First-Principle Study Of Structure And Superconductivity Of MgB₂ At High Pressure

The discovery of superconductivity for magnesium diboride (MgB2) in 2001 has extensively stimulated worldwide attentions in the scientific field. The superconducting transition temperature of MgB2 reaches about 39 K. Pressure is one of the basic parameters (Pressure, Temperature, Chemical component) which can change the properties of material. The crystal structure volume reduces quickly under high pressure, thus the distances between atoms in materials reduce effectively, as well as the overlap of electron orbits between two adjacent atoms changes at high pressure, which results in the change of bonding behavior significantly and induces structural phase transitions. Therefore, the high pressure research enormously develops the understanding of the material world, and high pressure technology has become a useful tool to discover new materials. Thus, the high pressure study of the crystal structures and superconductivity for MgB2 has stimulated worldwide excitement in the scientific field. Indeed, experiments demonstrated that superconducting transition temperature (Tc) of MgB2 decreases under pressure at a rate from -0.8 to -2.0 K/GPa. This occurs continuously within the same structure and experimental measurements up to 57 GPa did not reveal any structural phase transitions. The discovery of new stable of MgB2 is of great fundamental interest and, in particular, would help to reveal the key factors responsible for the formation of superconducting states- a knowledge that can even be used for design of new high-Tc materials. In this dissertation, we have performed detailed studies of the electronic structure, lattice dynamics and superconducting properties of MgB2 within the DFT under high pressure, obtaining original results in below: (1) We present a first-principle investigation of structural optimization on theAlB2 -type structure and get the lattice parameters under high pressure. Our ab-initiocalculations ruled out the transition from AlB2-type structure to the UHg2-typestructure. According to the optimized lattice parameters, we have calculated electronicproperties, lattice dynamics and electronic-phonon interaction of MgB2 at highpressures. We have computed the full phonon-dispersion curves at 0 and 200 GPa andfound no imaginary phonon frequencies, i.e., this structure is dynamically stable. Inaddition, we find that in MgB2 at zero pressure the frequency associated with E2gphonon mode, which plays a pivotal role in sustaining superconductivity, at is 15THz, which goes up to 40THz at 200 GPa. The Px,y densities of states (DOS) ,whichplay a crucial role in determining the superconducting properties of MgB2, aroundFermi level (EF) is significantly reduced as pressure increasing. According to ourcalculation, the strength of the electron-phonon coupling (EPC) and Tc bothmonotonous decrease until 200 GPa. Due to the harden-lattice of E2g phonon modeand dramatically reduction of B Px,y DOS at EF, the superconductivity in MgB2vanishes at 200GPa.(2) We report a high-pressure orthorhombic KHg2-type polymorph of MgB2stable above 190 GPa predicted through our newly developed ab-initio evolutionarysimulations (USPEX) for the first time. The formation of this new phase results fromthe strong out-of-plane distortions of the two-dimensional honeycomb boronsublattice of the low-pressure AlBB2-type structure creating a peculiarthree-dimensional cage compound. At the transition, 4 B-Bσbonds (sphybridization) are constructed for the KHg2 instead of 3 B-Bσbonds (sphybridization) formed in the AlB2. Moreover, we simulate electronic properties,phononic properties, and electron-phonon interaction of high-pressure KHg2-typestructure of MgB2 in the framework of linear response theory. In order to probe thisstructure by experiments, we do Raman analysis for the high pressure phase. Theelectronic properties calculation demonstrates that the high-pressure phase of MgB2 isa semi-metal, and the metallic property become weaker as pressure increasing. There are no imaginary phonon frequencies at the full phonon-dispersion curves from 0 to 500 GPa. Thus,the high pressure phase (KHg2) is dynamic stable from 0 to 500 GPa, The EPC calculation shows that strength of EPC is weak. It is about 0.24 (Tc ~ 0 K) at 200GPa. Therefore, the MgB2 at high pressure phase is not a good superconductor. The B-layered structure is advantageous for high Tc, this is useful for the design of novel superconductors related B.