First-principles Study Of Typical Light Element Superhard Multifunctional Materials

Superhard materials are popular in modern engineering, semiconductor industryand high-tech fields because of the high hardness, high bulk modulus and high heatconductivity. So the design and research of the superhard materials are very importantfor the development of the modern industry and technology. Diamond is known as thehighest hardness material. The harness of diamond is⁹6GPa; the bulk modulus ofdiamond is⁴43GPa. However, the （110） plane of diamond will happen themicroscopic cleavage fracture under the action of external force. At temperatureabove800℃, diamond are prone to oxidation, especially when diamond tool is usedin grinding or cutting the ferrous materials. The carbon reacts with the iron-basedmaterials. So the service life of the diamond cutting tool shortens greatly. For decades,people always hope to develop novel superhard multifunctional materials to replacethe diamond.It is found that the light element （B, C, N, O and so on） can form strong covalentbond materials which show the intrinsic superhard features. The superhard cold-compressed graphite, γ-B, c-BN, and c-BC₂N have been successfully theoreticalpredicted and synthesized. The cold-compressed graphite is another carbon allotropeswith superhard characteristics expect diamond in the carbon materials. The discoveryof the cold-compressed graphite greatly broadened the research field of vision of thecarbon materials. A series of new carbon materials with the multifunctional featureshave been designed by the theoretical method, such as the high density carbon （hP3,tP12and tI12）, the chiral carbon fame, cubic-like carbon （C₆and C₂0）, cage-like cubiccarbon （fcc-C₃2，bcc-C₂0，fcc-C₁2，and fcc-C₁0）, low density T-carbon-like structures（T carbon, Y carbon and TY carbon） and so on. The research of new superhardmultifunctional carbon materials has become a frontier topic in the field of physicsand materials. Therefore, we have systematically studied cold-compressed graphiteand T carbon-like structures by first principle methods, structure prediction, andmaterials design method. The electronic properties, lattice dynamic behavior,mechanical properties, and other properties of the cold-compressed graphite and Tcarbon-like structures have been explored deeply.Firstly, we have extensively explored the polymorph of cold-compressedgraphite by ab initio evolutionary algorithm based on DFT. We have predicted a novelcarbon polymorph C carbon that has the space group Cmcm. This structure is moreenergetically favorable than previously proposed bct-C4, M carbon and W carbon. Ithas a lowest phase transition pressure at about6.5GPa among previously proposedcold-compressed graphite phases. This is very important in the synthesis of superhardmaterials. The synthesis condition can be achieved easily. The calculations on theEOS and energy of the C carbon show that it is superhard material with a much higherincompressibility and a bulk modulus （427.8GPa）, and a direct band gap about4.4eV. The Vickers hardness of C carbon （56GPa） is found, smaller than diamond butcomparable with that of cubic boron nitride. The simulated XRD can match theexperimental data well. The calculated Raman spectra indicate that the C carbon has aunique feature which is different to other cold-compressed graphite phases. Twoespecial peaks in600-800cm-1were found. This makes it can be distinguished clearlyin Raman spectra.Secondly, we have predicted a novel modulated T carbon-like carbon allotropewith space group Pn-3m. We named it as T-II carbon, which has the smallest carbontetrahedron unit of the T carbon. The calculations on the mechanical and electronicproperties of T-II carbon show that it is a semiconductor with an indirect band gapabout0.88eV. And the T-II carbon has a larger density and a more compact atomsarrangement than T carbon. The ideal strength calculations indicate that the T-IIcarbon has better resistant ability to shear strain among theＴcarbon-like structures.The electron localization function calculations confirm that the bond of T carbonbreak much earlier than that of T-II carbon in the （100）<001> shear direction. So theVickers hardness of T-II carbon is much larger than that of T carbon. The presentresults provide insights for understanding the hydrogen storage materials.Over the past decades, it has been proposed that the intercalation of lightelements into transition metals might be a good strategy for search of potentialsuperhard materials. The transition metal carbides, nitrides, and borides are a largeand complex group of industrially relevant compounds with outstanding physicalproperties. The researchers have found that the combination of metals with lightcovalent-bond forming atoms like B, C, and N often leads to materials which not onlyhave a high melting point, but also have a very low compressibility and high hardness compared with the pure metal. The four noble metal nitrides PtN, PtN₂, IrN₂, andOsN2have been synthesized successfully under extreme conditions of high pressureand high temperature by diamond anvil techniques. The tantalum has similar electronarrangement and bulk modulus to osmium, iridium, and platinum. Tantalum andnitrogen can form semiconductor or low metallic materials with different composition.The binary Ta-N system displays rich crystal chemistry. The study of Ta-N system isextremely helpful for future experiments and is helpful for achieving more insight onthe superhard multifunctional materials.Therefore, a systematic study has been put on the Ta-N system. The phasestability, elastic properties and metallic properties of the tantalum nitrides （TaN_x, x>=1） TaN, Ta₅N₆, Ta₄N₅, Ta₂N₃, and Ta₃N₅were investigated using first principlesmethod. Considering the formation enthalpy and convex hulls, the experimentallyobserved TaN-WC, Ta₅N₆, Ta₄N₅, Ta₂N₃-P-4m₂and Ta₃N₅-I lie on the convex hull atzero pressure. This indicates that those structures are stable at ambient condition. Anovel TaN3is predicted at50GPa. The potential synthesis routes are suggested. Atthe ambient condition, the feature of covalent bonds of Ta-N system can be enhancedby increasing the nitrogen concentration. But the degrees of directionality of thecovalent bonds were reduced. These make the hardness of the tantalum nitridesreduced and the ductility of the tantalum nitrides improved with the nitrogencomposition increasing. At high pressure, the hardness of the tantalum nitrides alsoreduced with the nitrogen composition increasing. However, when the nitrogencomposition reaches to Ta:N=1:3, remarkable N-N bonds are found in the TaN₃. Thismake the hardness of TaN₃increases obviously. The metallic properties of Ta-Nsystem can be effectively reduced by controlling the nitrogen composition and pressure. The TaN has the excellent mechanical properties in the Ta-N system. And itcan be expected to be synthesized by high pressure technique.