Shuli Wei

Cubic C96: a novel carbon allotrope with a porous nanocube network

A novel cubic porous carbon allotrope C96 carbon with intriguing physical properties was predicted. It has 96 atoms in the conventional cell, possessing a Pmm space group. The basic building block of C96 carbon is a planar six-membered carbon ring. The structural stability, mechanical properties, and dynamical properties of C96 carbon were extensively studied. It is a semiconductor (1.85 eV) with a lower density (2.7 g cm−3) and a larger bulk modulus (279 GPa) and is stable under ambient conditions. The hardness of C96 carbon (25 GPa) is larger than that of T carbon (5.6 GPa). Due to the structural porous feature and lower density, C96 carbon can also be expected to be a good hydrogen storage material.

Bonding Properties of Aluminum Nitride at High Pressure

Exploring the bonding properties and polymerization mechanism of stable polymeric nitrogen phases is the main goal of our high-pressure study. The pressure versus composition phase diagram of the Al-N system is established. In addition to the known Fm3̅m phase of AlN, a notable monoclinic phase with N66- anion polymeric nitrogen chains for AlN3 in the pressure range from 43 to 85 GPa is predicted. Its energy density is up to 2.75 kJ·g-1, and the weight ratio of nitrogen is nearly 61%, which make it potentially interesting for the industrial applications as a high energy density material. The high-pressure studies of atomic and electronic structures in this predicted phase reveal that the formation of N66- anion is driven by the sp2 hybridization of nitrogen atoms. The resonance effect between alternating π-bonds and σ-bonds in polymeric nitrogen chains are all responsible for the structural stability. Because of the electrons transfer from aluminums to polymeric nitrogen chains, there is a pseudogap in the electronic structures of AlN3. The N_p electrons form π-type chemical bonds with the neighboring atoms, resulting in the delocalization of π electrons and charge transfer in polymeric nitrogen chains. Furthermore, disparities of charge density distribution between nitrogen atoms in polymeric nitrogen chains are the principal reason for the metallicity.

A first-principles investigation of a new hard multi-layered MnB2 structure

ReB2-type MnB2 has always been considered to be the ground-state structure of MnB2. However, subsequent theoretical study has revealed that this structure is easy to decompose into elemental Mn and B under ambient conditions, which motivated us to look for a stable MnB2 structure at high pressures. Using structure prediction algorithm USPEX and density functional theory calculations, we found a stable multi-layered MnB2 structure with space group Immm at high pressure. The calculated hardness of Immm-MnB2 is 22.5 GPa, which makes it a potential hard multifunctional material along with its conductive and magnetic properties. The hexagonal graphene-like boron networks of Immm-MnB2 contribute to its hardness and stability.

Structural phase transition and bonding properties of high-pressure polymeric CaN3

Alkaline-earth metal polynitrides can be used as a type of starting materials in the synthesis of polymeric nitrogen, which is used as a potential high-energy-density material. The structural evolutionary behaviors of nitrogen in CaN3 were studied at the pressure up to 100 GPa using a particle-swarm optimization structure search combining with density functional theory. Two new stable phases with P and C2/m space groups at the pressures of 26 and 60 GPa were identified for the first time. Throughout the stable pressure range, these two structures are semiconductors and consist of the N atoms in sp2-hybrid states. To the best of our knowledge, this is the first time an N6 chain is reported in case of alkaline-earth metal polynitrides. The stable polynitrogen compounds and polymeric nitrogen as high-energy-density materials have potential applications. The present results open a new possible avenue to synthesize high-energy-density polynitrogen.

Insights into Antibonding Induced Energy Density Enhancement and Exotic Electronic Properties for Germanium Nitrides at Modest Pressures

Here, the electronic and bonding features in ground-state structures of germanium nitrides under different components that not accessible at ambient conditions have been systematically studied. The forming essence of weak covalent bonds between the Ge and N atom in high-pressure ionic crystal Fd-3 m-Ge3N4 is induced by the binding effect of electronic clouds originated from the Ge_ p orbitals. Hence, it helps us to understand the essence of covalent bond under high pressure, profoundly. As an excellent reducing agent, germanium transfer electrons to the antibonding state of the N2 dimer in Pa-3-GeN2 phase at 20 GPa, abnormally, weakening the bonding strength considerably than nitrogen gap (N≡N) at ambient pressure. Furthermore, the common cognition that the atomic distance will be shortened under the high pressures has been broken. Amazingly, with a lower range of synthetic pressure (∼15 GPa) and nitrogen contents (28%), its energy density is up to 2.32 kJ·g-1, with a similar order of magnitude than polymeric LiN5 (nonmolecular compound, 2.72 kJ·g-1). It breaks the universal recognition once again that nitrides just containing polymeric nitrogen were regarded as high energy density materials. Hence, antibonding induced energy density enhancement mechanism for low nitrogen content and pressure has been exposed in view of electrons. Both the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) are usually the separated orbitals of N_π* and N_σ*, which are the key to stabilization. Besides, the sp2 hybridizations that exist in N4 units are responsible for the stability of the R-3 c-GeN4 structure and restrict the delocalization of electrons, exhibiting nonmetallic properties.

The stability of B6 octahedron in BaB6 under high pressure

We have performed in situ synchrotron X-ray diffraction and first-principles calculations to explore the compression behavior of barium hexaboride (BaB6) under high pressure. No phase transitions in our experiment are observed up to 49.3 GPa at ambient temperature. It is found that the ambient cage structure (Pmm) is still stable with a basic covalent network during the experimental pressure run. The results of our theoretical calculations show that the ambient structure might transform into three dynamically stable structures (Cmmm, Cmcm and I4/mmm) at 78 GPa, 97 GPa and 105 GPa respectively. The energy band calculations indicate that the sample is still a semiconductor with a narrow gap at 50 GPa.