The discovery of superconducting H₃S with a critical temperature T_c∼200 K opened a door to room temperature superconductivity and stimulated further extensive studies of hydrogen-rich compounds stabilized by high pressure. Here, we report a comprehensive study of the yttrium-hydrogen system with the highest predicted T_cs among binary compounds and discuss the contradictions between different theoretical calculations and experimental data. We synthesized yttrium hydrides with the compositions of YH₃, YH₄, YH₆ and YH₉ in a diamond anvil cell and studied their crystal structures, electrical and magnetic transport properties, and isotopic effects. We found superconductivity in the Im-3m YH₆ and P6₃/mmc YH₉ phases with maximal T_cs of ∼220 K at 183 GPa and ∼243 K at 201 GPa, respectively. Fm-3m YH₁₀ with the highest predicted T_c> 300 K was not observed in our experiments, and instead, YH₉ was found to be the hydrogen-richest yttrium hydride in the studied pressure and temperature range up to record 410 GPa and 2250 K.

Laser heating plays an essential role for in-situ high pressure high temperature studies into the physical and chemical properties of materials in the diamond anvil cell (DAC) and minerals at conditions relevant to the Earth's deep interior. High temperature experiments in the multi-Mbar (over 100 GPa) pressure range require the use of very small samples and consequently the utmost stability and controllability of the laser heating is crucial. To accomplish this, we have modified the laser heating system at GSECARS employing newly developed beam shaping optics combined with two diode-pumped, single mode fiber lasers. Varying the settings of the laser heating system, we were able to shape the beam to almost any desired intensity profile and size on the surface of the sample in the DAC, including tight focus, flat top, trident and doughnut types. The advantages and excellent performance of the flat top laser heating (FTLH) technique were demonstrated in melting experiments on germanium in the DAC at pressures up to 40 GPa.

Encapsulating Earth's deep water filter: Small inclusions in diamonds brought up from the mantle provide valuable clues to the mineralogy and chemistry of parts of Earth that we cannot otherwise sample. Tschauner et al. found inclusions of the high-pressure form of water called ice-VII in diamonds sourced from between 410 and 660 km depth, the part of the mantle known as the transition zone. The transition zone is a region where the stable minerals have high water storage capacity. The inclusions suggest that local aqueous pockets form at the transition zone boundary owing to the release of chemically bound water as rock cycles in and out of this region. Science , this issue p. 1136

High-pressure chemistry is known to inspire the creation of unexpected new classes of compounds with exceptional properties. Here, we employ the laser-heated diamond anvil cell technique for synthesis of a Dirac material ${\mathrm{BeN}}_4$. A triclinic phase of beryllium tetranitride $\mathrm{tr}\text{−}{\mathrm{BeN}}_4$ was synthesized from elements at $\sim 85\mathrm{GPa}$. Upon decompression to ambient conditions, it transforms into a compound with atomic-thick ${\mathrm{BeN}}_4$ layers interconnected via weak van der Waals bonds and consisting of polyacetylene-like nitrogen chains with conjugated $\pi$ systems and Be atoms in square-planar coordination. Theoretical calculations for a single ${\mathrm{BeN}}_4$ layer show that its electronic lattice is described by a slightly distorted honeycomb structure reminiscent of the graphene lattice and the presence of Dirac points in the electronic band structure at the Fermi level. The ${\mathrm{BeN}}_4$ layer, i.e., beryllonitrene, represents a qualitatively new class of 2D materials that can be built of a metal atom and polymeric nitrogen chains and host anisotropic Dirac fermions.

Group V elements in crystal structure isostructural to black phosphorus with unique puckered two-dimensional layers exhibit exciting physical and chemical phenomena. However, as the first element of group V, nitrogen has never been found in the black phosphorus structure. Here, we report the synthesis of the black phosphorus–structured nitrogen at 146 GPa and 2200 K. Metastable black phosphorus–structured nitrogen was retained after quenching it to room temperature under compression and characterized in situ during decompression to 48 GPa, using synchrotron x-ray diffraction and Raman spectroscopy. We show that the original molecular nitrogen is transformed into extended single-bonded structure through gauche and trans conformations. Raman spectroscopy shows that black phosphorus–structured nitrogen is strongly anisotropic and exhibits high Raman intensities in two A_g normal modes. Synthesis of black phosphorus–structured nitrogen provides a firm base for exploring new type of high-energy-density nitrogen and a new direction of two-dimensional nitrogen.