A systematic computational study on the structural, electronic, and bonding properties of binary sulfur nitrides has been performed using the projector augmented wave method based on density functional theory. The pressure–composition phase diagram of the S–N system has been established. The simulated pressure–temperature phase diagram and X-ray diffraction pattern of (SN) x explain the experimentally observed two-phase coexistence. The crystal structure of experimentally observed orthorhombic (SN) x is predicted. The high-pressure phase transition of (SN) x has been studied. Sulfur–sulfur interactions induced by localized sulfur 3p z electrons are found in the high-pressure phase of (SN) x . With increasing nitrogen composition, the coordination number of sulfur atoms increases from two to six in the S–N system. Furthermore, two nitrogen-rich sulfur nitrides SN2 and SN4 have been found at high pressure. SN4 exhibits a high energy density (2.66 kJ·g–1), which makes it potentially interesting for industrial applications as a high energy density material.

A novel carbon allotrope C96 with a low density and porous nanocube network has been predicted.

The evolutionary structure-searching method discovers that the energetically preferred compounds of germane can be synthesized at a pressure of 190 GPa. New structures with the space groups Ama2 and C2/c proposed here contain semimolecular H2 and V-type H3 units, respectively. Electronic structure analysis shows the metallic character and charge transfer from Ge to H. The conductivity of the two structures originates from the electrons around the hydrogen atoms. Further electron-phonon coupling calculations predict that the two phases are superconductors with a high Tc of 47-57 K for Ama2 at 250 GPa and 70-84 K for C2/c at 500 GPa from quasi-harmonic approximation calculations, which may be higher than under actual conditions.

Crystal structures of silane have been extensively investigated using ab initio evolutionary simulation methods at high pressures. Two metallic structures with P21/c and C2/m symmetries are found stable above 383 GPa. The superconductivities of metallic phases are fully explored under BCS theory, including the reported C2/c one. Perturbative linear-response calculations for C2/m silane at 610 GPa reveal a high superconducting critical temperature that beyond the order of 102 K.

A systematic computational study on the crystal structure of n-diamond has been performed using first-principle methods. A novel carbon allotrope with hexagonal symmetry R32 space group has been predicted. We name it as HR-carbon. HR-carbon composed of lonsdaleite layers and unique C3 isosceles triangle rings, is stable over graphite phase above 14.2 GPa. The simulated x-ray diffraction pattern, Raman, and energy-loss near-edge spectrum can match the experimental results very well, indicating that HR-carbon is a likely candidate structure for n-diamond. HR-carbon has an incompressible atomic arrangement because of unique C3 isosceles triangle rings. The hardness and bulk modulus of HR-carbon are calculated to be 80 GPa and 427 GPa, respectively, which are comparable to those of diamond. C3 isosceles triangle rings are very important for the stability and hardness of HR-carbon.

Tantalum-boron compounds, which are potential candidates for superhard multifunctional materials, may possess multiple stoichiometries and structures under pressure. Using first-principle methods, ground-state TaB3 with the monoclinic C2/m space group and high-pressure TaB4 with the orthorhombic Amm2 space group have been found. They are more stable than the previously proposed structures. High-pressure boron-rich Amm2-TaB4 can be quenched to ambient pressure. The ground-state C2/m-TaB3 and high-pressure Amm2-TaB4 are two potential ultra-incompressible and hard materials with a calculated hardness of 17.02 GPa and 30.02 GPa at ambient pressure, respectively. Detailed electronic structure and chemical bonding analysis proved that the high hardness value of Amm2-TaB4 mainly stems from the strong covalent boron-boron bonds in graphene-like B layers as well as B-B bonds between layers.

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Despite credible reports of several high-pressure electrides of alkali earth metal elements, high-pressure electrides composed of two elements of the alkali earth metal group remain a mystery. In this paper, taking the typical intermetallic compound BaMg 2 as a representative, we systematically investigate the highpressure electronic behavior of alkali earth metal intermetallic compounds by using first-principles calculations in conjunction of particle swarm optimization. BaMg 2 undergoes phase transitions from the ambient P6 3 /mmc phase to the high-pressure P4/nmm phase at 19.6 GPa and then to the high-pressure I4/mmm phase at 260 GPa, accomplishing an electron state transformation from nonelectride to electride. Although some electrons are trapped in the interstitial lattice of BaMg 2 , the high-pressure P4/nmm and I4/mmm phases exhibit different anionic electron-dominated metallicity. Although Mg and Ba have similar valence electron configurations, the electride state of BaMg 2 is completely dominated by Mg atoms because its electride state orbital has more favorable energy than the atomic orbital of Mg under high pressure. Different from that of Mg, the d orbitals of Ba have energy advantages over its electride state. This work not only enriches the knowledge of the family of electrides but also provides new insight into the electride formation mechanism.