First-Principles Study on Structure and Ideal Strength of Superhard Material BC2N Under Pressure
|School||Shanghai Jiaotong University|
|Course||Condensed Matter Physics|
|Keywords||first-principles BC2N pressure ideal strength|
In this dissertation, the first-principles calculation method based on density function theory is used to study the novel light elements superhard material of BC2N, and determine its physical properties such as structural parameters, electronic bands and ideal strength, as well the effects of high pressures on these physical properties.The research background and basic theories for this study are introduced in chapter one and two.In chapter three, with total-energy and dynamic phonon calculations, we study the structural transformation and stability of different cubic BC2N phases under pressure. We show that different starting material forms lead to distinct synthesis routes, yielding end products with drastically different physical properties. While a high-density phase with no B-B or N-N bonding shows remarkable structural stability at high pressure; a lower density, lower symmetry phase containing a broken covalent N-N bond undergoes a dramatic structural transformation, with the increase of pressure, the volume and bond-length collapse and a transition from a semimetal to a semiconductor occurs, first with an indirect band gap and then a direct gap. The present work clarifies a puzzling experimental situation in BC2N synthesis and characterization, compare the calculation results of phonon dispersions and velocities with the Brillouin scattering experiment, we strongly identify the low-density phase as the products in Solozhenko’s experiment. These results provide a comprehensive understanding for the high-pressure behaviour of the cubic BC2N phases and reveal their interesting properties that can be verified by experiments.Recently synthesized low-density cubic BC2N exhibits surprisingly high hardness inferred by nanoindentation in stark contrast to its relatively low elastic moduli. In chapter four, by first-principles calculation we consider both normal stress and shear stress beneath the indenter in hardness experiment, and show that this intriguing phenomenon can be ascribed to a novel structural hardening mechanism due to the compressive stress beneath the indenter. It significantly strengthens the weak bonds connecting the shear planes, yielding a colossal enhancement in shear strength. The resulting biaxial stress state produces atomistic fracture modes qualitatively different from those under pure shear stress. These results provide the first consistent explanation for a variety of experiments on the low-density cubic BC2N phase which show results that are seemingly contradictory to each other. This work also provides a reliable calculation method for the hardness study of materials with nanoindentation technology.In chapter five, we examine the cubic-graphitic transformation and relative stability of two high-density phases of cubic BC2N that have nearly identical crystal structure, lattice parameters, energetics and lattice dynamics, and equation of states over a large range of hydrostatic pressure. The calculated energy barriers for the formation and breaking of the bonds between the graphitic layers along different crystallographic directions clearly distinguish these two phases and provide important insights for the structural assignment of the experimentally synthesized high-density cubic BC2N. We also identify a graphitic structure as a possible precursor for the synthesis of the high-density cubic phase that has the highest critical tensile strength among the BC2N polymorphs.In chapter six, we first establish several possible stacking sequences of layer-structured graphitic BC2N, then identify four stable equilibrium structures, and calculate their band structures. The calculated results clearly indicate the indirect nature of the band gap and provide a quantitative account for the band gap and the dispersion of the conduction band near the valence band top observed in recent experiments, and resolve the dispute on the nature of the electronic band gap of the graphitic BC2N. These results demonstrate the crucial role of the interlayer interaction and the stacking in determining the electronic property of graphitic BC2N.