Theoretical Studies On Prediction Of Structure And Physical Properties Of Iridium Nitride Under High Pressure

The design of new materials with a hardness comparable to that of diamond poses a considerable experimental challenge.It is well known that the introduction of smaller atoms such as nitrogen and boron into interstitial sites in close packed transition metals(TMS)lattices leads to dramatic changes of the strength,durabil-ity,electronic and magnetic properties of the compound with respect to that of the metal.Herein,combining density functional theory and swarm structure searching technique in Combination,the phase transition sequences of Ir-N compounds with different chemical ratios in the pressure range of 0 to 100 GPa were studied,and the phase diagrams of binary Ir-N compounds were presented for the first time.At high pressure,three stable phases of IrN2-P21/c,IrN3-Im3 and IrN4-R-3c were discovered.The negative values of formation enthalpy for IrN2-P21/c,IrN3-Im3 and IrN4-R-3c compounds we calculated illustrate that these compounds are all thermodynamic stable phases.We have carefully performed the phonon dispersion calculations for IrN2-P21/c,IrN3-Im3 and IrN4-R-3c,respectively.There is no virtual frequency in the entire Brillouin zone,indicated the dynamic stability of these phases.The mechanical stability requires the all of elastic constants satisfy the Born stability criteria,we can confifirm that the Cij we calculated for IrN2-P21/c,IrN3-Im3 and IrN4-R-3c,are all fulfifills the stability criteria listed.previously,illustrating that all of these Ir-N compounds are mechanically stable.In order to understand the electronic nature and chemical bonding behavior between Ir and N,research the electron band structure and density of states(DOS)of IrN2-P21/c,IrN3-IM3 and IrN4-R-3c.For IrN2-P21/c and IrN4-R-3c,we can see that the two small band gaps of 0.297 eV and 0,660 eV,which suggests that IrN2-P21/c and IrN4-R-3c exhibit semiconductor.However,the valence and conduction bands cross over the Fermi level and as a result there is no band gap,which suggests that IrN3-Im3 exhibit metallic features.The use of the HSE functional in moving the bottom(top)of the conduction(valence)band upward(downward)significantly enlarged the band gap to 0.536 eV.So,IrN3-Im3 is a semiconductor.The DOS at Fermi level of mainly composed of the iridium d-state,which is moderately mixed with the nitrogen p-state.The electron localization function and Bader change analysis,show that covalent and ionic bonds coexist in these three crystal structures.In order to explore the elastic properties of IrN2-P21/c,IrN3-Im3 and IrN4-R-3c,the elastic constanof these Ir-N compounds were calculated and summarized.The bulk modulus of IrN2-P21/c,IrN3-IM3 and IrN4-R-3c are calculated to be 319.6 GPa,200.3 GPa and 271.9 GPa,respectively.The hardness of IrN2-P21/c,IrN3-Im3 and IrN4-R-3c were calculated to be 22.3 GPa,20.3 GPa and 22.2 GPa,respectively.Since the hardness is less than 40 GPa,these structures are not classified as super hard materials,but,they should be a relatively hard hard material.The current theoretical results enrich the crystal structures and chemical stoi-chiometry of the binary Ir-N system and benefifit the further understanding of the corresponding physical and chemical properties.