High Pressure Structures And Properties In Typical Elements Magnesium And Chlorine

The elements are the most fundamental materials and their high-pressure behavior has always attracted the attention of many researchers. At ambient conditions, the majority of the elements adopt very simple high symmetry structures. High pressure can reduce the distance between atoms, so that the crystal structure of the materials will be rearranged and the phase transition will be occurred and the new phases will be formed. For elements, there may be structural transitions to phases with lower symmetry and less close packing than found at ambient pressure. For example, iodine and bromine form incommensuration modulated structures under pressure. Changes of structure and electronic energy levels can also give rise to dramatic changes in physical phenomena and behaviors. Elements that are insulators at ambient pressure exhibit structural phase transitions accompanied by metallization and the onset of superconductivity. Indeed, the number of known elemental superconductors has been increased by over 70% by high-pressure studies. Applying pressure can also induce transitions from molecular to non-molecular structural forms. The more intriguing is metals Na will transform under pressure into insulating states. And the conversion of gaseous molecular elements to a monatomic metallic superconductor phase is, for many, the''holygrail''of high-pressure physics. Combined with modern computational methods, much is now understood about the behavior of elements at high pressure. However, there are still a number of outstanding questions and further studies are needed. Of particular interest would be the discovery of further new incommensurate structures.Most recently, with the help of improved theory and computational capability, the first principle method base on the density functional theory has been used widely in the high pressure structures and physics behaviors. The newly developed crystal structure prediction method extended the research field for theorist once again. CALYPSO (Crystal structure AnaLYsis by Particle Swarm Optimization) is a package for crystal structure prediction through particle swarm optimization algorithm. CALYPSO requires only chemical compositions for a giver compound to predict stable or metastable structures. It is proved to be powerful with high efficiency and a high success rate, such as the high pressure structure research of Li.Within this thesis, we studied the high pressure structures and properties of two typical elements, Mg and Cl, by using first principle method. We first determined their high pressure crystal structures based on the crystal structure prediction CALYPSO algorithm; then we studied their physical properties under high pressure.The first part of our work is to find high pressure structures of Mg, through the CALYPSO code. Then, we studied their electronic properties, chemical bonding features and the phase diagrams. We predicted correctly the hcp and bcc structures at low pressures. The predicted transition pressures, EOS, and temperature-dependent phase diagrams of the hcp and bcc structures are in excellent agreement with the experiments. A high-pressure face-centred cubic (fcc) phase and simple hexagonal (sh) phase has been uncovered to be stable 456-756 GPa and above 756 GPa. The sh structure can be derived from the fcc lattice by distortion of the ? andγangles from 60°to 90°. The fcc and sh structures are both metallic, but the metallicity is weaken than the low-pressure hcp structure. We also calculated the electronic DOS for Cl at different pressures. We found that the weakening of metallicity is attributed to the drop of 3d bands in energy relative to the 3p bands and the increased p-d hybridization upon compression. The ELF indicated that the valence electrons of fcc and sh are mostly localized in the interstitial sites, similar to alkali and other alkaline-earth elements, such as Na, Li, K, and Ca. The fcc and sh structures can be described as electrides formed by Mg ions cores and localized interstitial electrons. The valence electron localization of sh-Mg is not as strong as hp4-Na, and the core-valence overlap is not as forceful as Na. This could be the reason why hp4-Na is an insulator and sh-Mg is a metal. We have carefully calculated the complete phase transition diagrams of Mg at high pressures and temperatures within the quasi-harmonic approximation. Our results suggest that the temperature contribution does not affect the phase transition order, but slightly changes the transition pressures. The calculated phase boundaries for both bcc to fcc and fcc to sh transition are found to have positive Chaperon slopes.The second part of our work is to study the crystal structure and properties of Cl under high pressure. It divided into three parts. In part one our predicted high pressures structures, especially the incommensuration modulated structures through CAYLPSO are shown. The structural stability and transition mechanism are discussion. In part two, we discussions the pressure-induced metallization and molecular dissociation of Cl. Finally, in part three we focus our study on the superconductivity of Cl under pressure.First of all, our CALYPSO simulation predicted the Cmca structures to be stable, in agreement with experiment. Our simulation predicted that the most stable structure at 157-372 GPa is the bco structure (phaseâ…¡), which is isomorphic to solid bromine and iodine. The low enthalpy structure found above 372 GPa is the fcc structure. No imaginary phonon frequencies are found in the pressure ranges of 0-142 and 157-372 GPa for the Cmca and bco structure repeatedly and above 372 GPa for the fcc structure in the whole Brillouin zone, suggesting that the three structures are dynamically stable. In addition, at 150 GPa we also predicted a series of similar structures with different of space group and similar low-enthalpy. They all have similar"modulation wave"feature as incommensurate modulated structure. The x-ray diffraction patterns of our structure are very similar with the simulated incommensurate phase. The main peaks are the same with fco. The distances between neighboring atoms are not the same value, but there is a change of interval. Accordingly, we believe that chlorine also exists similar incommensurate structures with iodine, we call this phase as phaseâ…¤. Through optimization, we found that modulated phase oF28 which wave vector is 2/7 has the lowest enthalpy in the pressure range from 142 GPa to 157 GPa. For the analysis of mechanical nature of chlorine, we found that the elastic constants C₆₆ < 0 at 150 GPa, where the stability criterion is violated, implying that the structure is unstable at a-b plane. The modulated structure (such as oF28) is most stability structure. When approaching 160 GPa, the elastic constants of Cmca structure all taken place obvious mutations, particularly the C 45and C 55 drop to zero at about 170 GPa, implying that the phase transition happens. We assume that incommensurate structure coexists with phaseâ…¡ over a pressure range of 157-170 GPa until fully transformation the bco above 170 GPa.Later on, we calculated the electronic band structure, the partial electronic density of states (DOS), and the valence electron localization function and the scaled volume versus pressure. It can be clearly seen that chlorine elements transformed from atmospheric pressure insulator structure to high pressure metal structures at 130 GPa, and this change is caused by the gap closure under pressure. The crystal structure remains the same as that of ambient pressure diatomic molecular phase. By the structural scaling rule for Cl, its semi-metallization will take place at 130 GPa. The ELF clearly shows the electrons localize condition of molecular phase, modulated phase and monatomic phases. Above 130 GPa, the electrons which located at the sides of atoms transfer to the intermolecular space, resulting in metallization. At 157 GPa, the molecular dissociation takes place and the structure transforms to a monatomic bco structure. On the basis of a comparison of interatomic distances, elastic stiffness coefficients and Raman patterns under pressures, we found that Ag mode and shear elastic stiffness coefficient C 55 soften induces the metallization. This will lead to the emergence of incommensurate modulated structure ultimately.Finally, we studied the superconductivity of chlorine bco and fcc metal phases under high pressure. We found that element chlorine become superconducting at high pressures. The superconducting temperatures of bco and fcc phases are 3.73 and 13.03 K at 160 and 380 GPa, respectively. The superconducting temperature of bco decreased slowly under pressure. The superconducting temperature of fcc decreased under pressure and then increased. The superconducting temperature is 11.2 K at 420 GPa and 13.21 K at 500 GPa, respectively. Although the trends in the calculated superconducting properties are similar with iodine and bromine, the superconducting transition temperature of fcc phase is higher in chlorine than in iodine and bromine, even within the same structural phase. This is probably because that the electronic crossing Fermi level features apparently satisfy the"flatband-steep band"scenario, which has been suggested to be a favorable condition for the occurrence of superconductivity.