Yong Pan

Correlation between hardness and pressure of CrB4

The correlation between hardness and pressure for two different structures of CrB4 is investigated by a first-principles approach. With increasing pressure, the hardness gradually decreases, in contrast to the bulk modulus, shear modulus, Youngs modulus and B/G ratio, which monotonically increase. Pressure gives rise to a hardness transition from a superhard to a hard material, which is in good agreement with the experimental data. Moreover, the pressure leads to a brittle-to-ductile transition at 200 GPa based on the analysis of the B/G ratio, which is consistent with the hardness trend. The analysis of the density of states and chemical bonding implies that pressure induces electronic compression and collapse in localized regions, and the variation in hardness originates from the bond reversal between the B–B (3) and B–B (2) covalent bonds, which are located at the applied load plane. Finally, we conclude that the hardness of CrB4 under pressure is related to the B/G ratio and bond characteristics.

Role of Vacancies on Electronic and Elastic Properties of RuAl2 Semiconducting Compound from First-Principles Calculations

RuAl2 is a fascinating intermetallic semiconducting compound. However, the influence of vacancies on the electronic and mechanical properties of RuAl2 is unknown. By means of first-principles calculations, we have investigated the influence of vacancies on the electronic properties, elastic modulus, brittle or ductile behavior and Vickers hardness of RuAl2. Two possible vacancy types, Ru-va and Al-va, are considered. The calculated results show that the Ru-va vacancy is more thermodynamically stable than that of the Al-va vacancy. Importantly, we find that vacancies can improve the electronic properties of RuAl2 because the removed Ru or Al atom enhances the charge overlap between conduction band and the valence band near the Fermi level. In addition, these vacancies weaken the resistance to volume deformation, shear deformation and the elastic stiffness of RuAl2 because the removed atom weakens the localized hybridization between the Ru atom and the Al atom. However, the Ru-va vacancy can improve the Vickers hardness and Al-va vacancies result in brittle-to-ductile transition of RuAl2. The variation of mechanical properties is attributed to the Ru-Al and Al-Al metallic bonds along the shear direction. Therefore, we can conclude that vacances are beneficial for improving the electronic and mechanical properties of RuAl2.

First-principles study of a new structure and oxidation mechanism of Pt3Zr

Zirconia (ZrO2)–metal interfaces are interesting for solid oxide fuel cells. Although the oxidation of Pt3Zr provides a new route for the formation of ZrO2–Pt interfaces, the crystal structure of Pt3Zr remains controversial and the oxidation mechanism of Pt3Zr is unclear. To solve these problems, we use first-principles calculations to explore the crystal structure of Pt3Zr. We demonstrate a stable structure of Pt3Zr based on phonon dispersion. Importantly, two new Pt3Zr structures, Ti3Pt-type (Pmm) and Fe3Al-type (Fmm), are predicted. To study the oxidation mechanism, two possible doped models are considered. The calculated results show that the O atom prefers to occupy the tetrahedral interstitial site (TI) in comparison to the octahedral interstitial site (OI). We find that the oxidizing capacity of the Fe3Al-type cubic structure is stronger than that of other structures. In particular, we predict that Pt3Zr exhibits better oxidation capacity in comparison to other metals because of the strong localized hybridization between Zr and O.

Correlation between hardness and bond orientation of vanadium borides

The relationship between hardness and bond characteristic of vanadium borides was investigated by first-principles approach. The calculated lattice parameters of V–B system are in good agreement with previous experimental data. The convex hull indicates that the VB are most stable at ground state. The vanadium borides have higher bulk modulus, shear modulus and Youngs modulus, and lower B/G ratio. These vanadium borides are brittle. We predict that the V5B6 and VB2 are potential superhard materials. The nature of hardness is related not only to covalent bonding but also to bond orientation. The B–B and V–B covalent bonds parallel to the load plane are the origin of high levels of hardness.

Role of Boron Element on the Electronic Properties of α -Nb 5 Si 3 : A First-Principle Study

Transition metal silicides (TMSis) are attracting increasing interest from the microelectronics and nanoelectronic industries. In this paper, we use the first-principles method to investigate the B-doped mechanism and the influence of B on the electronic properties of α-Nb5Si3. The calculated results show that B-doped Nb5Si3 is thermodynamically stable at the ground state. The calculated electronic structure shows that the thermodynamically stable B-doped Nb5Si3 is attributed to the 3D-network B-Si bonds and B-Nb bond. In particular, B element prefers to occupy B-IT4 site in comparison to other sites. Moreover, the calculated band structure indicates that Nb5Si3 exhibits metallic behavior at the ground state. We find that B-doping can improve charge overlap between conduction band and the valence band, which effectively improves the electronic properties of Nb5Si3.

Insulator-to-metal transition of lithium–sulfur battery

Li2S is a promising battery material due to the high theoretical capacity and high energy density. However, the improvement of insulation of Li2S is a challenge for its application. Naturally, the insulator-to-metal transition strongly depends on the electronic overlap between the conduction band and the valence band at the Fermi level (EF). To solve this key problem, this work investigates the insulator-to-metal transition of Li2S under high pressure. We identify a stable structure based on the phonon dispersion and thermodynamic model. It is found that Li2S with CaF2-type, Br2O-type and Cs2S-type structures are dynamically stable in the ground state. Importantly, the band gap of Li2S decreases gradually with increasing pressure. We predict that pressure leads to the insulator-to-metal transition of Li2S owing to the Li atomic pairing and the existence of Li–Li metallic bonds.

DFT prediction of a novel molybdenum tetraboride superhard material

Although transition metal borides (TMBs) are promising superhard materials, the research and development of new TMB superhard materials is still a great challenge. Naturally, the Vickers hardness of TMBs is related to the 3D-network chemical bonding, in addition to the valence electron density and covalent bonds. In this paper, we apply ab initio calculations to explore the structural stability, Vickers hardness and hardening mechanism of MoB4 tetraboride. Four possible tetraborides are predicted based on the phonon dispersion model. We find that MoB4 with monoclinic structure (C2/m) and orthorhombic structure (Immm) are dynamically stable at the ground state. The calculated Vickers hardness of MoB4 with monoclinic and orthorhombic structures is 41.3 GPa and 40.0 GPa, respectively. We suggest that the high hardness is derived from the 3D-network B–B covalent bond owing to bond synergistic effects. On the other hand, the Vickers hardness of MoB4 decreases gradually with increasing pressure. The calculated results show that the hardness of MoB4 is attributed to the B/G ratio and c/a ratio. Finally, we predict that MoB4 is a new superhard material.