The Study On High Pressure Phase Transitions And Electronic Properties Of The Alkali Peroxides And Sodium Superoxide

The same kind of material has different crystal structures;structure determines the nature,so the different structure as material properties have a greater impact.On this basis,the structural phase transition and its phase transition mechanism in many areas such as earth science,material science and chemical physics are of great significance.Pressure as a one-dimensional conditions in addition temperature,for a variety of physical and chemical properties of substances have a degree of impact.Pressure on the material,can effectively reduce the atoms or molecules distance,the material crystal structure,energy band structure and its electronic orbital structure modulation.High-pressure phase-change is the phase change of matter caused by high-pressure environment.At high pressure,the interatomic distance shortens the instability of the system caused by the interaction of electrons and lattices,which leads to the change of energy in the material.In the high-pressure or high-pressure high-temperature conditions,the crystal structure,atomic arrangement of atoms and electronic structure will change,and changes in physical properties,such as from the insulator to metal,volume and resistance to a large degree of change,this phenomenon is high-pressure phase change.In this paper,the structure and electronic properties of alkali metal peroxides and sodium superoxide were calculated using the first principles.The main work includes:(1)Here,we reported the structural phase transition,lattice dynamics,and electronic properties of alkali metal(Li,Na,K,and Rb)peroxides by using the unbiased structure searching techniques and first-principles density functional calculations in the pressure range of 0–100 GPa.The predicted first-order phase transitions pressures in Li2O2,Na2O2,K2O2 and Rb2O2 occur at 84,28,7 and 6 GPa,respectively,which closely correlates with the electronegativity of alkali metals.These predicated high-pressure phases are thermodynamically stable against decomposition into alkali metal oxides plus O2 or alkali metals plus O2.Interestingly,the character of the peroxide group(O22-)is maintained under the considered pressure range.Phonon calculations using the quasi harmonic approximation confirm that these structures are dynamically stable.The band gaps for the studied alkali metal peroxides increase with increasing pressure.(2)By utilizing first-principles swarm structure-searching predictions,herein,we for the first time investigate the structures and electronic properties of NaO2 in the pressure range of0–20 GPa.It is found that the orthorhombic Pnnm structure at ambient pressure transforms to another orthorhombic Immm structure at approximately 4.6 GPa,and subsequently to thetetragonal P4/mbm structure at 6.7 GPa.The pressure-induced structural transitions are mainly derived from the denser polyhedral packing and higher coordination number.It is interesting to find that the superoxide group(O2)is maintained over the entire pressure range considered.Analysis of the electronic band structure and density of states shows that the structures found exhibit intriguing half-metallic magnetism.