| Looking for new super-hard multifunctional materials is one of the research hotspots in physics and materials science.The hardness test found that materials mainly produced volume deformation and shear deformation.Scientific research has shown that transition metals have high valence electron concentration,strong ability to resist volume deformation,as well as metallic and magnetic properties,etc;light elements are easy to form highly directional covalent bonds,and have a strong ability to resist shear deformation,so transition metal-light elements compounds(TM-LEs)are formed by combining this two ingredients,which are likely to have strong resistance to volume deformation and shear deformation and will be super-hard multifunctional materials.In many TM-LEs systems,transition metal borides and transition metal carbides are good candidate systems for super-hard multifunctional materials.These two compounds not only have rich chemical ratios,but also have high hardness,superconductivity and magnetic properties,etc.Therefore,looking for new super-hard multifunctional materials in these two types of compounds is not only efficient,but also find more novel physical and chemical properties hopefully.High pressure has many effects,such as the phase transformation of material structure,the appearance of exotic chemical ratio and the increase of chemical activity among elements,etc.Due to these effects,new materials can be produced.These new materials usually have dense structures and excellent functionality.For examples,metallic sodium becomes a wide band gap insulator under high pressure;element oxygen and sulfur become superconducting materials under high pressure;our research group found the structure of H3 S with high temperature superconductivity at high pressure for the first time,the superconducting temperature is as high as 200 K at 200 GPa,which has been confirmed by foreign experiments.All these examples can show that high pressure is an effective way to discover novel functional materials.Based on the favorable condition of high pressure,the systematic theoretical studies of several transition metal borides and transition metal carbides have been carried out.In this thesis,we have studied Mn-B,Ru-C and Ti-C systems by using structural search softwares and first-principles calculations in a certain pressure range,including crystal structures,dynamical properties,mechanical properties,magnetic properties,and hardness,etc.We found:1.We perfected the zero-temperature phase diagram of Mn-B system at 0 GPa and 50 GPa,and proved that the Al B2-type Mn B2 synthesized in the experiment was metastable,and found the new high-pressure phase Immm-Mn B2.According to the calculations of electronic structure,magnetism and hardness,Immm-Mn B2 was a conductive and ferromagnetic material with high hardness.The hardness value of Immm-Mn B2 was 22.5 GPa,which was far higher than that of traditional magnetic materials.There was no imaginary frequency in the phonon spectrum of Immm-Mn B2 and the elastic constants satisfied the stability criterion of Born-Huang at zero pressure,indicating that Immm-Mn B2 could stabilize under ambient conditions.According to the analysis of chemical bonds,boron atoms formed graphene-like boron structures due to the strong covalent interaction,which was the main reason for the structural stability and high hardness of Immm-Mn B2.2.We found a stable manganese boron compound with chemical ratio of 2:3,determined its phase sequence,and studied its material properties.At 200 GPa,a chemical ratio of 2:3 in manganese boron compound-Mn2B3 was firstly found by using variable component structure search method in USPEX code.Further studies on the structure of Mn2B3 showed that it was stable in the structure of Cmcm under low pressure,and then changed to the structure of C2/m around 28 GPa.By analyzing the chemical bonds,we found boron atoms formed the six-member ring structures because of covalent interaction,which played an important role in structural stability and high hardness,so that these two structures of Mn2B3 were hard materials.As the pressure increased to 80 GPa,the boron six-member rings were broken and formed a zigzag boron chain.The crystal structure of Mn2B3 became C2/c,and the hardness also changed from hard to soft.According to the calculations of the dynamical properties,mechanical properties,electronic structures and magnetism,the above three structures would quench to ambient conditions and possess metallicity and ferromagnetism.The hardness of Mn2B3 with structures of Cmcm and C2/m was 26.2 GPa and 18.0 GPa,respectively,far higher than that of traditional magnetic materials,and would be hard,conductive and ferromagnetic materials potentially.3.We studied the zero-temperature and high-pressure phase diagram of Ru-C system with three synthetic routes,updated the ground state structure of Ru C,and confirmed the new chemical ratios Ru2C3 and Ru C4.There was a structural phase transition from P-3m1 to R-3m in Ru C4 with a phase transition pressure point of 98 GPa.Without R-3m phase of Ru C4,other structures remained dynamically and mechanically stable at zero pressure.As the carbon concentration increased,carbon atoms formed graphene-like carbon six-membered rings in Ru2C3 and diamond-like carbon network structures in Ru C and Ru C4.The analysis of mechanical properties,Debye temperature and electronic structures showed that the new ruthenium carbides were hard materials with metallicity,the hardness of Ru C and Ru C4(P-3m1)was 25.9 GPa and 26.8 GPa,respectively,higher than that of wear-resistant materials commonly used in the industries.By studying the relationship between bonding characteristics,carbon concentration and material hardness,we found that the strong isotropic structure,diamond-like carbon network structures and the Ru-Ru bonding with covalency resulted in the high hardness of Ru C,and then the hardness of carbon-rich ruthenium carbides changed less obviously with increasing carbon concentration.4.We established the zero-temperature phase diagram of the Ti-C system within the pressure range of 0~100 GPa.The carbon-rich titanium carbides were predicted firstly: Ti C2,Ti C3 and Ti C4,and these three structures were determined to be dynamically and mechanically stable.According to the analyses of bonding and st ructures,we found that these structures were stacking alternatly by layers of titanium atoms and carbon atoms.The layers of carbon atoms successively showed the puckered graphene carbon structure,diamond-like carbon structure and double diamond-like carbon structure,respectively,therefore,they were called diamond net analogues.In the calculations of hardness value,Ti C2 was predicted to be potential hard material,while Ti C3 and Ti C4 were predicted to be potential superhard materials.Because of the diamond-like carbon network structures and anisotropic crystal structures in Ti C3 and Ti C4,their hardness respectively was as high as 40.8 GPa and 49.1 GPa,which was similar to that of super-hard material c-BN and showed super-hard features.We also found that Ti C4 could be used to be a structural model of ? and the subgroup?,and then five stable super-hard materials TMC4(TM= V,Zr,Nb,Hf and Ta)were obtained at zero pressure. |
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