A strategy to synthesize a 2D graphenic but ternary monolayer containing atoms of carbon, nitrogen, and boron, h-BCN, is presented. The synthesis utilizes bis-BN cyclohexane, B₂N₂C₂H₁₂, as a precursor molecule and relies on thermally induced dehydrogenation of the precursor molecules and the formation of an epitaxial monolayer on Ir(111) through covalent bond formation. The lattice mismatch between the film and substrate causes a strain-driven periodic buckling of the film. The structure of the film and its corrugated morphology is discussed based on comprehensive data from molecular-resolved scanning tunneling microscopy imaging, X-ray photoelectron spectroscopy, low-energy electron diffraction, and density functional theory. First-principles calculations further predict a direct electronic band gap that is intermediate between gapless graphene and insulating h-BN.

Evolutionary structure searches are used to predict stable phases with unique stoichiometries in the hydrogen-rich region of the magnesium/hydrogen phase diagram under pressure. MgH${}_4$, MgH${}_{12}$, and MgH${}_{16}$ are found to be thermodynamically stable with respect to decomposition into MgH${}_2$ and H${}_2$ near 100 GPa, and all lie on the convex hull by 200 GPa. MgH${}_4$ contains two H${}^{-}$ anions and one H${}_2$ molecule per Mg${}^{2+}$ cation, whereas the hydrogenic sublattices of MgH${}_{12}$ and MgH${}_{16}$ are composed solely of H${}_2^{\delta -}$ molecules. The high-hydrogen content stoichiometries have a large density of states at the Fermi level, and the $T_c$ of MgH${}_{12}$ at 140 GPa is calculated to be nearly three times greater than that of the classic hydride, MgH${}_2$, at 180 GPa.

A phase of nonmolecular nitrogen, found by first-principles density functional theory simulations at high pressures is reported. The phase has a $P{2}_1∕m$ symmetry and consists of two-dimensional layers of fused ${\mathrm{N}}_6$-rings where the rings are all in the boat conformation. The phase has been designated the layered boat or LB phase. Properties of LB are presented in comparison with other computationally studied polymeric phases of nitrogen. In particular, the structure of LB is compared to two other layered phases, black-phosphorus (BF) and $\alpha$-arsenic $\left(A7\right)$. A calculated enthalpy versus pressure phase diagram reveals that LB lies between BF and $A7$ in enthalpy at pressures less than $200\mathrm{GPa}$. At pressures higher than this, the enthalpy of LB edges above that of $A7$. The metastability of LB at low pressures is probed by a few simple tests and the band structure is calculated.

Recognition of helical structural motifs in the experimentally observed cubic gauche (CG) crystal lattice has led to the discovery of a single-bonded nonlayered nitrogen structure that we have named chaired web (CW). First-principles density functional theory calculations reveal that CW, which was originally identified at high pressures, possesses metastability at ambient conditions as well. The metastability is demonstrated by both high-quality phonon dispersion calculations and finite-temperature first-principles molecular dynamics simulations. In addition, the CW phase is thermodynamically more stable than the CG phase in the ambient pressure regime.