Study On The Structures And Physical Properties Of Heavy Alkali Metals Rubidium And Cesium And Their Compounds Under High-pressure

Alkali metal elements adopt a highly symmetrical body-centered cubic structure under ambient pressure,their electronic structures can be described by the free electron model.However,alkali metals show many unusual new phenomena under the effect of pressure,such as the new complex structure with a larger number of atoms and lower or higher symmetry,pressure-induced transformation into a superconductor,abnormal changes in physical properties such as electrical resistance under high-pressure,as well as the transformation of metals to insulators.The novel physical phenomena of these new phases of high-pressure have greatly enriched people's cognition of high-pressure science.Under environmental pressure,Cs has a simple body-centered cubic(bcc)structure.Carrying out high-pressure research on cesium is conducive to exploring its phase transition sequence and hidden physical phenomena under high-pressure,and it promotes the high-pressure research of other alkali metal elements.In this thesis,the structure search technology is used to predict the high-pressure crystal structure with the simulation cells ranging from 2 to 16 atoms at 200,250 and300 GPa.At the same time,based on density functional theory,systematically calculate the electronic structures and electronic properties of the alkali metal element Cs under high-pressure.Alkali metal cesium(Cs)possesses the body-centered cubic(bcc)phase under ambient pressure,and the phase transition of bcc?fcc will be completed at 2.3GPa.Further compression,the determined transition sequence for cesium is:fcc?o C84?t I4?o C16?double hexagonal close-packed(dhcp).In this thesis,we report the re-transformation of fcc phase above 180 GPa beyond the dhcp phase using structure search and total-energy calculation.Keeping this in mind,the fcc structure is actually a known structure of Cs at lower pressures(2.3-4.2 GPa),hence a recurrence of the same structure.Our calculations of transition state indicate that the transition from dhcp to fcc phase at 200 GPa exists by overcoming the energy barrier(144me V/atom),with volume change of 0.3%.The electronic states near Fermi level for fcc at high-pressure are primarly evolved from d electron.Moreover,the strong compression forces electrons of inner shell with a large overlap to act as valence electrons,making a distinction between the recurrent fcc Cs and the first one at low pressure.It must be emphasized that the dhcp?fcc phase transition similarly takes place in K and Rb,however,with higher transition pressures.Based on the research work of Rb and Cs,we studied the crystal structures and electronic properties of RbCs_n(n=1,2,3)compounds in the pressure range of 20-200GPa,and predicted their stable structures under different pressures.We found that they follow different phase transition sequences.The coordination number of RbCs and RbCs₃ structures increases from six-fold coordination to eight-fold coordination and from ten-fold coordination to twelve-fold coordination with the structural phase transition,respectively.The calculation of the electronic band structures of the stable phases of RbCs_n(n=1,2,3)under the corresponding pressures shows that these phases exhibit metallic behavior,and the calculation of the density of states suggests that the density of states of Cs-d state contributes the most,indicating that the density of state of Rb-Cs binary compound system may be largely occupied by the d orbitals of Cs.