Theoretical Design And Property Study On The Nitrides Of Several Typical Transition Metals

Transition metal (TM)–nitrogen (N) compounds are always a research focusof materials science and physics due to their high melting point, high chemicalstability, corrosion resistance, high hardness, and semiconductor property. Exploringthe possible high pressure structures is an effective approach to design newmaterials with novel physical properties, because of the effectiveness of tuning theinternal structure of material with high pressure. In this thesis, we systematicallystudied the structures, mechanical properties, electronic structure, and latticedynamic behavior of typical3d,4d, and5d TM-N compounds with first-principlescalculations.First, the zero-temperature high-pressure phase diagram of rhenium-nitrogensystem at0-100GPa has been studied with first-principles calculation and structureprediction method. We predicted that the low-energy structure of ReN4should bespace group Cmmm; Two new candidate phases of Re3N2and ReN3ratios were alsoidentified. It is found that high-pressure is an effective approach to access N-richRe-N compounds, both of ReN2and ReN3phases can be synthesized at highpressures and recoverable to ambient pressure. With the study on the evaluation ofcrystal configurations, electronic structures, elastic properties, and hardness as afunction of the N concentration, we prove that the3D polyhedral stacking withstrong covalent N-N and Re-N bonding can be formed at higher N contentsavoiding the direct Re-Re metal interactions, which remarkably improving the mechanical performance of Re-N compounds. We prove that ReN2and ReN3aretwo potential superhard materials, with the calculated hardness of38and44GPa,respectively.And then, we investigated the structure and properties of4d technetiumnitrides between0–60GPa with first-principles method. It is found that there couldbe many stable technetium nitrides including Tc3N, Tc2N, TcN, Tc2N3, TcN2, TcN3,and TcN4, among them Tc3N and Tc2N subnitrides are synthesizable at zeropressure and could be applied to nuclear waste management, such as separateradioactive99Tc from nuclear fuel cycle. And, N-rich Tc-N compounds can besynthesized at high pressures, and display outstanding mechanical properties. N-richTcN3and TcN4exhibit remarkable hardness comparable to hard WC and can bepotential ultrastiff and hard materials.Next, we studied the structure and properties of tungsten-nitrogen (W-N)compounds with first-principles calculations and variable-composition evolutionarystructure searches. We discovered new ground state candidates and high-pressurephases at3:2,1:1, and5:6compositions for possible synthesis. We found for theW-N system that compared with the conventional6-fold phases (rs-WN and δ-WN),the4/5-fold N coordination structures (i.e., NbO-WN and W5N6) are more favouredat low-pressures. With detailed electronic structure analysis, we attribute the low Ncoordination feature of W-N ground states to the enhanced W5d–N2p orbitalhybridization and strong covalent W-N bonding, which can improve remarkably themechanical strength and hardness.Finally, we studied the magnetic ordering effects and electron correlations onthe stability of3d magnetic material FeN. We found that the competition betweendirect magnetic exchange and superexchange interactions drives theantiferromagnetic phase transition of rs-FeN at high pressure, while zb-FeN is stillnon-magnetic. The relative energy of rs and zb FeN changed with the different exchange-correlation functionals, the unstable rs-FeN under PBE can be stable withthe HSE and LDA+U methods. Therefore the discrepancy of FeN’s ground state inprevious studies lies mainly in the treatment of the correlations between Fe3delectrons.In brief, the zero-temperature high-pressure phase diagrams of Re-N, Tc-N,and W-N system, and the structure and the magnetic ground state of FeN have beenstudied. More than a dozen of new materials such as Re3N2, ReN3, ReN4, TcN,Tc2N3, TcN2, TcN3, TcN4, W3N2, NbO-WN, and W5N6have been established forthe first time for possible synthesis. Among them, TcN3, TcN4, NbO-WN, andW5N6displays outstanding incompressibility and hardness; ReN2and ReN3arepotential superhard materials. We establish the correlations between Nconcentration and hardness of Re-N compounds, and discuss the novel low Ncoordination feature of W-N compounds. Besides, the effects of magnetic orderingand electron correlations on the stability of FeN have also been studied. Newapproaches for designing novel transition metal–nitrogen superhardmultifunctional materials are therefore covered throughout the current research.