First-principles Study On The Crystal Structure Prediction And Mechanical Properties Of Binary Transition Metal Carbides

Superhard materials are of great significance in both science and technology field due to their wide applications, such as cutting and polishing tools, abrasives and coating. Consequently, they have attracted wide attention because of their superior properties, such as high stiffness, high hardness, high thermal conductivity and high melting point. Current search for superhard materials mainly centers on two classes of materials. The first class, which is generally accepted, is composed of light elements（B, C, N, and O） containing strong covalent bonds, such as diamond, cubic BN, BC2 N,B2CO etc. The second class, formed by heavy transition metal（TM） and light element（LE）, is considered to be potential superhard/hard materials as well, such as Pt N2,Re B2 and WB4.As a member of TM-LEs, transition metal carbides（TMCs） generally show very high bulk modulus, for example, 439 GPa of WC, 303 GPa of Pt C and 316 GPa of Ru C. Recently, two new transition metal carbides with stoichiometry of Ru₂C and Os₂C were synthesized at high pressure and high temperature by a group of Indian researchers. Based on the temperature quenched high pressure X-ray diffraction（HPXRD） combined with transmission electron microscope（TEM） imaging and diffraction as well as first-principles calculations, two hexagonal structures with space groups P-3m1 and P63/mmc were proposed for Ru₂C and Os₂C, respectively. As is often the case in the TM-LE compounds, it is difficult to determine the crystal structure of Ru₂C in experiment due to the small scattering cross section of carbon,which leads to controversial results. Moreover, since previous calculations are only based on selecting a number of known structures, the question whether the ground state of Os₂C under ambient condition is the P63/mmc phase as previously proposed is still open. In addition, the calculated hardness of Os₂C with P63/mmc phase is quite different in two independent studies which needs to be clarified.In this thesis, the crystal structures and physical properties, including thermodynamic, dynamic, mechanical properties and electronic structures, of Ru₂C and Os₂C have been extensively investigated using the structure searching methodcoupling with the first-principles calculations.Firstly, the dynamic stability of the previously proposed P-3m1 phase of Ru₂C was checked and it was found that in accordance to a previous research, the P-3m1 phase is dynamically unstable at ambient pressure. Then, systematic studies on the crystal structure and physical properties of Ru₂C were performed both at ambient and high pressures. Some innovative results were obtained as follows:1) Two new energetically stable phases with space groups P63/mmc and P-31 m were obtained using the swarm-intelligent particle swarm optimizations（PSO）.2) Enthalpies calculations reveal that the two newly predicted phases are energetically much more competitive than the previously proposed P-3m1 phase.The P-31 m structure is the most stable phase at 0 GPa, indicating that the P-31 m phase is the ground state of Ru₂C at ambient pressure. The P-31 m structure is stable up to 32 GPa, above which the P63/mmc structure takes over. Therefore,according to our enthalpy results, a P-31 m â†' P63/mmc phase transition was uncovered for Ru₂C.3) The two novel phases were checked to be dynamically stable both at ambient and high pressures by phonon dispersions.4) Mechanical calculations indicate that the P63/mmc and P-31 m phase are ultra-incompessible.5) Electronic structure and charge density calculations show that the two newly predicted phases are metallic due to strong covalent Ru-C bond arising from strong hybridization between Ru-d orbital and C-p orbital. It partly explains why these two phases exhibit high bulk modulus.In the next part, the crystal structures of Os₂C were extensively searched to find its groundstate at ambient pressure. In addition, the hardness of the P63/mmc phase was estimated using another two empirical models different from previous studies to clarify the discrepancy. Some important conclusions are listed as follows:1) In contrast to other searched structures including the previously proposed P63/mmc structure, we found a new energetically competitive structure with space group P-6m2. Moreover, the P-6m2 structure is more stable than the P63/mmc structure, indicating that the P-6m2 phase is the ground state of Os₂C at ambient pressure.2) A P-6m2 â†' P63/mmc phase transition was obtained for Os₂C at 12 GPa.3) Unlike the results proposed by the Indian researchers, the hardness value of the P63/mmc phase was found to be much smaller. Further ideal strength calculations also reveal the P63/mmc phase is not hard.4) The electron localization function（ELF） reveals that in contrast to many of the TM-LEs with a strong covalent bonding between TM and LE, Os₂C is quite exceptional due to its weak metallic bonding character between TM and LE.