale that is nearly two orders of magnitude greater. The electric dipole moments of cyanide ions in Cd(CN)2 assume the role of magnetic pseudospins, with the difference in energy scale reflecting the increased strength of electric vs magnetic dipolar interactions. As a result, spin-ice physics influences the structural behaviour of Cd(CN)2 even at room temperature.