The Structural Prediction And Experimental Investigation On Novel Transition-Metal Diborides

Transition metal diboride(TMB₂)plays an important role in the national economy and national defense because of its unique properties such as high melting point,high hardness,excellent chemical stability,high thermal conductivity,low resistivity,and light-weight.Therefore,to explore the new microstructure and properties of TMB₂materials has become the common goal of science and industry.In this thesis,we investigated the issues concerning new structures and phase transitions of TMB₂ via a variety of calculations methods;Then,the structural changes of ZrB₂ and La-doped ZrB₂were studied experimentally via high-pressure and high-temperature(HPHT)methods.The structure and phase transition mechanism of TMB₂(TM=Zr,Sc,Ti,Nb,and Y)have been theoretically studied.A novel TMB₂ polytype,tP6,was revealed through the CALYPSO search.Upon subjecting to pressure,these TMB₂ compounds underwent universal phase transitions from an Al B₂-type to a new high-pressure phase tP6 structure.The analysis of the atomistic mechanism suggests that the tP6 phases result from atomic layer folds of the Al B₂-type parent phases under pressure.Stability studies indicate that the tP6-structured ZrB₂,Sc B₂,and Nb B₂ are stable and may be observed under high pressure and the tP6-structured Ti B₂ phase may be recovered at ambient pressure.This study may enrich the common understanding of Al B₂-type TMB₂ high-pressure transitions.The superconductivity of three new ZrB₂ structures has been theoretically studied.The structural stability of the three novel ZrB₂ phases was confirmed on the basis of elastic constant and phonon dispersion calculations.In addition,tP6 phase is dynamically stable at above 20 GPa,and it is the most stable structure over 800 GPa because the energy is even lower than the known Al B₂-type structure of ZrB₂.Theoretical analysis of their superconductivity shows that only the tP6 phase is superconductive.The T_c values of the superconductivity transition temperature at 20 GPa and 600 GPa are 12.7 K and 34.4K,respectively,indicating that the pressure may enhance the interaction of electron-phonon coupling.The bonding properties,mechanical properties,hardness,and Debye temperature of three new ZrB₂ structures are studied theoretically.It is found that the chemical bonds of three new ZrB₂ structures are mainly covalent,including some ionic bonds and metallic components.The?-Ti B₂-type and?-Ti B₂-type ZrB₂ are brittle materials and tP6 phase exhibits ductility,and tP6 phase has the best compressibility and the smallest shear modulus.The results show that the hardness values of?-Ti B₂-type and?-Ti B₂-type ZrB₂are close to 23.8 GPa at ambient pressure,and that of tP6 phase is 17.4 GPa at 20 GPa.The Debye temperature of?-Ti B₂-type and?-Ti B₂-type ZrB₂ is very close,905.1 K and908.6 K respectively,and that of tP6 phase is the lowest at ambient pressure,about602.6K.The structural changes of ZrB₂ and La-doped ZrB₂ were studied experimentally via high-pressure and high-temperature(HPHT)methods.The variation of ZrB₂ resistance in the pressure range of 30 GPa was studied by in-situ resistance measurement under high-pressure.The results show that:the resistance of ZrB₂ suddenly increases as the pressure increases to 22 GPa at room temperature,indicating that there is an obvious phase transition process;Then the resistance of the sample gradually decreases and stabilizes to a value higher than that of the initial raw material when the pressure continues to increase to the target pressure of 30 GPa;During the subsequent decompression to ambient pressure,the resistance of the sample remains unchanged.The experiment of La-doped ZrB₂ was carried out by the HPHT method,and there is an unknown new phase in the sample synthesized.The structure of the new phase needs further determination.