First-principles Study Of The Properties Of Noble Metal Nitrides TMN₂（TM=Ru,Rh,Os,Ir）

Transition metal nitrides are attracting increasing attention from both experimental and theoretical studies for their low compressibility, high melting point,high Tc transition temperature, excellent mechanical, optical and magnetic properties in both fundamental science and technological applications. Most of the transition metal nitride are super hard material or superconducting materials with high hardness. The elastic modulus of IrN₂ is 428 GPa, and the superconducting temperature of NaCl-structure NbN is up to 17.8 K, and the superconducting transition temperature of cubic MoN even reaches 29 K. It is generally believed that the noble metal does not form a nitride. While since 2006, PtN₂, PdN₂, OsN₂, and IrN₂ have been synthesized,which raised the research upsurge of noble metal nitrides, and many of the results about the above four substances have been reported. The research shows that the noble metal nitride may be a potential new（super） hard materials. But RuN₂ and RhN₂ have not been synthesized. There are few reports about them. Yu et al have theoretical and systematic research on the structure and bulk modulus of RuN₂ and RhN₂. In this paper,the structural, elastic, and thermodynamic properties of RuN₂ under high pressure have been studied by using the first principle based on the density functional theory within the generalized gradient approximation（GGA） and local density approximation（LDA）for the exchange of correlation functions. And for further study, we studied the structural, elastic, electrical, thermodynamic properties and hardness of potential superhard material-TMN₂（TM=Ru, Rh, Os, Ir） at 0 GPa and 20 GPa. The research results of OsN₂ and IrN₂ are in good agreement with the existing experimental studies,which provide the theoretical support for the further experimental study. In addition, it provides theoretical guidance for the experimental synthesis of RuN₂ and RhN₂ and the prediction of the material properties. This paper is divided into five chapters, of whichthe third and fourth chapters of the core part of this article.Using the first principle, the structure constants of RuN₂ are obtained and they are in good agreement with other theoretical values. It is suggesting that RuN₂ is a potential candidate to be one of the ultra-incompressible and hard materials; RuN₂ possesses brittle nature at 0 GPa and when the pressure increases to 13.4 GPa（for LDA） or 20.8GPa（for GGA）, it begins to prone to ductility; The ratio of bulk modulus and shear modulus of RuN₂ is low and Poisson’s ratio is relatively small, which show that RuN₂ has excellent elastic properties; When T=0 K, P= 0 GPa, the Debye temperature calculated by bulk modulus and shear modulus method is 688.94 K（GGA） and 736.74K（LDA）, and the Debye temperature obtained by the quasi harmonic Debye model is956 K. Through the quasi-harmonic Debye model, we calculated the thermodynamic properties of RuN₂. It can be seen that increasing the pressure or decreasing temperature are equivalent to change elastic modulus; Thermal expansion coefficient α changes quickly at low temperatures（T<600 K）, low pressure（P<40 GPa）, and it increases with temperature and decreases with increasing pressure. At high temperatures（T>600 K）,high pressure（P>40 GPa）, it gradually close to change linearly. The heat capacity CV is influenced by temperature and pressure. When the temperature rose, the anharmonic effect was suppressed, and the heat capacity is gradually close to the Dulong-Petit limit（74.85 J?mol-1?K-1）. The Debye temperature Θ, entropy S and Grüneisen parameter γ, on pressure and temperature have also been obtained successfully.In addition, structural parameters of TMN₂（TM=Ru, Rh, Os, Ir） are investigated and the calculated values are in excellent agreement with other available experimental and theoretical results. The compression ratio of TMN₂ follows the order of RhN₂>RuN₂> IrN₂> OsN₂, which reveal that RhN₂ is the easiest one to be compressed of the four compounds. Only RhN₂ possesses ductile nature at 0 GPa, RuN₂, OsN₂ and IrN₂ are prone to brittleness under around 20.8 GPa, 20 GPa, and 4.3 GPa respectively. The order of ductility is: RhN₂> IrN₂> RuN₂> OsN₂. The order of metallicity from high to low is: RhN₂> IrN₂> RuN₂> OsN₂. Based on Mulliken overlap population in first principles technique, the calculated hardness of orthorhombic RuN₂, RhN₂, OsN₂ and IrN₂ are 13.66 GPa, 11.54 GPa, 16.33 GPa, 17.92 GPa, respectively. The hardness values of the four marcasite-structure compounds are lower than the standard of superhard materials（40 GPa）, so they should be a relatively hard material. IrN₂ and OsN₂ are the most likely candidate for superhard materials. We analyzed the thermodynamic properties through the quasi-harmonic Debye model. Unfortunately, forthese thermodynamic properties, there are no previous calculations and experimental values for our comparison.