The Synthesis And Physical Properties Characterizations Of Binary Metal Borides/Nitrides

Binary metal borides/nitrides is a kind of functional materials with excellent physical and chemical properties.Due to the small atomic radius,diverse structure and electron-deficient of boron and nitrogen,these two elements are easily to be filled into the metal lattice or bond with metal atoms through born/nitrogen atomic cluster.Such metal borides/nitrides often have high hardness,high elastic modulus,and excellent electrical and thermodynamic stability.Moreover,some metal borides/nitrides also have magnetism and superconductivity.Theoretical calculation shows that pressure may promote the synthesis of borides and nitrides.Meanwhile it is also a very simple and effective tool to explore the physical properties of materials without introducing impurities.Therefore,in this paper,we synthesized AlB₁₅,MnN and?-MoN_0.67.67 single crystals under high pressure and investigated the physical properties.The main innovative results were following:?1?We synthesized AlB₁₅5 single crystal.Vickers hardness test indicated that AlB₁₅ single crystal belongs to the superhard materials,which is also one of the hardest semiconductor compound that has been ever found.With the direct band gap of 2.3 eV,AlB₁₅ exhibits excellent optical transmittance?>90%?,covering the visible range from 459nm to 760 nm and part of infrared range.Moreover,AlB₁₅ is also a p-type semiconductor with a hole carrier concentration of 7.5×10¹⁵ cm^-3.It has excellent physical and chemical stability.No phase transition was observed under high pressure up to 40 GPa.Such an assembly of various excellent properties within one material has great implication for high power electronic design and application.?2?We synthesized MnN single crystal.We performed high-pressure resistance,magnetic resistance and synchrotron radiation on MnN single crystal/polycrystalline and observed a first-order phase transition accompanied antiferromagnetic?AFM?to ferromagnetic?FM?magnetic structure transition under 34 GPa.MnN systems have been widely used to investigate Mn incorporated into?–V compounds,which combines the spin and electron charge as promising diluted magnetic semiconductors.However,few experiments have demonstrated the magnetic transformation of MnN with lattice constant changes,although it has been predicted many times in previous theoretical calculations.First-principle calculations revealed that the magnetic transition from the AFM to FM state with the volume compressed to 16?³,which is inconsistent with the experiment result.Hamiltonian indicates a strong FM coupling in the[001]plane of double exchange mechanism from Mn e_g orbitals.With the reducing of Mn-N bond length under high pressure,the enhanced FM double exchange interactions could induce the magnetic transition from AFM to FM and subsequently lead to a structural transition from a face-centered tetragonal structure to a cubic structure.These findings provide the solid foundations for the research of GaMnN or MnN layer DMS.?3?We synthesized?-MoN_0.67 single crystal.X-ray powder/single crystal diffraction,X-ray energy dispersion,transmission electron microscopy and other experimental methods determined the?-MoN_0.67.After the hardness test,we found that the?-MoN_0.67 has high hardness.Moreover,the?-MoN_0.67 showed metal behavior without superconductivity,while the first principle calculation predicted that the superconductivity of?-MoN_0.67 would occur around 29 K and it was an unstable phase.Based on our experimental results and density functional theory calculation,we found that with the increase of nitrogen content,the vacancy of nitrogen in the crystal decreased.However,the vacancy of nitrogen can effectively inhibit the spin fluctuation inside,which will destroy the cooper pair.Therefore,we can see that?-MoN_x can show superconductivity when x is relatively smaller,and no superconductivity was observed when the x relatively is larger.In addition,the first principles calculations show that?-MoN has an antiferromagnetic ground state,which is a very stable structure both dynamically and thermodynamically,easily converted to the paramagnetic state.