The High Pressure-high Temperature Synthesis And Characterization Of Cubic Niobium Nitride

Transition metal nitrides（TMN_x）are well-knownin physical and material fields for their various mechanical,electrical,magnetic,superconducting and catalytic properties,among which hard superconducting niobium nitride（δ-NbN）is a typical representative.However,because of the poor thermodynamic stability at atmospheric pressure for TMN_x,nitrogen tends to escape in high temperature,making it more difficult to synthesize stoichiometric δ-NbN.As a result,most of the synthesized samples are nonstoichiometric,partly affecting the investigations of δ-NbN.For example,the bulk modulusB₀=348GPa for δ-NbN was just measured by high pressure synchrotron X-ray diffraction experiment with nonstoichiometric NbN0.90（1）samples in non-hydrostatic pressure condition,leaving enormous controversy.Hence,it is of great importance tofind an effective way to synthesize stoichiometric δ-NbN and further study its physical properties.Recently,high-pressure technology is gradually being employed to synthesize TMN_x,with which a lot of stoichiometric and nitrogen-rich TMN_x have been successfully synthesized.Superior to ambient pressure,high pressure condition could strengthenthermodynamic stability of TMN_x,making it easier to synthesize stoichiometric and nitrogen-richTMN_x.Therefore,we employed high pressure-high temperaturetechnology to synthesize stoichiometric δ-NbN,with which we further carried out high pressure synchrotron X-ray diffraction experiment and resistivity,magnetisability,Seebeck coefficient measurements.In addition,combining first principles calculations,we studied its elastic properties,plastic properties and energy band structure.The specific results are as follows:Pressure and temperature calibration experiments were conducted first,obtaining a high pressure-temperature condition of 15.5GPa and 1700°C.Approximativelystoichiometric δ-NbN was successfully synthesized then at 5.5GPa and 1400°C with KNbO3 and hBN as starting materials,solving the synthesis problem for stoichiometric δ-NbN,and ensuring high quality sample for characterization.With as-synthesized approximatively stoichiometricδ-NbN,high pressure X-ray diffraction experiment was carried out in hydrostatic pressure condition,deriving the exact bulk modulusB₀=348GPa,illustrating the strong incompressibility of δ-NbN,and providing a referenceforstudyingthe originof incompressibility.The measured superconducting transition temperature and Seebeck coefficient were T_c=15.3K and S=4.37uV/K,respectively.Through first principles calculations,elastic constants,bulk modulus,shear modulus,and Poison ratio were derived as C11=706.1GPa,C12=110.3GPa,C44=93.1GPa,BH=308.9GPa,GH=151.7GPa,and v=0.29,respectively.According to the calculated stress-strain relations upon shear,the minimum shear strength of δ-NbNwas determined as 23.4GPa for the（111）1 1 2 slip system.On the basis of the calculated band structure and partial density of states,the metallic behavior of δ-NbN was illustrated.