Theoretical And Experimental Investigation On Novel Structures And Properties Of Silicon And Boron Nitride

Both silicon(Si)and boron nitride(BN),which consist of the nearest-neighbor elements of carbon,exhibit a multitude of microstructures similar to carbon.Moreover,the microstructure is the most basic elements of the material and determines the properties of the material.Therefore,it is very important to study the structures and phase transitions of Si and BN.In this thesis,we investigated the issues concerning new structures and phase transitions of silicon and boron nitride via ab-initio calculations and high-pressure and high-temperature(HPHT)methods.We investigated the structure and phase transition mechanism of the experimental unidentified silicon phase.A sp3 Si allotrope,Si10,was revealed through CALYPSO search.This structure,with an indirect bandgap of 1.01 eV,starts to absorb the sunlight at lower energy than that of Si-I,indicating its great potential in solar cell applications.The d-spacings and diffraction angle relationships of Si10 match those of the structural unsolved metastable silicon phase recently synthesized by an ultrafast laser-induced confined microexplosion,indicating the viability to achieve Si10.Further analysis suggests that Si10 may originate from the assembly of Si5 clusters probably produced by a radiation-induced disintegration of SiO2 initially used as a confined cover above the Si surface.A new metallic metastable silicon allotrope,hP12-Si,was predicted.The metallicity might be due to the delocalization of valence electrons induced by the existence of five coordinated atoms.Further analysis suggests that hP12-Si might be achieved by the method analogous with the recent preparation of Si24.To investigate structural defects in hexagonal BN(hBN)samples,eight BN allotropes with triple-layer periodicity were theoretically designed.All of the proposed structures are energetically more favorable than the experimentally observed AA structure,implying that the proposed structures may exist in hBN samples or other disordered BN phase.The layer-structured AAb-and ABc-BN with direct bandgap were predicted,providing direct evidence for direct bandgaps in hBN samples.Using a series of layered BN structures as the structural model of disordered layered BN,we theoretically investigated the structural transition in disordered layered BN at high pressures,and a novel sp3 BN allotrope(tR3-BN)was discovered in the high-pressure simulation.The tR3-BN has lower energies than the known wurtzite BN(wBN),implying that it can be observed experimentally.It is superhard with the same Vicker hardness as that of cBN.There are significant structural features of twin boundaries in tR3-BN structure,which may be used to study the formation mechanism of nanotwinned cBN(nt-cBN).After the theoretical exploration of structural transition of disordered layered BN,we investigated the phase transition behavior of onion-like BN(oBN)nanoparticles under HPHT conditions.Combining in-situ synchrotron x-ray diffraction(XRD)techniques with HPHT methods,we obtained the phase transition diagram of oBN nanoparticles with pressure and temperature less than 22 GPa and 2200?,respectively.It is found that the results of HPHT phase transition are only hBN and cBN,not the well-known wBN.And oBN is easier than h BN for transition into cBN.The phase transition behavior revealed oBN having reversible polymorphic transition under pressure.