Structural independence of hydrogen-bond symmetrisation dynamics at extreme pressure conditions

Pressure-Induced Hydrogen Bond Symmetrization in Babingtonite

Hydrogen bond symmetrization and electronic spin transition of iron under pressure are key factors in understanding the physical properties and behaviors of Fe- and H-bearing minerals. However, the coexistence of these two effects may often result in inconsistent interpretations. Babingtonite is characterized by prominent hydrogen bonds and diverse iron oxidation states within its crystal structure. These features make it an ideal subject for studying the interactions between hydrogen bond symmetrization and spin transitions of iron. Here, we conducted high-pressure infrared absorption spectroscopy, single-crystal X-ray diffraction, and Mössbauer spectroscopy experiments on babingtonite to investigate the behaviors of hydrogen bonds and spin states of iron. Pressure-induced hydrogen bond symmetrization occurs at approximately 22 GPa in babingtonite, while no spin transition of iron is detected up to 40 GPa. Our results show that the notable deformation of the unit cell and the kink in the trend of the hydrogen-bonded O-O distance with increasing pressure provide insightful evidence for hydrogen bond symmetrization. The symmetrical rigid configuration of the hydrogen bond leads to increased distortion in the neighboring Fe3+-O octahedron. These findings will contribute to the determination of hydrogen bond symmetrization through crystallographic structural analysis, partly overcoming the challenge of accurately locating hydrogen positions using X-ray diffraction.

The effect of Fe incorporation on the single-crystal elasticity of δ-AlOOH

The seismic mapping of hydrous materials in the Earth's deep interior requires experimental constraints on the elastic anisotropy of hydrous minerals and phases. Oxyhydroxides like δ-(Al,Fe)OOH are arguably the main hosts of water in the lower mantle. Therefore, constraints on the single-crystal elastic tensor of δ-(Al,Fe)OOH solid solutions are crucial to quantify the elastic anisotropy of this material, and advance the current understanding of the recycling of water into the lower mantle. Yet, experimental data for intermediate compositions are scarce, limiting the understanding of how Fe incorporation affects the single-crystal elastic properties of δ-AlOOH. In this study, we provide experimental constraints on the single-crystal elasticity of two δ-(Al,Fe)OOH solid solutions, with Fe/(Al + Fe) of 0.06(1) and 0.133(3). Large single-crystal samples of δ-(Al,Fe)OOH were synthetized at high pressures and temperatures using a multi-anvil press, and the full elastic stiffness tensors were determined at ambient conditions by combining X-ray diffraction and Brillouin scattering measurements. We show that replacing Al3+ with Fe3+ in δ-(Al,Fe)OOH lowers the magnitude of most coefficients of the elastic stiffness tensor (c ij), which translates into a substantial reduction of aggregate moduli and acoustic wave velocities. We further show that, at ambient conditions, the acoustic anisotropy of δ-(Al,Fe)OOH displays no sensitivity to Fe-Al substitution.

Diffusion-driven transient hydrogenation in metal superhydrides at extreme conditions

In recent years, metal hydride research has become one of the driving forces of the high-pressure community, as it is believed to hold the key to superconductivity close to ambient temperature. While numerous novel metal hydride compounds have been reported and extensively investigated for their superconducting properties, little attention has been focused on the atomic and electronic states of hydrogen, the main ingredient in these novel compounds. Here, we present combined 1H- and 139La-NMR data on lanthanum superhydrides, LaHx, (x = 10.2 - 11.1), synthesized after laser heating at pressures above 160 GPa. Strikingly, we found hydrogen to be in a highly diffusive state at room temperature, with diffusion coefficients in the order of 10-6cm2s-1. We found that this diffusive state of hydrogen results in a dynamic de-hydrogenation of the sample over the course of several weeks, approaching a composition similar to its precursor materials. Quantitative measurements demonstrate that the synthesized superhydrides continuously decompose over time. Transport measurements underline this conclusion as superconducting critical temperatures were found to decrease significantly over time as well. This observation sheds new light on formerly unanswered questions on the long-term stability of metal superhydrides.

Emerging Two-Dimensional Materials for Proton-Based Energy Storage

The rapid diffusion kinetics and smallest ion radius make protons the ideal cations toward the ultimate energy storage technology combining the ultrafast charging capabilities of supercapacitors and the high energy densities of batteries. Despite the concept existing for centuries, the lack of satisfactory electrode materials hinders its practical development. Recently, the rapid advancement of the emerging two-dimensional (2D) materials, characterized by their ultrathin morphology, interlayer van der Waals gaps, and distinctive electrochemical properties, injects promises into future proton-based energy storage systems. In this perspective, we comprehensively summarize the current advances in proton-based energy storage based on 2D materials. We begin by providing an overview of proton-based energy storage systems, including proton batteries, pseudocapacitors and electrical double layer capacitors. We then elucidate the fundamental knowledge about proton transport characteristics, including in electrolytes, at electrolyte/electrode interfaces, and within electrode materials, particularly in 2D material systems. We comprehensively summarize specific cases of 2D materials as proton electrodes, detailing their design concepts, proton transport mechanism and electrochemical performance. Finally, we provide insights into the prospects of proton-based energy storage systems, emphasizing the importance of rational design of 2D electrode materials and matching electrolyte systems.

Trace element detection in anhydrous minerals by micro-scale quantitative nuclear magnetic resonance spectroscopy

Nominally anhydrous minerals (NAMs) composing Earth's and planetary rocks incorporate microscopic amounts of volatiles. However, volatile distribution in NAMs and their effect on physical properties of rocks remain controversial. Thus, constraining trace volatile concentrations in NAMs is tantamount to our understanding of the evolution of rocky planets and planetesimals. Here, we present an approach of trace-element quantification using micro-scale Nuclear Magnetic Resonance (NMR) spectroscopy. This approach employs the principle of enhanced mass-sensitivity in NMR microcoils. We were able to demonstrate that this method is in excellent agreement with standard methods across their respective detection capabilities. We show that by simultaneous detection of internal reference nuclei, the quantification sensitivity can be substantially increased, leading to quantifiable trace volatile element amounts of about 50 ng/g measured in a micro-meter sized single anorthitic mineral grain, greatly enhancing detection capabilities of volatiles in geologically important systems.

Hydrogen bond symmetrisation in D2O ice observed by neutron diffraction

Hydrogen bond symmetrisation is the phenomenon where a hydrogen atom is located at the centre of a hydrogen bond. Theoretical studies predict that hydrogen bonds in ice VII eventually undergo symmetrisation upon increasing pressure, involving nuclear quantum effect with significant isotope effect and drastic changes in the elastic properties through several intermediate states with varying hydrogen distribution. Despite numerous experimental studies conducted, the location of hydrogen and hence the transition pressures reported up to date remain inconsistent. Here we report the atomic distribution of deuterium in D2O ice using neutron diffraction above 100 GPa and observe the transition from a bimodal to a unimodal distribution of deuterium at around 80 GPa. At the transition pressure, a significant narrowing of the peak widths of 110 is also observed, attributed to the structural relaxation by the change of elastic properties.

Molecular anchoring of free solvents for high-voltage and high-safety lithium metal batteries

Constraining the electrochemical reactivity of free solvent molecules is pivotal for developing high-voltage lithium metal batteries, especially for ether solvents with high Li metal compatibility but low oxidation stability ( <4.0 V vs Li+/Li). The typical high concentration electrolyte approach relies on nearly saturated Li+ coordination to ether molecules, which is confronted with severe side reactions under high voltages ( >4.4 V) and extensive exothermic reactions between Li metal and reactive anions. Herein, we propose a molecular anchoring approach to restrict the interfacial reactivity of free ether solvents in diluted electrolytes. The hydrogen-bonding interactions from the anchoring solvent effectively suppress excessive ether side reactions and enhances the stability of nickel rich cathodes at 4.7 V, despite the extremely low Li+/ether molar ratio (1:9) and the absence of typical anion-derived interphase. Furthermore, the exothermic processes under thermal abuse conditions are mitigated due to the reduced reactivity of anions, which effectively postpones the battery thermal runaway.

The curious case of proton migration under pressure in the malonic acid and 4,4'-bipyridine cocrystal

In the search for new active pharmaceutical ingredients, the precise control of the chemistry of cocrystals becomes essential. One crucial step within this chemistry is proton migration between cocrystal coformers to form a salt, usually anticipated by the empirical ΔpKa rule. Due to the effective role it plays in modifying intermolecular distances and interactions, pressure adds a new dimension to the ΔpKa rule. Still, this variable has been scarcely applied to induce proton-transfer reactions within these systems. In our study, high-pressure X-ray diffraction and Raman spectroscopy experiments, supported by DFT calculations, reveal modifications to the protonation states of the 4,4'-bipyridine (BIPY) and malonic acid (MA) cocrystal (BIPYMA) that allow the conversion of the cocrystal phase into ionic salt polymorphs. On compression, neutral BIPYMA and monoprotonated (BIPYH+MA-) species coexist up to 3.1 GPa, where a phase transition to a structure of P21/c symmetry occurs, induced by a double proton-transfer reaction forming BIPYH22+MA2-. The low-pressure C2/c phase is recovered at 2.4 GPa on decompression, leading to a 0.7 GPa hysteresis pressure range. This is one of a few studies on proton transfer in multicomponent crystals that shows how susceptible the interconversion between differently charged species is to even slight pressure changes, and how the proton transfer can be a triggering factor leading to changes in the crystal symmetry. These new data, coupled with information from previous reports on proton-transfer reactions between coformers, extend the applicability of the ΔpKa rule incorporating the pressure required to induce salt formation.

Silica-water superstructure and one-dimensional superionic conduit in Earth's mantle

Water in Earth's deep interior is predicted to be hydroxyl (OH-) stored in nominally anhydrous minerals, profoundly modulating both structure and dynamics of Earth's mantle. Here, we use a high-dimensional neuro-network potential and machine learning algorithm to investigate the weight percent water incorporation in stishovite, a main constituent of the subducted oceanic crust. We found that stishovite and water prefer forming medium- to long-range ordered superstructures, featuring one-dimensional (1D) water channels. Synthesizing single crystals of hydrous stishovite, we verified the ordering of OH- groups in the water channels through optical and nuclear magnetic resonance spectroscopy and found an average H-H distance of 2.05(3) Å, confirming simulation results. Upon heating, H atoms were predicted to behave fluid-like inside the channels, leading to an exotic 1D superionic state. Water-bearing stishovite could feature high ionic mobility and strong electrical anisotropy, manifesting as electrical heterogeneity in Earth's mantle.