Theoretical Studies Of The Behavior Of Alkali Metals Sodium,potassium And Its Compounds Under High Pressure

Each alkali metal only has a valence electron. There is only a weak interaction between its valence electron and the core, so alkali metals are considered"simple metal"for a long time. However, under compression, these simple systems exhibit unexpected complexity. Firstly, they exhibit structural diversity. Under high pressure, the alkali metals show many complex structures. Secondly, they show unexpected electronic structures. Under high pressure, the light metals, such sodium and lithium show a transition trend towards insulator. And thirdly, the light alkali metals show unexpected melting curves. Measurements of the sodium melting curve reveals a pressure-induced dropping in melting temperature from 1,000K at about 30 GPa down to room temperature at about 120 GPa. And to today's knowledge, sodium is the only system with a decreasing melting temperature over large pressure ranges to about 90 GPa. According to previous work, all the complexity of alkali metals under high pressure relates to the changes of their electronic structures. So our work focuses on the electronic structure of alkali metals and its relative properties. The detail of our work is as following:Firstly, we used first-principles calculations to investigate the electronic structure of the new oP8 phase of sodium which was experimentally reported recently. Our results show the transition from I -43d to oP8 structure, which happens at room temperature, can also happen at 0 K. The I -43d structure will change to the oP8 structure at about 155 GPa and 0 K, rather than the CsIV structure at 190 GPa and 0 K, as the previous studies predicted. It is also found that the oP8 structure forms a new nonequilateral triangle Na3 structure and mainly distributes charge accumulation in the voids of the structure, rather than within the Na3 triangles. Electronic density of states analysis shows that the oP8 structure opens a deeper pseudogap close to the Fermi level through symmetry breaking of the structure compared with that of the I-43d structure. Together with its unusual charge density distribution, it is found that the Peierls mechanism works for the transition to the oP8 structure. Different from previous results on the Peierls mechanism of light alkali metals, the unit which produces a one-dimensional charge density wave is the Na3 cluster instead of the pairing mechanism.Secondly, we investigated extensively the melting curve of cI16 sodium at high pressure by means of Lindemann melting criterion and ab initio molecular dynamics simulations. Our results show that the melting point of cI16 sodium continues to decrease with increasing pressure, different from previous theoretical results. It is found that the negative melting curve of the cI16 phase is not due to the Peierls-like liquid transition which was reported to be the origin of negative melting curve in f.c.c. sodium. The elastic constant softening plays an important role and results in a negative Lindemann melting curve. Electronic structure analysis shows that more electrons in cI16 liquid localize in the voids of lattice just like in its solid state, the exclusionary effect of atomic cores on valence electrons in the cI16 liquid still takes effect.Thirdly, by means of ab initio molecular dynamics and Kubo-Greenwood formula, we investigate the conductivity of fluid potassium under high pressure. Our results show the conductivity of fluid potassium increases first and then decreases with increasing pressure. The increase of conductivity at low density is related to the Mott transition. At high density, the electron is more scattered by ion, together with exclusionary effect of atomic cores on valence orbitals finally make the conductivity decrease.Finally, we searched for candidate high-pressure phases of KHn using a combination of ab initio calculations and'random structure searching'technique, we find that six hydrogen and one potassium can combine together and form a stable structure at a pressure above 75 GPa. At lower pressure, the C2/m structure of KH6 is stable. Further compressed to 166 GPa, it changes to the C2/c structure, which is metallic. The hydrogen atoms in the two structures forms ring atomic arrangements. By adding small amounts of potassium to hydrogen, the formed KH6 can metalize at a pressure within 166 GPa. The superconducting transition temperature in the layered metallic phase C2/c is found to be in the range of 58~69 K.