Structures And Properties Of Transition Metal Borides/nitrides From First Principle Investigations

In this dissertation,the crystal structure,stability,chemical properties and electronic properties of transition metal borides and nitrides were studied by means of first-principles calculations.It will be helpful to predicting the properties of materials and designing new materials.Employing Particle Swarm Optimization(PSO)combined with first-principles calculations,we systemically studied high-pressure behaviors of Cr B4 and Fe B4.Our predictions reveal a distinct structural evolution under pressure for Cr B4 despite having the same initial structure with Fe B4.Cr B4 is found to adopt a new P2/m structure above 196 GPa,another Pm structure at a pressure range of 261-294 GPa and then a Pmma structure beyond 294 GPa.Instead of puckering boron sheets in initial structure,the high-pressure phases have the planar boron sheets with different motifs upon compression.Comparatively,Fe B4 prefers an I41/acd structure over 48 GPa with terahedron B4 units and a P213 structure above 231 GPa having equilateral triangle B3 units.Significantly,Cr B4 exhibits the persistent metallic behaviors in contrast with the semiconducting features of Fe B4 upon compression.Mechanical property calculations indicate that I41/acd-Fe B4 is a potential superhard material and the hig-pressure phases of Cr B4 can be classified into hard materials.By means of first-principles calculations,the competing Fe B4 and Fe2B7 in experiment were compared by the enthalpy and structural features.The close enthalpy of Fe2B7+B and Fe2B8(Fe B4)may explain the coexistence and tight mutual intergrowth of these two phases.The thermodynamic stability,mechanical stability,and dynamical stability of hypothetical Ru2B7 and Os2B7 are checked by calculating the formation enthalpy,elastic constants,and phnon spectra.The stable Ru2B7 and Os2B7 show interesting metallic property and great mechanical property due to the hybridization of metal-d and B-p orbitals and B-B covalent bonding.First-principles calculations were performed to understand the structural stability,synthesis routes,mechanical and electronic properties of diverse ruthenium nitrides in the pressure range of 0-50 GPa.Ru N with a new I-4m2 symmetry stabilized by pressure is found to be energetically preferred over the experimental Na Cl-type and Zn S-type ones.The Pnnm-Ru N2 is found to be stable above 1.1 GPa,in agreement with the experimental results.Specifically,new stoichiometries like Ru N3 and Ru N4 are proposed firstly to be thermodynamically stable,and the dynamical and mechanical stabilities of the newly predicted structures have been verified by checking their phonon spectra and elastic constants.A phase transition from P4/mmm-Ru N4 to C2/c-Ru N4 is also uncovered at 23.0 GPa.Drawn from bonding and band structure analysis,P4/mmm-Ru N4 exhibits semi-metal-like behavior and becomes a semiconductor for the high-pressure C2/c-Ru N4 phase.Meanwhile the P21/c-Ru N3 shows metallic feature.Highly directional covalent N-N and Ru-N bonds are formed and dominating in N-enriched Ru nitrides,making them promising hard materials.Using First-principles calculations,we systemically investigated structures and electronic properties of the stable phases in Os-N system in the pressure range of 0-50 GPa.It is found that pressure is an effective route to stabilize Os nitrides.Except the Os N2 in experiment,four new stable stoichiometries,Os3N8,Os N4,Os N6,and Os N8 were identified.The mechanical and dynamical stabilities of the newly proposed phases have been verified by checking their elastic constants and phonon spectra.These phases contain various polymeric nitrogen forms,such as N2 dumbbells,N(N3)units,and N? chains.Strong covalent N-N bonds exist in these polymeric nitrogens.The electronic structure calculations reaveled that Immm-Os N6 is semiconductor,while C2-Os3N8,P4/mmm-Os N4,R-3c-Os N4,C2/m-Os N6,P21/c-Os N8,and Pnnm-Os N8 exhibit metallic feature.Among these phases,the Pnnm-Os N8 phase has the highest energy density,7.91 k J/cm3,which is slightly higher than that of the 2,4,6-trinitrotoluene(TNT).Moreover,the stabilization pressure for Pnnm-Os N8 is only 41.8 GPa.High energy density and low stabilization pressure make Pnnm-Os N8 a candidate for high energy density material.