The search for high-temperature superconductors has been focused on compounds containing a large fraction of hydrogen, such as SiH₄(H₂)₂, CaH₆ and KH₆. Through a systematic investigation of yttrium hydrides at different hydrogen contents using an structure prediction method based on the particle swarm optimization algorithm, we have predicted two new yttrium hydrides (YH₄ andYH₆), which are stable above 110 GPa. Three types of hydrogen species with increased H contents were found, monatomic H in YH₃, monatomic H+molecular “H₂” in YH₄ and hexagonal “H₆” unit in YH₆. Interestingly, H atoms in YH₆ form sodalite-like cage sublattice with centered Y atom. Electron-phonon calculations revealed the superconductive potential of YH₄ and YH₆ with estimated transition temperatures (T_c) of 84–95 K and 251–264 K at 120 GPa, respectively. These values are higher than the predicted maximal T_c of 40 K in YH₃.

Polymeric nitrogen, stabilized by compressing pure molecular nitrogen, has yet to be recovered to ambient conditions, precluding its application as a high-energy density material. Here we suggest a route for synthesis of a tetragonal polymeric nitrogen, denoted t-N, via He-N compounds at high pressures. Using first-principles calculations with structure searching, we predict a class of nitrides with stoichiometry HeN₄ that are energetically stable (relative to a mixture of solid He and N₂) above 8.5 GPa. At high pressure, HeN₄ comprises a polymeric channel-like nitrogen framework filled with linearly arranged helium atoms. The nitrogen framework persists to ambient pressure on decompression after removal of helium, forming pure polymeric nitrogen, t-N. t-N is dynamically and mechanically stable at ambient pressure with an estimated energy density of ~11.31 kJ/g, marking it out as a remarkable high-energy density material. This expands the known polymeric forms of nitrogen and indicates a route to its synthesis.

The pressure-induced transformation of diatomic nitrogen into nonmolecular polymeric phases may produce potentially useful high-energy-density materials. We combine first-principles calculations with structure searching to predict a new class of nitrogen-rich boron nitrides with a stoichiometry of ${\mathrm{B}}_3{\mathrm{N}}_5$ that are stable or metastable relative to solid ${\mathrm{N}}_2$ and $h$-BN at ambient pressure. The most stable phase at ambient pressure has a layered structure ($h{\text{−}\mathrm{B}}_3{\mathrm{N}}_5$) containing hexagonal ${\mathrm{B}}_3{\mathrm{N}}_3$ layers sandwiched with intercalated freely rotating ${\mathrm{N}}_2$ molecules. At 15 GPa, a three-dimensional $C222_1$ structure with single N-N bonds becomes the most stable. This pressure is much lower than that required for triple-to-single bond transformation in pure solid nitrogen (110 GPa). More importantly, $C222_1\text{−}{\mathrm{B}}_3{\mathrm{N}}_5$ is metastable, and can be recovered under ambient conditions. Its energy density of $\sim 3.44\mathrm{kJ}/\mathrm{g}$ makes it a potential high-energy-density material. In addition, stress-strain calculations estimate a Vicker’s hardness of $\sim 44\mathrm{GPa}$. Structure searching reveals a new clathrate sodalitelike BN structure that is metastable under ambient conditions.

While exploring potential superconductors in the C-S-H ternary system using first-principles crystal structure prediction methods, we uncovered a class of hydride perovskites based on the intercalation of methane into an ${\mathrm{H}}_3\mathrm{S}$ framework. These intriguing ${\mathrm{H}}_3\mathrm{S}\text{−}{\mathrm{CH}}_4$ structures emerge as metastable at $\sim 100$ GPa. Electron-phonon coupling calculations indicate that phases with ${\mathrm{CSH}}_7$ stoichiometry are potential superconductors with $T_c$ values ranging from 100 K to 190 K at megabar pressures. The results are expected to guide the experimental search for new high-$T_c$ superconductors, including those stable at lower pressures than previously documented superconducting hydrides such as ${\mathrm{H}}_3\mathrm{S}$ and ${\mathrm{LaH}}_{10}$.

Controlling magnetism of two-dimensional multiferroics by an external electric field provides special opportunities for both fundamental research and future development of low-cost electronic nanodevices. Here, we report a general scheme for realizing a magnetic phase transition in 2D type-I multiferroic systems through the reversal of ferroelectric polarization. Based on first-principles calculations, we demonstrate that a single-phase 2D multiferroic, namely, ${\mathrm{ReWCl}}_6$ monolayer, exhibits two different low-symmetric ($C_2$) phases with opposite in-plane electric polarization and different magnetic order. As a result, an antiferromagnetic-to-ferromagnetic phase transition can be realized by reversing the in-plane electric polarization through the application of an external electric field. These findings not only enrich the 2D multiferroic family, but also uncover a unique and general mechanism to control magnetism by electric field, thus stimulating experimental interest.