The 6th International Conference on Matter and Radiation at Extremes (ICMRE2023)

41 / 2023-04-07 02:39:33

Nuclear magnetic resonance in lanthanum polyhydrides up to 1.5 Mbar

lanthanum polyhydrides，NMR

High Pressure Physics and Materials Science > Hydrogen and high-energy density material-poster

Lanthanum hydride LaH₁₀ is a superhydride with the highest critical temperature (T_C ≈ 250 K) among known polyhydrides [1]. For this reason, the La-H system is one of the most attractive objects for research. In this work, the formation of lanthanum polyhydrides was studied by nuclear magnetic resonance (NMR) in diamond anvil cells at pressures up to 150 GPa at room temperature. To detect nuclear induction signals, from the hydrogen spin-system in our microscopic samples (d ≈ 30 μm, t ≈ 5 μm), we used a system of Lenz lenses sputtered on pavilions of diamond anvils. La metal and LaH₃ were used as a starting material, while ammonia borane complex (NH₃BH₃, AB) served as a hydrogen source.

We found that the ¹H NMR spectra of LaH₃ recorded at pressures from 5 to 150 GPa, contain two signals of different widths, the NMR shift of which practically does not change with increasing pressure. A wide signal (FWHM ≈75-100 ppm) corresponds to hydrogen atoms rigidly localized in the LaH₃ crystal structure, and a much narrower signal (10-15 ppm) possibly corresponds to a sub-system of hydrogen atoms with enhanced mobility. This is in good agreement with theoretical predictions of a diffusion coefficient of hydrogen of about 10^-6 – 10^-7 cm²/s in high-pressure polyhydrides at 300 K [2, 3]. Such high diffusion coefficients are achieved due to the presence of defects in the nonstoichiometric crystal lattice of hydrides under pressure.

Laser heating of our LaH₃/NH₃BH₃ sample was carried out at 121 (DAC #2) and 150 GPa (DAC #1). As a result, a new narrow spin-echo signal with a shorter relaxation time (T₁) appeared, which was not previously observed. This new signal exhibits surprisingly sharp spectral features as well as spin-spin relaxation times (T₂) approaching spin-lattice relaxation times (T₁), a tell-tale indication that this new spin system lies in the “extreme narrowing limit” of NMR, characteristic for liquids and high mobility ionic conductors. This indicates that in the structure of lanthanum hydrides exists a fraction of diffusion-active hydrogen atoms even at room temperature. This observation of a high mobility hydrogen sub-species in lanthanum hydrides underlines previous similar findings in iron- and copper hydrides using NMR [4, 5].