Solid hydrogen sulfide is well known as a typical molecular crystal but its stability under pressure is still under debate. Particularly, Eremets et al. found the high pressure superconductivity with $T_{c}\approx$ 190 K in a H$_{2}$S sample [arXiv: 1412.0460 (2014)] which is associates with the elemental decomposition into H$_{3}$S [Sci. Rep. 4, 6968 (2014)]. Therefore, on what pressure H$_{2}$S can decompose and which kind of the products of decomposition urgent need to be solved. In this paper, we have performed an extensive structural study on different stoichiometries H$_{n}$S with ${n> 1}$ under high pressure using $ab$ $initio$ calculations. Our results show that H$_{2}$S is stable below 50 GPa and decomposes into H$_3$S and sulfur at high pressure, while H$_{3}$S is stable at least up to 300 GPa. The other hydrogen-rich H$_{4}$S, H$_{5}$S, and H$_{6}$S are unstable in the pressure range from 20 to 300 GPa

Based on an ab initio evolutionary algorithm, a novel carbon polymorph with an orthorhombic Cmcm symmetry is predicted, named as C carbon, which has the lowest enthalpy among the previously proposed cold-compressed graphite phases.

Stabilization of the pentazole anion only by acidic circumstances entrapment impedes the realization of a full-nitrogen substance; however, compression of nitrogen-rich nitrides has been recommend as an alternative way that has more controllable advantages to acquire the atomic nitrogen states. Through the structure searches are in conjunction with first-principle calculations, moderate pressure stabilized nitrogen-rich zinc nitrides with abundant extended nitrogen structures, e.g., cyclo-N 5 , infinite −(N 4 ) n − chains, three-point stars N(N 3 ), and N 2 dumbbells, are predicted. The resonance between alternating σ bonds and π bonds in poly nitrogen sublattices takes charge of the coexistence of single and double bonds. The Zn(N 5 ) 2 salt has a noteworthy energy density (6.57 kJ/g) among the reported binary metal nitrides and synthesized pentazolate hydrates. An excellent Vicker's hardness (34 GPa) and detonation performance is unraveled. Although Zn(N 5 ) 2 salt is not expected to be recoverable at ambient conditions, it is worth noting that Zn(N 5 ) 2 is found to be stable at a very low pressure of ∼30 GPa, which is only half of those pressures required to synthesize CsN 5 . We clarified that the metalcentering octahedral pentazolate framework was entrapped by dual ionic−covalent bonds. More importantly, the covalent bonding can effectively enhance the chemical insensitivity and thermal stability, further preventing the autodecomposition of monatomic solid N 5 − anions into dinitrogen. Meanwhile, a unique topological pseudogap that attached to a metastable phase of ZnN 4 salt is exposed for the first time, due to the dual effects of strong covalent sp 2 hybridization interaction and the origin of ionic states.

We report on a first-principles study of the phase diagram, structures and properties of the Ru-H system in the H-rich regime over a wide range of pressures. The results show that RuH is thermodynamically stable and can coexist with RuH3 and RuH6 under pressure. RuH and RuH3 stoichiometries exhibit metallic character as a result of notable band structures, while RuH6 is a semiconductor. Strikingly, some hydrogen atoms pairwise couple into H2 units in the RuH6 compound. An estimation of superconducting transition temperature Tc is carried out by applying the Allen-Dynes modified McMillan equation for Fm3[combining macron]m (RuH), Pm3[combining macron]m (RuH3), and Pm3[combining macron]n (RuH3) structures and the resulting Tc reaches 0.41, 3.57 and 1.25 K at different pressures, respectively.