Structure Prediction And Physical Properties Of ZnS-MgS Semiconductor Alloys And W-B Superhard Compounds

ZnS is an important wide bandgap semiconductor material,after forming an alloy with MgS,the bandgap can be adjusted,which can greatly expand its application range.Tungsten borides are a new class of potential superhard,with excellent mechanical properties comparable to traditional superhard materials,and can be synthesized without the need of high pressure.However,the in-depth understanding of these two systems is hindered by the uncertainty of their phase relations and complex crystal structures.In this thesis,firstly,we explored the ZnS-MgS semiconductor alloys and the W-B superhard compounds systematically by combining the crystal structure prediction and the first-principles,and then analyzed the electronic and mechanical properties of the predicted structures.And the innovative results are as follows:1)For binary semiconductor ZnS and MgS semiconductor,we systematically explored the crystal structures at pressures from 0 to 180 GPa,and analyzed the electronic properties of these structures.We successfully found the known structures of ZnS and MgS under low pressure,and discovered the high-pressure phases P4/nmm of ZnS,and P213 and R3?of MgS for the first time.P4/nmm is a metal phase,indicating that the traditional semiconductor ZnS eventually becomes metal under pressure.We revealed the phase transition pressures between these structures through detailed enthalpy calculations,in which the known Cmcm of ZnS transforms to P4/nmm at 67GPa,and the known B1 of MgS transforms to P213 at 128 GPa and then P213transforms to R3?at 157 GPa.Note that the phase transition pressures between the known phases of ZnS and MgS we calculated was consistent with the experimental values.2)For the ZnS-MgS alloys,we found four stable structures with different Mg components?Pmm2-MgZn₇Mg₈,P21/m-MgZn₃S₄,P4/nmm-MgZnS₂,and R3-Mg₈ZnS₉?by the crystal structure prediction in the pressure range 0-180 GPa,and constructed the complete pressure-composition phase diagram.Further investigations reveal that the high-pressure phases of binary compounds play the dominant roles in constructing the alloying structures:the structures of Zn-rich alloys?MgZn₇S₈,MgZn₃S₄,and MgZnS₂?rely on the metallic P4/nmm phase of ZnS and those of Mg-rich alloy?Mg₈ZnS₉?on the R3?phase of MgS.This leads to that the band gap increases in the Zn-rich range and decreases in the Mg-rich range with Mg concentration.3)We explored the W-B system systematically by ab initio variable-composition evolutionary simulations at pressures from 0 to 40 GPa.Our calculations successfully found all known stable compounds and discovered two novel stable phases P4?2₁m-WB,P21/m-W₂B₃,and three nearly stable R3m-W₂B₅,Ama2-W₆B₅,Pmmn-WB₅ phases at ambient pressure and zero kelvin.Interestingly,P4?2₁m-WB is much harder than the known?and?phases,while Pmmn-WB₅ shows the highest hardness.Furthermore,it is revealed that the debated WB₄ becomes stable as P6₃/mmc?2 f.u.per unit cell?phase at pressures above 1 GPa,not at ambient pressure as reported previously.Our findings provide important insights for understanding the rich and complex crystal structures of tungsten borides,and indicate WB₂,WB₄,and WB₅ as compounds with the most interesting mechanical properties.