First-principles Study Of The Elastic Propetries Of N-defect And O-doping In TaN And Ta₃N₅

Transition metal nitride has a wide range of industrial application and attracts many people’s attentionfor its special properties. Transition metal nitride is a class of compounds that N element is inserted intothe transition elements’ lattice and it has the properties of covalent compounds, ionic crystal and transitionmetal. Transition metal element has a high electron density, after synthesizing with covalent element N, itexhibits various special properties. Because of the N element insertion, transitional metal’s lattice expands,and space of metal and lattice constant increase, interaction between the metal atoms are weak, the numberof valence electron increases, and then the structure changes. After these changes, transition metal nitrideacquires unique physical and chemical properties.As in the films, TiN is a kind of new ceramic material with high hardness, high strength, corrosionresistance, high temperature resistance, thermal shock, low density, wear resistance, good electricalconductivity, thermal conductivity and other excellent performance. It is widely used in the preparation ofmetal ceramic, cutting tool, mold, and so on. It can be used in lining material of electrode of electrolyticmetal of molten salt, electrical contacts and metal surface coating materials. It also can be used as a hightemperature lubricant, a new generation of advanced refractory materials. When transition metal nitridemake into nanometer multilayer film, it will produce super modulus effect and super hardness effect. Themechanical properties of nanometer multilayer film become one of the research hotspots of films in recentyears. In super hard materials, diamond is the hardest material available in nature, however, naturaldiamond is very rare and cost of synthetic diamond is too high. CBN is the second hard material, justbehind synthetic diamond, however there are not natural CBN and synthesis of cubic boron nitride is very difficult. Because of high electron density, strong hybridization and small elastic anisotropy between the delectron of transition metal nitride and p electron of N, transition metal nitrides show the properties ofstrong incompressible and high hardness. Thus the people who research super hard materials started to payattention to the transition metal nitrides. Such as OsN₂,PtN₂, IrN₂, they all have high bulk modulus andhigh shear modulus.The calculations presented in this study were performed within the density functional theory, usingthe project-augmented wave （PAW） method as implemented in Vienna ab initio simulation package（VASP）. The generalized gradient approximation （GGA） was used with the PAW potential. The structurewas optimized with the conjugate-gradient algorithm method. A plane wave cutoff energy of500eV wasemployed throughout. Appropriate Monkhorst-Pack mesh was chosen to guarantee the sufficient k-pointsin the irreducible Brillouin zone. The strain-stress method was used to obtain the elastic constant. From thecalculated elastic constant, the bulk modulus and shear modulus were estimated using the Voigt-Reuss-Hillapproximation.Using first-principles method, this paper has investigated the influence of N defect and doping of O tocmcm phase and Pnma phase of Ta₃N₅. This paper first introduces the theoretical calculation method usedhere, and the density functional theory, elastic mechanics and the knowledge of VASP. The cmcm phase ofTa₃N₅, the most stable modification of Ta₃N₅up to9GPa has been synthesized. The Pnma phase has notbeen synthesized but is predicted to be the new high pressure modification at about9GPa. N defect anddoping of O in transition metal nitride are common situation, but the influence of N defect and doping of Oto the two phases of Ta₃N₅has not been reported. This paper systematically studied the influence to thetwo phases of Ta₃N₅. We calculate the defect information energy of the two phases of Ta₃N₅,the resultsshow that: if there is no external energy, it is difficult for Ta₃N₅-cmcm to generate N defect. When pressure is lower than5GPa, Ta₃N₅-cmcm is easy to generate N defect, however when pressure is higher than5GPa, it is difficult to generate N defect without external energy. After generating N defect and doping of O,the two phases of Ta₃N₅still have high structural stability and maintain hardness. Through the comparisonwe conclude that after generating N defect and doping of O, the two phases of Ta₃N₅maintain highstructural stability. It is also show that these two phases have high structural stability.We also investigate the influence of N defect and doping of O to ε phase and δ phase of TaN. Thereare some literatures which have investigated the phase transition of TaN but for N defects and doping of Oof TaN there aren’t any relevant reports. This paper investigate the influence of N defect and doping of Oto ε phase and δ phase of TaN in detail. We calculate the defect formation of the two phases of TaN, theresults showed that: if there is no external energy the TaN will be difficulty to produce N defect. At apressure less than15GPa, the defect formation energy of δ TaN defect formation energy is low, so it is easyto produce N defect. Through the comparison we can see that: after N-defect and O-doping the TaN is alsostability, it shows that the tow phases of TaN have high stability.