Meiguang Zhang

Structural, electronic and mechanical properties of Imma-carbon

A theoretical investigation on mechanical and electronic properties of Imma-carbon was performed by employing first-principles calculations based on the density functional theory. The stability at ambient condition is approved by phonon dispersion and elastic constants calculations. The analysis of elastic anisotropy and hardness anisotropy shows that Imma-carbon is nearly mechanical isotropy. The large elastic constants and ideal strength indicate Imma-carbon is a potential superhard material. Ideal strength studies show that (010) plane is the easy cleavage plane for Imma-carbon, and the cleavage mechanism was discussed in detail. The calculated band structure is a direct band gap semiconductor with 2.97 eV gap at Γ-point.

Influences of carbon concentration on crystal structures and ideal strengths of B2CxO compounds in the B-C-O system

The search for novel superhard materials with special structures and improved thermal stability and hardness remains considerably experimental and theoretical challenges. Recent reports proposed that higher carbon content in ternary B2CxO compounds, which are isoelectronic with diamond, would lead to increased strength and hardness. This notion was derived from the calculated elastic parameters and empirical hardness formulas based on structural and electronic properties of the equilibrium structures. In present work, we introduce three potential ultra-incompressible and thermodynamically stable B2CxO (x ≥ 2) phases via a systematic particle swarm optimization algorithm structure searches. By evaluating the trends of the crystal configuration, electronic structure, and mechanical properties as a function of the C concentration, it is found that the high carbon concentration benefits the formation of the sp3 C-C covalent bonds and leads to the enhanced elastic moduli and ideal strengths in these B2CxO compounds. Studies of strain-stress behavior at large deformation, however, indicate that all these B2CxO compounds possess substantially lower ideal shear strengths than those of diamond and c-BN, suggesting that they may not be intrinsically superhard.

Cubic C3N: A New Superhard Phase of Carbon-Rich Nitride

Using the particle swarm optimization technique, we proposed a cubic superhard phase of C3N (c-C3N) with an estimated Vicker’s hardness of 65 GPa, which is more energetically favorable than the recently proposed o-C3N. The c-C3N is the most stable phase in a pressure range of 6.5–15.4 GPa. Above 15.4 GPa, the most energetic favorable high pressure phase R3m-C3N is uncovered. Phonon dispersion and elastic constant calculations confirm the dynamical and mechanical stability of c-C3N and R3m-C3N at ambient pressure. The electronic structure calculations indicate that both c-C3N and R3m-C3N are indirect semiconductor.

Crystal Structures and Mechanical Properties of Ca2C at High Pressure

Recently, a new high-pressure semiconductor phase of Ca2C (space group Pnma) was successfully synthesized, it has a low-pressure metallic phase (space group C2/m). In this paper, a systematic investigation of the pressure-induced phase transition of Ca2C is studied on the basis of first-principles calculations. The calculated enthalpy reveals that the phase transition which transforms from C2/m-Ca2C to Pnma-Ca2C occurs at 7.8 GPa, and it is a first-order phase transition with a volume drop of 26.7%. The calculated elastic constants show that C2/m-Ca2C is mechanically unstable above 6.4 GPa, indicating that the structural phase transition is due to mechanical instability. Both of the two phases exhibit the elastic anisotropy. The semiconductivity of Pnma-Ca2C and the metallicity of C2/m-Ca2C have been demonstrated by the electronic band structure calculations. The quasi-direct band gap of Pnma-Ca2C at 0 GPa is 0.86 eV. Furthermore, the detailed analysis of the total and partial density of states is performed to show the specific contribution to the Fermi level.

A new high-pressure polymeric nitrogen phase in potassium azide

To explore new stable polymeric nitrogen phases in alkali metal azides, the crystalline structures of potassium azide KN3, are systematically investigated up to 400 GPa by using unbiased structure searching methods combined with first principles density functional calculations. Two high-pressure phases of KN3, insulator C2/m phase with N3− anions and metallic P6/mmm phase with “N6” rings were uncovered above 20 GPa and 41 GPa, respectively, which are consistent with recent theoretical works. Above 274 GPa, a stable C2/m-N phase featuring polymerized N is identified for the first time and it is energetically much superior to the previously proposed C2/m-II structure. This C2/m-N structure consists of zig-zag N polymer nets which can be naturally viewed as the polymerization of “N6” molecules rings in the low-pressure P6/mmm phase under increasing pressure. Furthermore, the structure evolutions and accompanied chemical bonding behavior of KN3 under pressure are also discussed.

Exploration on pressure-induced phase transition of cerium mononitride from first-principles calculations

Recently, pressure-induced phase transition of CeN from ambient B1 phase to the B2 phase has been experimentally reported at the pressure of 65–70 GPa. Nevertheless, the full transformation of the high-pressure B2 phase has not been observed in the experiment as the authors said. Here we predict an unexpected anti-B10 high-pressure phase which is energetically more preferable than the B2 phase at the studied pressure range, disproving the experimental result. Our argument has been supported by the softening of both elastic constant C44 in B1 phase and transverse acoustic mode at the zone boundary M point of B2 phase.Recently, pressure-induced phase transition of CeN from ambient B1 phase to the B2 phase has been experimentally reported at the pressure of 65–70 GPa. Nevertheless, the full transformation of the high-pressure B2 phase has not been observed in the experiment as the authors said. Here we predict an unexpected anti-B10 high-pressure phase which is energetically more preferable than the B2 phase at the studied pressure range, disproving the experimental result. Our argument has been supported by the softening of both elastic constant C44 in B1 phase and transverse acoustic mode at the zone boundary M point of B2 phase.

Elastic and thermodynamic properties of Re2N at high pressure and high temperature

Abstract First principles calculations are preformed to systematically investigate the elastic and thermodynamic properties of Re 2 N at high pressure and high temperature. The Re 2 N exhibits a clear elastic anisotropy and the elastic constants C 11 and C 33 vary rapidly in comparison with the variations in C 12 , C 13 and C 44 at high pressure. In addition, bulk modulus B , elastic modulus E , and shear modulus G as a function of crystal orientations for Re 2 N are also investigated for the first time. The tensile directional dependences of the elastic modulus obey the following trend: E [0001] E [ 2 1] > E [10 0] > E [10 1] . The shear moduli of Re 2 N within the (0001) basal plane are the smallest and greatly reduce the resistance of against large shear deformations. Based on the quasi-harmonic Debye model, the dependences of Debye temperature, Gruneisen parameter, heat capacity and thermal expansion coefficient on the temperature and pressure are explored in the whole pressure range from 0 to 50 GPa and temperature range from 0 to 1600 K.

Pressure-induced phase transition and mechanical properties of molybdenum diboride: First principles calculations

Using newly developed particle swarm optimization algorithm on crystal structural prediction, we first predicted that MoB2 undergoes a phase transition from the low-pressure rhombohedral phase to a tetragonal α-ThSi2-type phase with a volume drop of 4.01% when the applied pressure is 68 GPa. Phonon calculations suggest the α-ThSi2-type phase can be quenchable to ambient pressure. Then, Young’s modulus E and shear modulus G as a function of crystal orientation for the α-ThSi2-MoB2 have been systematically investigated. Further mechanical properties demonstrated that α-ThSi2-MoB2 possesses large bulk modulus of 322.3 GPa and high Vickers hardness of 32.1 GPa, exceeding the hardness of α-SiO2 (30.6 GPa) and β-Si3N4 (30.3 GPa). The excellent mechanical properties are attributed to the three-dimensional networks linked by strong covalent B-B bonding and Mo-B covalent bonds in MoB12 polyhedrons.

Electronic bonding analyses and mechanical strengths of incompressible tetragonal transition metal dinitrides TMN2 (TM = Ti, Zr, and Hf)

Motivated by recent successful synthesis of transition metal dinitride TiN2, the electronic structure and mechanical properties of the discovered TiN2 and other two family members (ZrN2 and HfN2) have been thus fully investigated by using first-principles calculations to explore the possibilities and provide guidance for future experimental efforts. The incompressible nature of these tetragonal TMN2 (TM = Ti, Zr, and Hf) compounds has been demonstrated by the calculated elastic moduli, originating from the strong N-N covalent bonds that connect the TMN8 units. However, as compared with traditional fcc transition metal mononitride (TMN), the TMN2 possess a larger elastic anisotropy may impose certain limitations on possible applications. Further mechanical strength calculations show that tetragonal TMN2 exhibits a strong resistance against (100)[010] shear deformation prevents the indenter from making a deep imprint, whereas the peak stress values (below 12 GPa) of TMN2 along shear directions are much lower than those of TMN, showing their lower shear resistances than these known hard wear-resistant materials. The shear deformation of TMN2 at the atomic level during shear deformation can be attributed to the collapse of TMN8 units with breaking of TM-N bonds through the bonding evolution and electronic localization analyses.

Evolution of structures, stabilities, and electronic properties of anionic [AunRb]− (n = 1–10) clusters: comparison with pure gold clusters

The geometric structures, stabilities, and electronic properties of small size anionic [AunRb]− and Aun+1− (n = 1–10) clusters have been systematically investigated by using density functional theory. The optimised geometries show that the structures of [AunRb]− clusters favour the three-dimensional structure at n ≥ 8. The Rb atoms tend to occupy the most highly coordinated position and form the largest probable number of bonds with gold atoms. One Au atom capped on [Aun-1Rb]− structures is the dominant growth pattern for n = 2–8 and Rb atom capped on Aun− structures for n = 9–10. The averaged atomic bonding energies, fragmentation energies, second-order difference of energies, and highest occupied molecular orbital–lowest unoccupied molecular orbital gaps exhibit a pronounced even–odd alternations phenomenon. The charges in [AunRb]− clusters transfer from the Rb atoms to Aun host. In addition, it is found that the most favourable dissociation channel of the [AunRb]− clusters is to eject a Rb atom and the highest energy dissociation path is Rb− anion ejection.