New results presented in the 2023 MRE HP Special Volume clearly demonstrate the cross-disciplinary synergistic progress in high-pressure physics and chemistry. The prevalence of pressure-induced crystal chemistry of clathrate-like host-guest cages in borides,1,2 nitrides,3 and hydrides4 has led to exotic compositions and physical properties.

Superionic and electride states are associated respectively with cationic and interstitial electron behaviors in materials. Their coexistence can provide access to exotic physical properties, but the exploration of such materials is challenging. Guo et al.1 reported that reaction of sodium and boron under pressure can produce the unusual Na-rich stoichiometry of Na₉B with host–guest Na–B structure and superionic and electride properties, representing the first example among alkali metal borides. Unexpectedly, it exhibits diffusion of the heavier Na atoms rather than the lighter B atoms, and an increase in bandgap with pressure. Boron-based compounds are excellent candidates for tunable multifunctional materials. Xie et al.2 performed structure searches coupled with first-principles calculations and discovered a new class of superconducting and hard pentaborides MB₅ (M = Na, K, Rb, Ca, Sr, Ba, Sc, and Y), in which B atoms are arranged in B20 cages to form a boron octahedral network and the metal atoms are located in the center of the B20 cages and act as “stabilizers” of the boron sublattice. The cage-like B20 structure can be viewed as a structural model for hard superconductors with an estimated highest T_c of 18.6 K and H_v of 35.1 GPa under ambient conditions. In particular, KB₅, RbB₅, and BaB₅ have been predicted to be both superconducting and hard, with T_c ranging from 14.7 to 18.6 K and H_v of around 32.4–35.1 GPa.

Pressure has a general tendency to induce polymerization. Tang et al.3 explored the dynamics of acetylene topochemical polymerization using a distance–vibration-based reaction model to explain the commencement of bonding in crystals of acetylene. Through computational analysis, their study has provided a deeper understanding of the factors influencing topochemical reactions, particularly the role of phonon modes in reaching the intrinsic threshold for intermolecular electronic interaction. Its findings offer insights into the prediction and design of topochemical reactions and enhance our understanding of the bonding process in solids, with implications for the development of crystalline polymeric materials. Polymeric nitrogen materials are environmentally friendly high-energy-density materials. The synthesis pressure and structural stability are two crucial factors for the application of polymeric nitrogen. Wang et al.4 reported a lamellar N₁₄ ring in P1̄-CeN₆, which can be quenched to ambient conditions. It possesses the lowest synthesis pressure of 32 GPa among the lamellar metal polynitrides, owing to the strong ligand effect of the element cerium. The cerium atom can effectively weaken the N≡N bond strength and promote the conversion of N₂ to N-chain and then to N-layer. In addition, it exhibits he outstanding energy density and explosive performance, which make it as a promising high-energy-density material.

The prediction and realization of high-temperature superconductivity in pressure-stabilized hydrogen-rich metal hydrides is a thriving field in condensed matter physics. However, the experimental characterization of high-pressure metal hydrides (MH) is challenging, especially with regard to the hydrogen content. Meier et al.5 investigated seven metal hydrides with different hydrogen contents (CuH_0.15, Cu₂H, CuH, YH₂, YH₃, H₆, and H_6±δS₅) and provided a way to directly access the hydrogen content of MH solids synthesized at high pressures in (laser-heated) diamond anvil cells using nuclear magnetic resonance spectroscopy. This method of hydrogen quantification could possibly be employed for superconducting superhydrides, such as LaH₁₀ or YH₉.

The quest for room-temperature superhydride superconductors at ultrahigh pressures6,7 has remained one of the hottest topics in condensed matter physics, but it is prudent to bear in mind the cautions raised by Hirsch and van der Marel.8 Recently, in a paper in Nature, Dasenbrock-Gammon et al.9 reported evidence of near-ambient superconductivity in Lu–H–N, which has attracted worldwide attention. If this result could be reproduced by other teams, it would be a major scientific breakthrough. Cai et al.10 synthesized samples following the protocol described in the Nature paper and also using an alternative laser heating technique. They have provided solid evidence of an absence of superconductivity in both samples. This result is an important contribution that adds to other recent experimental and theoretical evidence questioning the validity of the results published in the Nature paper. In addition, the raw data of resistance vs temperature just display a drop, instead of a zero-resistance state, at near room temperature. Clarifying whether the resistance drop is truly relevant to a superconducting transition is crucial. By using a controllable experimental protocol, Peng et al.11 synthesized Lu–H–N and reproduced the sudden change in resistance near room temperature. They showed that this dramatic resistance change is most likely caused by a metal-to-poor-conductor transition rather than by superconductivity, thus clarifying the confusion surrounding the Nature report. The room-temperature superconductivity has been attributed to Fm3m-LuH_3−δN_ε. Open questions remain, such as the exact stoichiometry and the positions of the hydrogen and nitrogen atoms. Using first-principles calculations and a virtual crystal approximation, Huo et al.12 systematically studied the phase diagram of Lu–N–H at 1 GPa. They found that no ternary Lu–H–N compounds can be stabilized at 1 GPa. In addition, they predicted that within the pressure range investigated in their study, the highest T_c of N-doped Fm3m-LuH₃ does not exceed 30 K, which is much lower than room temperature. These conclusive studies led to eventual retraction of the paper by Dasenbrock-Gammon et al.9

With the advent of fourth-generation synchrotron radiation X-ray sources that provide significantly enhanced coherent flux, Zeng13 pioneered high-energy X-ray photon correlation spectroscopy measurements that can be routinely performed on various materials in a regular diamond anvil cell. Essential experimental information on atomic dynamics that was previously inaccessible could be obtained for various novel phenomena arising under extreme conditions, such as the glassy nature of pressure-induced amorphous phases (ice, silicon, and others) and their relationship to liquid polymorphism, the novel superionic states under the conditions at the Earth’s core, atomic mechanisms for pressure-induced phase transitions, and charge/spin ordering dynamics in quantum materials, opening up unprecedented opportunities for cutting-edge research.

Our natural samples of high-pressure minerals are mostly discovered in terrestrial impact craters that were formed by hypervelocity impacts of asteroids or other celestial objects onto the Earth’s surface. Owing to subsequent modification by active internal tectonics and surficial weathering and erosion, however, such impact craters are often hard to preserve and recognize. Chen et al.14 reported a circular-shaped impact crater at the top of Baijifeng Mountain about 700 m high. The impact origin of this crater is confirmed by the widespread distribution of impact-excavated rock debris and shock-metamorphic features in minerals. Microscopic examination of planar deformation features in quartz grains from the crater reveal shock pressures of 10–35 GPa. Baijifeng impact crater is only the third known impact crater in the entire Chinese territory, and it is also the world’s first-known mountaintop impact crater. This study provides a new vision for investigation of impact craters in mountain areas, where these craters may occur with uncommon morphological characteristics.

Acknowledgment. H.-K. Mao acknowledges financial support from the Shanghai Key Laboratory of MFree, China (Grant No. 22dz2260800) and the Shanghai Science and Technology Committee, China (Grant No. 22JC1410300).