The knowledge of stoichiometries of alkaline-earth metal nitrides, where nitrogen can exist in polynitrogen forms, is of significant interest for understanding nitrogen bonding and its applications in energy storage. For calcium nitrides, there were three known crystalline forms, CaN2, Ca2N, and Ca3N2, at ambient conditions. In the present study, we demonstrated that there are more stable forms of calcium nitrides than what is already known to exist at ambient and high pressures. Using a global structure searching method, we theoretically explored the phase diagram of CaNx and discovered a series of new compounds in this family. In particular, we found a new CaN phase that is thermodynamically stable at ambient conditions, which may be synthesized using CaN2 and Ca2N. Four other stoichiometries, namely, Ca2N3, CaN3, CaN4, and CaN5, were shown to be stable under high pressure. The predicted CaNx compounds contain a rich variety of polynitrogen forms ranging from small molecules (N2, N4, N5, and N6) to extended chains (N∞). Because of the large energy difference between the single and triple nitrogen bonds, dissociation of the CaNx crystals with polynitrogens is expected to be highly exothermic, making them as potential high-energy-density materials.

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A new allotrope of nitrogen in which the atoms are connected to form a novel N6 molecule is predicted to exist at ambient conditions. The N6 molecule is a charge-transfer complex with an open-chain structure containing both single and triple bonds. The charge transfer induces ionic characteristics in the intermolecular interactions and leads to a much higher cohesive energy for the predicted crystal compared to solid N2. The N6 solid is also more stable than a previously reported polymeric solid of nitrogen. Because of the kinetic stability of the molecules and strong intermolecular interactions, the N6 crystal is shown by metadynamics simulations to be dynamically stable around room temperature and to only dissociate to N2 molecules above 700 K. The N6 crystal can likely be synthesized under high-pressure high-temperature conditions, and the considerable metastability may allow for an ambient-pressure recovery of the crystal. Because of the large energy difference between the single and triple bonds, the dissociation of the N6 crystal is expected to release a large amount of energy, placing it among the most efficient energy materials known today.

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Hydrostatic pressure is an effective tool that can give rise to novel crystal structures and physical properties. It has proven to be an alternative to chemical pressure. Therefore new functional...

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An allotrope of nitrogen formed solely by N−N single bonds is predicted to exist between 100 and 150 GPa. The crystal structure has the Pccn symmetry and is characterized by a distorted tetrahedral network consisting of fused N 8 , N 10 , and N 12 rings. Stability of this structure is established by phonon and vibrational free energy calculations at 0 K and finite temperatures. Simulated x-ray diffraction pattern of the Pccn phase is compared to the pattern of a recently synthesized nitrogen phase at the same P-T conditions, which suggests that the Pccn phase is likely a minor component of the latter. The Pccn phase is expected to form above the stability field of cubic gauche (cg) phase. The outstanding metastability of this phase is attributed to the intrinsic stability of the sp 3 bonding as well as the energetically favorable dihedral angles between N−N single bonds, in either gauche or trans conformation. The prediction of another single-bonded phase of nitrogen after the lab-synthesized cg phase will stimulate research on metastable phases of nitrogen and their applications as high-energy-density materials.

Transport properties like diffusivity and viscosity of melts dictated the evolution of the Earth’s early magma oceans. We report the structure, density, diffusivity, electrical conductivity and viscosity of a model basaltic (Ca11Mg7Al8Si22O74) melt from first-principles molecular dynamics calculations at temperatures of 2200 K (0 to 82 GPa) and 3000 K (40–70 GPa). A key finding is that, although the density and coordination numbers around Si and Al increase with pressure, the Si–O and Al–O bonds become more ionic and weaker. The temporal atomic interactions at high pressure are fluxional and fragile, making the atoms more mobile and reversing the trend in transport properties at pressures near 50 GPa. The reversed melt viscosity under lower mantle conditions allows new constraints on the timescales of the early Earth’s magma oceans and also provides the first tantalizing explanation for the horizontal deflections of superplumes at ~1000 km below the Earth’s surface.