High-temperature And High-pressure Syntheses And Property Study Of Manganese Nitrides

Manganese nitride materials have unique advantages due to their unique structure and physical properties.In particular,the coexistence of various bonding states,atomic spatial configurations,and complex electronic structures in manganese nitride compounds lead to their multi-functions disclose novel physical laws,such as their diverse hardness and magnetism states,which makes it an important research subject in material science and physics.However,due to nitrogen being a gas under normal conditions,it is challenging to prepare high-quality bulk samples of manganese nitrides,making them generally regarded as materials with limited functionality.High-temperature and high-pressure synthesis is an efficient means for preparing bulk transition metal nitrides.This method not only ensures the stability of the nitrogen content and crystal structure of the samples but also enables the preparation of high-purity samples with enhanced material density,meeting the requirements for various measurements to uncover their intrinsic properties.Facing the challenges such as unclear phase diagrams and the need for post-processing due to the nitrogen content,we employed the high-temperature and high-pressure decomposition method to study manganese nitride compounds.With this method,we prepared various high-quality samples of manganese nitrides,and characterized their structures and properties through multiple techniques such as X-ray diffraction,thermogravimetric and differential scanning calorimetry,and magnetic measurements.Moreover,we performed computational simulations to investigate the correlation between the microscopic structures and macroscopic properties.Through this research,we have obtained innovative results as follows:1.Using the high-temperature and high-pressure method under the conditions of5 GPa and 1400℃,a bulk sample of Mn₂N_0.86 with the P6₃22 structure was synthesized,and high-quality samples suitable for characterization,were obtained through a second sintering process.The measured hardness was 7.5 GPa,which is significantly lower than the theoretical simulated value.The oxidation process was obtained through variable-temperature XRD testing in air,and the denitrogenation process and final formation of Mn₄N were obtained through thermogravimetric and differential scanning calorimetry testing under N₂ protection,revealing a new method for preparing Mn₄N.This indicated that Mn-N-O system catalysts could be prepared by controlling the pressure and gas environment.Unlike the antiferromagnetism predicted by first-principles calculations,a coexistence of antiferromagnetism and weak ferromagnetism was observed in the sample,which might be caused by trace nitrogen vacancies.2.High-quality Mn₃N₂ bulk samples with room-temperature antiferromagnetic properties were synthesized under high-temperature and high-pressure conditions of 5GPa and 1600°C,and their oxidation resistance was not lower than 700°C.The measured hardness was 9.9 GPa,that was higher than the theoretically predicted value of 7.01 GPa.Contrary to previous knowledge,the presence of metallic bonds enhances its hardness.Electronic structure analysis revealed that the metallic property mainly come from the Mn 3d orbital electrons,and significant electron localization between Mn atoms forms electron pathways.Ionic bonds dominate between Mn and N atoms,while metallic bonds exist between Mn and Mn atoms.3.Through high-temperature denitriding and secondary sintering,Mn₄N samples were obtained.Using high-temperature and high-pressure synthesis method,anti-perovskite Mn₄N,Mn₃Ni N,and Mn₃Pd N samples were prepared with Ni powder and Pd powder and Mn₂N_0.86,and the samples’quality was improved through secondary sintering.The space groups of the three compounds are all Pm-3m,and their oxidation resistance is no less than 600℃.Magnetic measurements showed that Mn₄N exhibits ferromagnetism below room temperature,and the magnetic saturation strength and coercivity at different temperatures were obtained,both of which are negatively correlated with temperature;Mn₃Ni N and Mn₃Pd N are paramagnetic at room temperature and become ferromagnetic and antiferromagnetic,respectively,at low temperatures,with transition temperatures of 244 K and 281 K,respectively.The measured hardness of the three compounds are 10.5 GPa,11.2 GPa,and 12.4 GPa,respectively,and the hardness increases with the concentration of valence electrons in the crystal.Existing models cannot describe the hardness and internal relationships of the three compounds well.Electronic structure analysis shows that the metallic properties of the three compounds all come from the coupling of the orbital electrons of the metal atoms at the six-fold coordinated corners and those at the face-centered positions.With the increase of the concentration of valence electrons,the bonding between the metal atoms at the face-centered positions and those at the corners weakens,while the bonding between the face-centered metal atoms strengthens.In summary,we successfully synthesized five transition metal nitrides and characterized their structures,hardness,oxidation resistance,and magnetic properties,obtaining reliable data.Especially in understanding hardness,we found that in ionic crystal compounds where ionic bonding is the main type of bonding,the concentration of valence electrons helps to improve the hardness of the samples.With the increase in valence electron concentration,hardness also increases.At this point,there exists a kind of interaction relationship similar to that of other metal atoms transferring electrons to the atoms forming the ionic bond,which strengthens the interaction between the atoms forming the ionic bond and thus improves the hardness.