Yu Dongli

First-Principles Calculations of Phase Transition and Stability of Si2CN4 under High Pressure

A pressure-induced phase transition and stability in Si2CN4 polymorphs under high pressure are studied by first-principles calculations. The result shows that the phase transition pressure of α- and β-Si2CN4 to the cubic spinal phase is 29.9 GPa and 27.5 GPa predicted by thermodynamic method respectively. Under ambient condition, all of the three Si2CN4 polymorphs are metastable with positive formation enthalpy. Unlike the stability of Si3N4 polymorphs, α-Si2CN4 is more stable than the β phase.

Theoretical Hardness of Wurtzite-Structured Semiconductors

Vickers hardness calculations of eleven wurtzite-structured semiconductors are performed based on the microscopic hardness model. All the parameters are obtained from first-principles calculations. There are two types of chemical bonds in wurtzite-structured crystals. The overlap populations of the two types of chemical bonds in lonsdaleite are chosen as Pc for wurtzite structure. The calculated bond ionicity values of the wurtzite-structured semiconductors are in good agreement with the ionicities from the dielectric definition. When the hardness of wurtzite-structured crystal is higher than 20GPa, our calculated Vickers hardness is within 10% accuracy. Therefore, the hardness of novel wurtzite-structured crystal could be estimated from first-principles calculations.

Chemical Synthesis of C3N and BC2N Compounds

A chemical reaction for the preparation of B–C–N compounds by using carbon tetrachloride (CCl4), boron tribromide (BBr3), lithium nitride (Li3N) and sodium as reactants has been carried out at the temperature of 400°C. Measurements of FTIR, XRD, TEM and EELS show that two kinds of compounds have been formed in the prepared sample. One is hollow sphere-like C–N with an amorphous structure; the other is piece-like polycrystalline B–C–N with the hexagonal structure. Their determined compositions are close to C3N and BC2N, respectively.

Ab Initio Study of Structural and Electronic Properties of Hexagonal BC2N

We investigate hexagonal BC2N in graphite unit cells using the first-principles method and calculate the total energies, lattice parameters, and electronic band structures after full relaxation. It is shown that stable hexagonal BC2N should be stacked sequentially with one graphite layer and one h-BN layer. The density of states indicates that this structure should have metallicity.

Investigations on ternary B-C-N materials

Isoelectronic BCxN compounds have been researched widely. However, electron-deficient boron-rich B-C-N solids have also attracted much interest both theoretically and experimentally. In this paper, we introduce the synthesis, theoretical prediction, and physical properties of crystalline ternary B-C-N compounds. Our recent work reveals that the novel B-C-N materials may have a wide variety of crystal structures with different characteristics.

Synthesis of Boron-Rich Cubic B(CxN1−x) Compounds

B2CN precursor is prepared by a mechanical vibration-milling process using amorphous boron, graphite and h-BN powders with mole ratio of 1:1:1. A mixture of precursor and Ca3B2N4 catalyst is treated under high pressure and high temperature. A boron rich cubic B(CxN1−x) phase is obtained after removing the catalyst by acid treatment. The average C content of the boron-rich cubic phase is about 6 at.% detected by energy-dispersive x-ray analysis spectroscopy. It is found that the highest carbon content in the cubic phase is as large as 16 at.%.