Benchmark Calculation Of The Enthalpy Of Formation Of MgH₂ And First-Principles Study For Heusler Alloys ZrMnVz And ZrCoFeZ（Z=Si,Ge）

The first part is my earlier work, focused on researching hydrogen storage material MgH2.Quantum Monte Carlo(QMC) is always limited by the finite size error which is caused by the limit size of crystal. We adopted the method of extending some finite size cluster to infinite size which have minimize the finite size error. This is proved better than the periodic boundary conditions(PBC). So we have obtained the dissociation energy of MgH2 solid very close to the experimental result. Therefore, we believe this method can be widely used.Then we focused on researching in the field of spintronics. As the developing of the microelectronics industry, spintronics has become a hot research field. It has been widely used in our everyday’s life. Half-metallic ferromagnetism is one of the most important properties in spintronics materials. So many researches have paid attention to searching for new materials which own this property. Heusler compounds are known for their special magnetic properties. Furthermore, Heusler compounds have specific crystals structure and wide component. Therefore the Heusler compounds have been a pool of searching for new Half-metallic ferromagnetism materials. Recently, many Heusler compounds with Half-metallic ferromagnetism have been found, which develops the spintronics.Next we have carried out density functional calculation using the scalar relativistic version of the full-potential local-orbital(FPLO) minimum-basis band-structure method on Heusler alloys ZrMnVZ and ZrCoFeZ(Z=Si,Ge).The second part of the thesis shows ZrMnVSi, ZrMnVGe and ZrCoFeSi are half-metallic ferromagnetic materials. Their half-metallic gaps are 0.14 eV, 0.18 eV and 0.22 eV respectively.They can keep their half-metallic ferromagnetic properties under high pressure. ZrCoFeGe is a nearly half-metallic material. Its spin polarization rates up to 98.99 % in the equilibrium lattice constant. It will transform into half-metallic material under 0.20 GPa pressure. They also have high stability, include: structural stability, physical stability, chemical stability and mechanical stability. And they have high values of Curie temperature. ZrMnVSi, ZrMnVGe have high value of Curie temperature than that of room temperature, while the Curie temperature of ZrCoFeSi and ZrCoFeGe are close to room temperature. So these four materials have the potential to became ideal spintronics materials in the future.In the third part we performed a systematic investigation on the stability between austenite phase and martensitic phase of these materials. The results show that they all have a better stability with austenite phase. Meanwhile within a certain range of variation in the Z-direction,ZrMnVSi, ZrMnVGe and ZrCoFeSi can maintain their half-metallicity. In addition, ZrMnVSi and ZrMnVGe exist minima value of energy in metastable state of martensitic phase, which has large differences between with the globally stable state energy of austenite phase, so phase transition will not easily occur.In the fourth part we studied ZrCoFeGe doped with the main group element As and transition metal element Nb. We found that when doped with a certain percentage of As or Nb,ZrCoFeGe transforms into half-metallic material. In addition, the system remains a high stability after doping with As or Nb.In the fifth part we studied the surface properties of ZrCoFeSi. The results show that Si- or Zr-terminated system also can keep its half-metallic property. Meanwhile Si-terminated system provide a higher half-metallic gap, making the quality of the materials greatly enhanced.Finally we summarize our present work, and give an outlook on our future work.