First-principles Study Of Structures And Physical Properties Of Mg And Mg-based Compounds At High Pressure

Within this thesis, the crystal structuresof magnesium (Mg) are extensivelyexploredbased on the crystal structure prediction USPEX package. Two structures withsimple trigonal (STG) and simpletetragonal (STT) symmetries are discovered to possiblyexist under high-pressure. First-principles calculation results show that the STG structureis more stable than the HCP structure when the pressure beyond190GPa. Meanwhile, theSTT structure may exist at least above600GPa. The STG structure hasthe bestcompressibility, whereas the STT structure has the worst compressibility.In addition, thepressure has a significant effect on the electronic structure, but both stuctures remainmetallic in the studied pressure range.We also explored the potential structures of magnesium nitride with a chemicalcomposition of Mg3N2.In addition to the previously proposedα-,-and-Mg3N2, we findfive high-pressure phases-,-,-,-and-Mg3N2，and the enthalpy of all these five newstructures will be lower than that of α-Mg3N2at27GPa,30GPa,53GPa,68GPa and115GPa, respectively. All phases are direct band gap semiconductors except thatthe ω-Mg3N2phase has an indirect band gap. Furthermore, the estimated Vickers hardness of newphasesis higher than that of the known phases.In addition, we also investigated the potential crystal structures of Mg2C. Twotetragonal structures (P42/mnm and I41/amd) and two hexagonal structures (P63/mmc andP6m2) are found. The effect of pressure and temperature on the phase transition amongdifferent Mg2C polymorphshas been studied, suggesting these new phases may besynthesized and found in high pressure and high temperature conditions.What s more,theratio value of B/G is also calculated in order to analyze the brittle and ductile nature ofthese Mg2C phases. Electronic structure calculations suggest that the I41/amd phase issemimetallic and the other three new phases are all metallic.