Liu Zhongyuan

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.

Double spin-glass-like behavior and antiferromagnetic superexchange interaction between Fe3+ ions in α-Ga2−xFexO3 (0 ≤ x ≤ 0.4)

Single phase of Fe3+-doped α-Ga2−xFexO3 (α-GFxO, x = 0.1, 0.2, 0.3, 0.4) is synthesized by treating the β-Ga2−xFexO3 (β-GFxO) precursors at high temperatures and high pressures. Rietveld refinements of the X-ray diffraction data show that the lattice constants increase monotonically with the increase of Fe3+ content. Calorimetric measurements show that the temperature of the phase transition from α-GFxO to β-GFxO increases, while the associated enthalpy change decreases upon increasing Fe3+ content. The optical energy gap deduced from the reflectance measurement is found to decrease monotonically with the increase in Fe3+ content. From the measurements of magnetic field-dependent magnetization and temperature-dependent inverse molar susceptibility, we find that the superexchange interaction between Fe3+ ions is antiferromagnetic. Remnant magnetization is observed in the Fe3+-doped α-GFxO and is attributed to the spin glass in the magnetic sublattice. At high Fe3+ doping level (x = 0.4), two evident peaks are observed in the image part of the AC susceptibility . The frequency dependence in intensity of these two peaks as well as two spin freezing temperatures observed in the DC magnetization measurements of α-GF0.4O is suggested to be the behavior of two spin glasses.