An impact structure 1400 m in diameter, formed by a bolide impact, has been discovered on Baijifeng Mountain in Tonghua City in Northeast China’s Jilin province. The impact structure takes the form of a cirque-shaped depression on the top of the mountain and is located in a basement mainly composed of Proterozoic sandstone and Jurassic granite. A large number of rock fragments composed mainly of sandstone, with a small amount of granite, are distributed on the top of Baijifeng Mountain. Planar deformation features (PDFs) have been found in quartz in the rock and mineral clasts collected from the surface inside the depression. The forms of the PDFs indexed in the quartz include among others, {101̄3}, {101̄2}, and {101̄1}. The presence of these PDFs provides diagnostic evidence for shock metamorphism and the impact origin of the structure. The impact event took place after the Jurassic Period and probably much later.

The recent report of superconductivity in nitrogen-doped lutetium hydride (Lu-H-N) at 294 K and 1 GPa brought hope for long-sought-after ambient-condition superconductors. However, the failure of scientists worldwide to independently reproduce these results has cast intense skepticism on this exciting claim. In this work, using a reliable experimental protocol, we synthesized Lu-H-N while minimizing extrinsic influences and reproduced the sudden change in resistance near room temperature. With quantitative comparison of the temperature-dependent resistance between Lu-H-N and the pure lutetium before reaction, we were able to clarify that the drastic resistance change is most likely caused by a metal-to-poor-conductor transition rather than by superconductivity. Herein, we also briefly discuss other issues recently raised in relation to the Lu-H-N system.

Topochemical reactions are a promising method to obtain crystalline polymeric materials with distance-determined regio- or stereoselectivity. It has been concluded on an empirical basis that the closest intermolecular C⋯C distance in crystals of alkynes, d_(C⋯C)min, should reach a threshold of ∼3 Å for bonding to occur at room temperature. To understand this empirical threshold, we study here the polymerization of acetylene in the crystalline state under high pressure by calculating the structural geometry, vibrational modes, and reaction profile. We find d_(C⋯C)min to be the sum of an intrinsic threshold of 2.3 Å and a thermal displacement of 0.8 Å (at room temperature). Molecules at the empirical threshold move via several phonon modes to reach the intrinsic threshold, at which the intermolecular electronic interaction is sharply enhanced and bonding commences. A distance–vibration-based reaction picture is thus demonstrated, which provides a basis for the prediction and design of topochemical reactions, as well as an enhanced understanding of the bonding process in solids.

On the basis of the current theoretical understanding of boron-based hard superconductors under ambient conditions, numerous studies have been conducted with the aim of developing superconducting materials with favorable mechanical properties using boron-rich compounds. In this paper, first-principles calculations reveal the existence of an unprecedented family of tetragonal pentaborides MB₅ (M = Na, K, Rb, Ca, Sr, Ba, Sc, and Y), comprising B₂₀ cages and centered metal atoms acting as stabilizers and electron donors to the boron sublattice. These compounds exhibit both superconductivity and high hardness, with the maximum superconducting transition temperature T_c of 18.6 K being achieved in RbB₅ and the peak Vickers hardness H_v of 35.1 GPa being achieved in KB₅ at 1 atm. The combination of these properties is particularly evident in KB₅, RbB₅, and BaB₅, with T_c values of ∼14.7, 18.6, and 16.3 K and H_v values of ∼35.1, 32.4, and 33.8 GPa, respectively. The results presented here reveal that pentaborides can provide a framework for exploring and designing novel superconducting materials with favorable hardness at achievable pressures and even under ambient conditions.

Recently, room-temperature superconductivity has been reported in a nitrogen-doped lutetium hydride at near-ambient pressure [Dasenbrock-Gammon et al., Nature 615, 244 (2023)]. The superconducting properties might arise from Fm3̄m-LuH_3-δN_ε. Here, we systematically study the phase diagram of Lu–N–H at 1 GPa using first-principles calculations, and we do not find any thermodynamically stable ternary compounds. In addition, we calculate the dynamic stability and superconducting properties of N-doped Fm3̄m-LuH₃ using the virtual crystal approximation (VCA) and the supercell method. The R3m-Lu₂H₅N predicted using the supercell method could be dynamically stable at 50 GPa, with a T_c of 27 K. According to the VCA method, the highest T_c is 22 K, obtained with 1% N-doping at 30 GPa. Moreover, the doping of nitrogen atoms into Fm3̄m-LuH₃ slightly enhances T_c, but raises the dynamically stable pressure. Our theoretical results show that the T_c values of N-doped LuH₃ estimated using the Allen–Dynes-modified McMillan equation are much lower than room temperature.

Synthesis pressure and structural stability are two crucial factors for highly energetic materials, and recent investigations have indicated that cerium is an efficient catalyst for N₂ reduction reactions. Here, we systematically explore Ce–N compounds through first-principles calculations, demonstrating that the cerium atom can weaken the strength of the N≡N bond and that a rich variety of cerium polynitrides can be formed under moderate pressure. Significantly, P1̄-CeN₆ possesses the lowest synthesis pressure of 32 GPa among layered metal polynitrides owing to the strong ligand effect of cerium. The layered structure of P1̄-CeN₆ proposed here consists of novel N₁₄ ring. To clarify the formation mechanism of P1̄-CeN₆, the reaction path Ce + 3N₂ → trans-CeN₆ → P1̄-CeN₆ is proposed. In addition, P1̄-CeN₆ possesses high hardness (20.73 GPa) and can be quenched to ambient conditions. Charge transfer between cerium atoms and N₁₄ rings plays a crucial role in structural stability. Furthermore, the volumetric energy density (11.20 kJ/cm³) of P1̄-CeN₆ is much larger than that of TNT (7.05 kJ/cm³), and its detonation pressure (128.95 GPa) and detonation velocity (13.60 km/s) are respectively about seven times and twice those of TNT, and it is therefore a promising high-energy-density material.

High-pressure metal hydride (MH) research evolved into a thriving field within condensed matter physics following the realization of metallic compounds showing phonon mediated near room-temperature superconductivity. However, severe limitations in determining the chemical formula of the reaction products, especially with regards to their hydrogen content, impedes a deep understanding of the synthesized phases and can lead to significantly erroneous conclusions. Here, we present 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. We show that this method can be used to investigate MH compounds with a wide range of hydrogen content, from MH_x with x = 0.15 (CuH_0.15) to x ≲ 6.4 (H_6±0.4S₅).

Clathrate-like or caged compounds have attracted great interest owing to their structural flexibility, as well as their fertile physical properties. Here, we report the pressure-induced reemergence of superconductivity in BaIr₂Ge₇ and Ba₃Ir₄Ge₁₆, two new caged superconductors with two-dimensional building blocks of cage structures. After suppression of the ambient-pressure superconducting (SC-I) states, new superconducting (SC-II) states emerge unexpectedly, with T_c increased to a maximum of 4.4 and 4.0 K for BaIr₂Ge₇ and Ba₃Ir₄Ge₁₆, respectively. Combined with high-pressure synchrotron x-ray diffraction and Raman measurements, we propose that the reemergence of superconductivity in these caged superconductors can be ascribed to a pressure-induced phonon softening linked to cage shrinkage.

Polymeric nitrogen has attracted much attention owing to its possible application as an environmentally safe high-energy-density material. Based on a crystal structure search method accelerated by the use of machine learning and graph theory and on first-principles calculations, we predict a series of metal nitrides with chain-like polynitrogen (P2₁-AlN₆, P2₁-GaN₆, P-1-YN₆, and P4/mnc-TiN₈), all of which are estimated to be energetically stable below 40.8 GPa. Phonon calculations and ab initio molecular dynamics simulations at finite temperature suggest that these nitrides are dynamically stable. We find that the nitrogen in these metal nitrides can polymerize into two types of poly-N42- chains, in which the π electrons are either extended or localized. Owing to the presence of the polymerized N₄ chains, these metal nitrides can store a large amount of chemical energy, which is estimated to range from 4.50 to 2.71 kJ/g. Moreover, these compounds have high detonation pressures and detonation velocities, exceeding those of conventional explosives such as TNT and HMX.

A structural search leads to the prediction of a novel alkaline earth nitride BeN₄ containing a square planar N₄^2- ring. This compound has a particular chemical bonding pattern giving it potential as a high-energy-density material. The P4/nmm phase of BeN₄ may be stable under ambient conditions, with a bandgap of 3.72 eV. It is predicted to have high thermodynamic and kinetic stability due to transfer of the outer-shell s electrons of the Be atom to the N₄ cluster, with the outer-shell 2p orbital accommodating the lone-pair electrons of N₄^2-. The total of six π electrons is the most striking feature, indicating that the square planar N₄^2- exhibits aromaticity. Under ambient conditions, BeN₄ has a high energy density (3.924 kJ/g relative to Be₃N₂ and N₂ gas), and its synthesis might be possible at pressures above 31.6 GPa.