Huafeng Dong

The phase diagram and hardness of carbon nitrides

Novel superhard materials, especially those with superior thermal and chemical stability, are needed to replace diamond. Carbon nitrides (C-N), which are likely to possess these characteristics and have even been expected to be harder than diamond, are excellent candidates. Here we report three new superhard and thermodynamically stable carbon nitride phases. Based on a systematic evolutionary structure searches, we report a complete phase diagram of the C-N system at 0–300 GPa and analyze the hardest metastable structures. Surprisingly, we find that at zero pressure, the earlier proposed graphitic-C3N4 structure () is dynamically unstable, and we find the lowest-energy structure based on s-triazine unit and s-heptazine unit.

Improving the optical absorption of BiFeO3 for photovoltaic applications via uniaxial compression or biaxial tension

First-principles computations are employed to investigate the electronic structures and optical absorption of rhombohedral BiFeO3 under uniaxial compression and biaxial tension. We find that the bandgap of BiFeO3 is reduced under uniaxial compression, and it can be tuned to the ideal value for photovoltaic applications; furthermore, the indirect-to-direct bandgap transition occurs, which would lead to much enhanced optical absorption near the band edge. Similar results are found for biaxial tensile strain. Strong optical absorption is critical to build efficient solar cells based on ferroelectric thin films; strain engineering is thus a practical route towards realizing this scheme, in which no junction is needed to separate charge carriers.

Novel lithium-nitrogen compounds at ambient and high pressures

Using ab initio evolutionary simulations, we predict the existence of five novel stable Li-N compounds at pressures from 0 to 100 GPa (Li13N, Li5N, Li3N2, LiN2, and LiN5). Structures of these compounds contain isolated N atoms, N2 dimers, polyacetylene-like N chains and N5 rings, respectively. The structure of Li13N consists of Li atoms and Li12N icosahedra (with N atom in the center of the Li12 icosahedron) – such icosahedra are not described by Wade-Jemmis electron counting rules and are unique. Electronic structure of Li-N compounds is found to dramatically depend on composition and pressure, making this system ideal for studying metal-insulator transitions. For example, the sequence of lowest-enthalpy structures of LiN3 shows peculiar electronic structure changes with increasing pressure: metal-insulator-metal-insulator. This work also resolves the previous controversies of theory and experiment on Li2N2.

Elastic properties of tetragonal BiFeO3 from first-principles calculations

Multiferroic BiFeO3 can exist in tetragonal G-type and C-type antiferromagnetic phases with a giant c/a ratio and polarizability. In this letter, the elastic constants cijs of these tetragonal BiFeO3 phases are studied as the function of pressure using first-principles density-functional theory. We find that, except for c44, the predicted cijs decrease with decreasing pressure (or increasing volume). When the volume is less than 11 A3/atom (or greater than 17 A3/atom), the c44 of these tetragonal phases tend to zero and the structures become unstable. The tetragonal phases are predicted to be softer than the rhombohedral antiferromagnetic phase. Other elastic properties, including bulk modulus, shear modulus, Youngs modulus, Poissons ratio, and elastic anisotropy ratios are also investigated.

Pressure-induced novel compounds in the Hf-O system from first-principles calculations

Using first-principles evolutionary simulations, we have systematically investigated phase stability in the Hf-O system at pressure up to 120 GPa. New compounds Hf5O2, Hf3O2, HfO and HfO3 are discovered to be thermodynamically stable at certain pressure ranges and a new stable high-pressure phase is found for Hf2O with space group Pnnm and anti-CaCl2-type structure. Both P62m-HfO and P4m2-Hf2O3 show semimetallic character. Pnnm-HfO3 shows interesting structure, simultaneously containing oxide O2- and peroxide [O-O]2- anions. Remarkably, it is P62m-HfO rather than OII-HfO2 that exhibits the highest mechanical characteristics among Hf-O compounds. Pnnm-Hf2O, Imm2-Hf5O2, P31m-Hf2O and P4m2-Hf2O3 phases also show superior mechanical properties, these phases can be quenched to ambient pressure and their properties can be exploited.

Nanotwinned Boron Suboxide (B6O): New Ground State of B6O

Nanotwinned structures in superhard ceramics rhombohedral boron suboxide (R-B6O) have been examined using a combination of transmission electron microscopy (TEM) and quantum mechanics (QM). QM predicts negative relative energies to R-B6O for various twinned R-B6O (denoted as τ-B6O, 2τ-B6O, and 4τ-B6O), consistent with the recently predicted B6O structure with Cmcm space group (τ-B6O) which has an energy 1.1 meV/B6O lower than R-B6O. We report here TEM observations of this τ-B6O structure, confirming the QM predictions. QM studies under pure shear deformation and indentation conditions are used to determine the deformation mechanisms of the new τ-B6O phase which are compared to R-B6O and 2τ-B6O. The lowest stress slip system of τ-B6O is (010)/⟨001⟩ which transforms τ-B6O to R-B6O under pure shear deformation. However, under indentation conditions, the lowest stress slip system changes to (001)/⟨110⟩, leading to icosahedra disintegration and hence amorphous band formation.

Prediction of a new ground state of superhard compound B6O at ambient conditions.

Boron suboxide B6O, the hardest known oxide, has an Rm crystal structure (α-B6O) that can be described as an oxygen-stuffed structure of α-boron, or, equivalently, as a cubic close packing of B12 icosahedra with two oxygen atoms occupying all octahedral voids in it. Here we show a new ground state of this compound at ambient conditions, Cmcm-B6O (β-B6O), which in all quantum-mechanical treatments that we tested comes out to be slightly but consistently more stable. Increasing pressure and temperature further stabilizes it with respect to the known α-B6O structure. β-B6O also has a slightly higher hardness and may be synthesized using different experimental protocols. We suggest that β-B6O is present in mixture with α-B6O, and its presence accounts for previously unexplained bands in the experimental Raman spectrum.

Superconductivity of novel tin hydrides (Sn n H m ) under pressure

With the motivation of discovering high-temperature superconductors, evolutionary algorithm USPEX is employed to search for all stable compounds in the Sn-H system. In addition to the traditional SnH4, new hydrides SnH8, SnH12 and SnH14 are found to be thermodynamically stable at high pressure. Dynamical stability and superconductivity of tin hydrides are systematically investigated. Im2-SnH8, C2/m-SnH12 and C2/m-SnH14 exhibit higher superconducting transition temperatures of 81, 93 and 97 K compared to the traditional compound SnH4 with Tc of 52 K at 200 GPa. An interesting bent H3–group in Im2-SnH8 and novel linear H in C2/m-SnH12 are observed. All the new tin hydrides remain metallic over their predicted range of stability. The intermediate-frequency wagging and bending vibrations have more contribution to electron-phonon coupling parameter than high-frequency stretching vibrations of H2 and H3.

Prediction of a stable post-post-perovskite structure from first principles

A novel stable crystal structure is discovered in a variety of

Explaining stability of transition metal carbides – and why TcC does not exist

AB{O}_{3},AB{F}_{3}