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ABSTRACT Using first-principles calculations and crystal structure search methods, we found that many covalently bonded molecules such as H2, N2, CO2, NH3, H2O and CH4 may react with NaCl, a prototype ionic solid, and form stable compounds under pressure while retaining their molecular structure. These molecules, despite whether they are homonuclear or heteronuclear, polar or non-polar, small or large, do not show strong chemical interactions with surrounding Na and Cl ions. In contrast, the most stable molecule among all examples, N2, is found to transform into cyclo-N5− anions while reacting with NaCl under high pressures. It provides a new route to synthesize pentazolates, which are promising green energy materials with high energy density. Our work demonstrates a unique and universal hybridization propensity of covalently bonded molecules and solid compounds under pressure. This surprising miscibility suggests possible mixing regions between the molecular and rock layers in the interiors of large planets. 

Compressed hydrides are promising candidates for high-temperature superconductivity, yet achieving simultaneous structural stability and high‑Tc at low pressures remains challenging. Here, we introduce a new mechanism for accomplishing this goal by mimicking the bonding characteristics of high-pressure H3S within metal hydrides. Using Li3CuH4 as an example, its Cu-H covalent interaction effectively mimics the core function of the S-H bonding in H3S. This interaction not only induces a high hydrogen-derived electronic density of states at the Fermi level, but also softens the hydrogen phonon modes, thereby significantly enhancing the electron-phonon coupling. Furthermore, embedding the strongly ionic Li3H lattice into the covalent Cu-H framework stabilizes the structure at significantly low pressures via a chemical-template effect, while maintaining high-Tc. Li3CuH4 exhibits excellent thermodynamic stability at 20 GPa, with a Tc of 39.25 K at 12 GPa that is further increased to 60.12 K when anharmonic effects are included. Further comprehensive high-throughput studies on Li3MH4 (M = transition metal) compounds uncover general principles applicable to a broader range of compounds. This work establishes a new paradigm for the simultaneous optimization of the stability and high-temperature superconductivity of metal hydrides through complementary sublattice interactions, thus advancing the search for practical and viable superconducting materials. 

We demonstrate a remarkable change in the chemical bonding of iron under pressure that underlies the distribution of elements in the Earth’s mantle and core. Using a massive-scale, first-principles study, we show that while reacting withp-block elements under increasing pressure from ambient to Earth core conditions, iron tends to reverse its redox nature, changing from an electron donor (reductant) to an electron acceptor (oxidant), and oxidizes manyp-block elements. Such reverse redox propensity significantly impacts the stoichiometries, bond types and strengths, structures, and properties of iron compounds under deep planetary conditions. This change transforms manyp-block elements (conventionally labeled lithophile or chalcophile) into highly siderophile species. The chemical binding strengths with iron show an inverse correlation with the depletion ofp-block elements in the silicate Earth. Furthermore, silicon shows a distinct anomaly in its bonding to iron, which suggests silicon may readily be incorporated into Earth’s core. 

Metal superhydrides, known for their high hydrogen content and polyhedral hydrogen cages, are promising candidates for high-temperature superconductivity. Despite extensive studies on binary metal superhydrides, there remains a large, unexplored chemical space, particularly regarding non-integer hydrogen-to-metal ratios. By integrating the "chemical template effect" with machine learning algorithms, we developed a specialized structure discovery workflow that significantly enhances the efficiency of predicting stable superhydrides. Within the compiled dataset used in this work, our method led to the identification of 13 new structural prototypes and 31 stable metal superhydrides, representing a 23% increase in discoveries. The 3D hydrogen clathrates in these compounds are significantly correlated with high superconducting transition temperatures (Tc), and our approach achieves a remarkable 65% increase. Most of these structures contain over 50 atoms per primitive cell, with the I4/m M10H84 prototype having the largest unit cell, containing 94 atoms. Additionally, 19 of the newly identified superhydrides exhibit Tc > 100 K, highlighting the potential for higher Tc materials within the 3D hydrogen clathrate structures. The method also shows good potential to search for ternary superhydrides on a large scale. 

Numerous experimental and theoretical studies showed that alkali and alkaline metals transform into various structures with large open interstitials under pressure, an intricate behavior that is caused by the electron transfer among s, p, and d atomic and quasi-atomic orbitals. However, little is known about the behavior of these metals beyond this pressure region. Using first-principles calculations and crystal structure search methods, we show that Cs returns to simple close-packed structures under ultrahigh pressure, including the transition from DHCP to FCC structure at 183 GPa and a further transition into HCP at 655 GPa. More significantly, under ultra-high pressure, the 5p electrons are activated and form strong chemical bonds between neighboring Cs atoms that govern the properties of Cs under very high pressure, including the transition from FCC to HCP. 

The request for extremely high pressures hinders the applications of hydrogen-rich compounds that can potentially achieve room-temperature superconductivity. Lowering the pressure while maintaining high-temperature superconductivity remains a challenge for hydrides. Here, we report an unprecedented high Tc of 245 K at a moderate pressure of 60 GPa in a ternary hydride, NaKH12. The H in this compound remains in molecular form, unlike the atomiclike hydrogen networks in most superhydrides. Both the excellent stability and superconductivity of NaKH12 originate from the fact that the localized electrons in the interstitial region of the metal lattice occupying the crystal orbitals well matched with the hydrogen lattice and forming chemical templates to assist the assembly of H2 units. These localized electrons weaken the H-H covalent bonds and improve the charge connectivity between the H2 units, ensuring the strong coupling between electrons and hydrogen-dominated optical phonons. In contrast to the traditional belief that atomiclike hydrogen is the key to reaching high Tc superconductivity, our findings demonstrate that molecular hydrogens connected by localized electrons in metal lattices offer a new route that might achieve this goal at much lower pressure. 

Electrides have emerged as promising materials with exotic properties due to the presence of localized electrons detached from all atoms. Despite the continuous discovery of many new electrides, most of them are based on atypical compositions, and their applications require an inert surface structure to passivate reactive excess electrons. Here, we demonstrate a different route to attain tunable electrides. We first report that monolayer transition metal dichalcogenides (TMDCs) exhibit weak electride characteristics, which is the remainder of the electride feature of the transition metal sublattice. By introducing chalcogen vacancies, the enhanced electride characteristics are comparable to those of known electrides. Since the precise tailoring of the chalcogen vacancy concentration has been achieved experimentally, we proposed that TMDCs can be used to build electrides with controllable intensities. Furthermore, we demonstrate that the electride states at the chalcogen vacancy of monolayer TMDCs will play an important role in catalyzing hydrogen evolution reactions.