Nuclear spin coupling crossover in dense molecular hydrogen

Abstract

One of the most striking properties of molecular hydrogen is the coupling between molecular rotational properties and nuclear spin orientations, giving rise to the spin isomers ortho- and para-hydrogen. At high pressure, as intermolecular interactions increase significantly, the free rotation of H2 molecules is increasingly hindered, and consequently a modification of the coupling between molecular rotational properties and the nuclear spin system can be anticipated. To date, high-pressure experimental methods have not been able to observe nuclear spin states at pressures approaching 100 GPa (Meier, Annu. Rep. NMR Spectrosc. 94:1–74, 2017; Meier, Prog. Nucl. Magn. Reson. Spectrosc. 106–107:26–36, 2018) and consequently the effect of high pressure on the nuclear spin statistics could not be directly measured. Here, we present in-situ high-pressure nuclear magnetic resonance data on molecular hydrogen in its hexagonal phase I up to 123 GPa at room temperature. While our measurements confirm the presence of ortho-hydrogen at low pressures, above 70 GPa, we observe a crossover in the nuclear spin statistics from a spin-1 quadrupolar to a spin-1/2 dipolar system, evidencing the loss of spin isomer distinction. These observations represent a unique case of a nuclear spin crossover phenomenon in quantum solids.

High-pressure synthesis of seven lanthanum hydrides with a significant variability of hydrogen content

“…The presence of phases other than the superconducting one has been considered as a potential explanation for the sudden drop of resistivity measured in some systems as they could enable a metallic conduction path to form upon temperature decrease 7 . The lack of a homogeneous sample would not only affect resistivity measurements but also magnetic susceptibility 1 , 12 , 13 and nuclear magnetic resonance 14 – 16 , serving as a strong impetus to accurately determine all phases present in these superconducting hydride systems. Single-crystal X-ray diffraction (SCXRD) coupled with ab initio calculations has already been demonstrated as a very effective tool to identify the formation of novel hydrides in laser-heated diamond anvil cells (DACs) experiments.…”

Chemically Assisted Precompression of Hydrogen Molecules in Alkaline-Earth Tetrahydrides

“…When pure molecular hydrogen is compressed to above 30 GPa, there is a turnover in the intramolecular vibron frequency and a lengthening of the H–H bond. 41 − 45 For instance, the H–H bond lengthens from 0.718 at 6 GPa in phase I to 0.84 Å when entering phase IV at 225 GPa, 3 , 46 , 47 and this is manifested by a 650 cm –1 drop in the H 2 -ν 1 frequency. 3 Comparing the vibron in the alkaline tetrahydrides with the vibron of pure H 2 -ν 1 , we observe that pressures of only 60 GPa for CaH 4 , 40 GPa for SrH 4 and 20 GPa for BaH 4 are sufficient to reach values seen in H 2 at 225 GPa (Phase IV).…”

In-situ High Pressure Nuclear Magnetic Resonance Crystallography

“…For efficient line narrowing in MAS, the rotation speed of the sample around the magic angle should be much greater than the dominant spin interaction frequencies. For high pressure experiments, line-widths in the order of several hundred kHz have repeatedly been reported [17][18][19] ; far outside of the capabilities of commercially available MAS probes.…”

“…Ideal LG-conditions were determined in both cases. First, we performed one-dimensional 19 F-LG-NMR experiments on a single crystal of CaF 2 by incrementing the saturation pulse in steps of 1 µs (maximum length of the LG-pulse was 100 µs) at ∆ω 100 kHz. The zero-time intensity sampling of both real and imaginary parts of the signals (Figure 3a) decay within approximately 125 µs, corresponding to a 5fold stretching of the FIDRF relative to the laboratory frame (FID).…”

“…YH 2 and YH 3 , in one experiment where a two dimensional analysis leads to a decomposition of the resonances and enables therefore not only an individual analysis, but also a simultaneous analysis at the same P and T , as well as, a quantification of the sample fractions. First LG-decoupling experiments in DACs have been published recently on molecular hydrogen with resulting spectral resolutions of about 3 ppm corresponding to a 1600-fold reduction of the H 2 NMR signals at pressures of up to 123 GPa 19 . Molecular hydrogen is predicted to show a coupling between molecular rotational properties and nuclear spin orientations, giving rise to the spin isomers ortho-and para-hydrogen.…”