High Pressure Study Of Phosphorus And Phosphorus Nitrides

Phosphorus has a variety of allotropes,among which the more special is black phosphorus（BP）crystalized in an orthorhombic structure at ambinent conditions.Similar to graphite,BP is formed with quasi-two-dimensional layers connected by van der Waals force.BP with a few layers and black phosphorus quantum dots（BPQDs）have been successfully synthesized by mechanical exfoliations.BPQDs exhibit unique electronic and optical properties because of the quantum confinement and small size effects.Besides that,phosphorus can also react with nitrogen to form a variety of phosphorus nitrides,which may have high dielectric constants or large band gaps.High pressure can modulate the structure and properties of materials.However,high pressure study on the structural phase transition and electronic structure transition of BPQDs has not been carried out yet.The selection of precursor can significantly affect final products and critical conditions in high-pressure synthesis of functional materials.Currently,high temperature and the high pressure synthesis of phosphorus nitrides mainly based on compressing（PNCl₂）₃ and NH₄Cl as precursors,and a more economical and environmentally-friendly synthesis approach has yet to be explored.In this study,we investigate the high-pressure behaviors of phosphorus and synthesis phosphorus nitrides under high pressure and high temperature.The research contents and results are as follows:1.We used high-pressure in-situ Raman spectroscopy and UV-Vis absorption spectroscopy methods to study the phase transition and band gap changes of BPQDs at room temperature.According to the Raman spectra of BPQDs upon compression,the BPQDs have experienced two structural phase transitions up to 41.1 GPa.At 12.5 GPa,the BPQDs transform from the orthogonal phase to the rhombohedral phase.The second structural phase transition occurs at 15.6 GPa from the rhombohedral to the simple cubic phase.Upon decompression,Raman spectra show that the structural phase transitions of BPQDs is reversible.Moreover,by using the UV-Vis absorption spectroscopic measurements,it is observed that the band gap of the BPQDs decreases gradually with increasing pressure.There is an inflection point of pressure-dependence of band gaps at 1.6 GPa,which is caused by the electronic phase transition.The second inflection point of pressure-dependence of band gaps appears at 11.4 GPa consistent with the pressure-induced phase transition from the orthogonal phase to rhombic phase.Compared with the previous studies on the high pressure of BP in bulk,it is found that the critical pressures of structural phase transitions of BPQDs have obvious hysteresis effect.Compared with bulk BP,BPQDs have large specific high surface energy,so their structural phase transitions needs more energy,which results in the hysteresis of phase transitions in BPQDs.2.The high pressure structural transformation of red phosphorus was investigated by high pressure Raman spectroscopy.Experimental results showed that red phosphorus transform into to orthorhombic black phosphorus at 6.9 GPa.Upon decompression,the phase transition is irreversible.It was observed that red phosphorus transforms into BP at 5.2 GPa with laser-heating.Further more,we adopted phosphorus and nitrogen as precursors in a laser-heating diamond anvil cell to synthesize phosphorus nitride.High-pressure Raman spectra show that,above 11.0 GPa with the laser-heating power exceeding 4 W,stable phosphorus nitride can be formed by reactions of red phosphorus and nitrogen.According to the structural refinement results,the synthesized compounds was confirmed to be a mixture ofα-P₃N₅ andγ-P₃N₅.