Theoretical Research On The Electronic Structure And Optical Properties Of Lithium Under High Pressure

Lithium has tremendous exotic and intriguing characteristics at high pressure,including intricate phase diagrams,abnormal low-temperature melting,superconducting,metal-insulator transition and so on,which entice broad research in recent years.These fascinating phenomena are essentially related to the behavior of electrons.On one hand,the electronic structure of dense lithium has changed dramatically companied by excitation of electron from 2s states to 2p states and the hybridization of s and p orbitals.As a consequence,the fermi surface distorts gradually and a nesting fermi structure with copper-like necks appears.On the other hand,valence electrons suffer a strong repulsion from core electrons and are accordingly less attracted by nuclear under compression,due to Pauli exclusion or the required orthogonality by quantum mechanics.Some valence electrons thus move from the neighborhood of ions to the interstitial positions of crystalline structure,and behave like anions,forming electride.Electride is a novel material different from common metals or ionic compounds,in which electrons are localized within specific regions while are able to be activated by low energy.It reveals huge potential in wide applications such as catalyst,electron emission,and transparent conductor.However,because the compositions and crystal structures of electrides at ambient pressure are complex,the dielectric responses and optical properties of electrides have not been systematically studied,especially for the theoretical part.The metal-semiconductor-metal transition in dense lithium is considered as an archetype of interplay between interstitial electron localization and delocalization induced by compression for studying electrides.In addition,theoretical study of dynamics is significant to understand the physics behind physical properties and phase transitions.For electrons,optical property is a basic dynamic characteristic to describe electron-photon interaction and electronic excited states.So far,theoretical research on optical properties of dense lithium is limited to relatively low-pressure range,mostly on the fcc structure.For the higher-pressure range including the metal-semiconductor transition,changes of optical properties along with the opening of gap still remain unclear.Therefore,we systematically studied the dielectric and optical properties in electride phases of dense lithium by first-principles calculations.The important role of localized electrons in electrides is investigated by analyzing the evolution of electronic structures through the whole pressure range of the transition process.The dynamic dielectric response and optical properties of the high-pressure electride phases of cI16,oC40 and oC24 in lithium spanning a wide pressure range from 40 to 200 GPa are reported.Both interband and intraband contribution to the dielectric function are deliberately treated with the linear response theory.The reflectivity of these electrides in the visible light range is predicted to decrease with pressure.However,due to the irregular effect of compression in electrides,one intraband and two interband plasmons in cI16 at 70 GPa induced by a structural distortion at 2.1,4.1,and 7.7 eV are discovered,which remarkably make the reflectivity of this weak metallic phase lower than the insulating phase oC40 at the corresponding frequencies.oC24 as a semi-metal elelctride behaves semitransparent due to unique electronic structure,signaling the diversity and rich physics in electrides.An intriguing reflectivity anisotropy in both oC40 and oC24 is also predicted,with the former being strong enough for experimental detection within the spectrum up to 10 eV.The important role of localized interstitial electrons is highlighted.These newly obtained results and findings might guide experiments on dense lithium and lay down the foundation for further exploration of electrides.Dynamics of ions also play an important role in phase stability and transition at high pressure,especially for light elements like hydrogen and lithium.However,current theoretical research in lattice dynamics of dense lithium is mostly based on the harmonic approximation,which is not accurate enough to describe the strong anharmonicity in lithium's vibration.To fully understand how strong the anharmonicity is and how it changes through various phases in dense lithium,the anharmonic effects of bee,fee,and cI16 phases of dense lithium are investigated in this work using temperature dependent effective potential method.The anharmonic phonon dispersion relationship and phonon lifetime are calculated,removing the instabilities calculated in harmonic approximation.Temperature dependent thermal properties are analyzed as well.The results show that the anharmonicity of these phases are all evident,especially for cIl6 and fee.The anharmonicity of the fee phase decreases as a function of pressure,leading a gradual increasing lattice thermal conductivity(LTC).A evident jump of LTC is predicted when phase transition from fee to cI16,due to the low group velocity of cI16.A thorough analysis to the changes of electronic structure through the metal-semiconductor-metal transition,and how they determine the optical properties of the electride phases in lithium,is provided in this work.It helps to get a better understanding to the physics images of electrides by revealing its uniqueness and diversity of electron behavior.This research of high pressure electrides could give new theoretical reference for research and application in electrides.Besides,this work exhibits the evident impact of phonon-phonon interaction in dense lithium,explains the origin of structural instability in fee phase,and predicts several essential temperature dependent thermal properties.These theoretical results complement the current theoretical development and facilitate further research on the dynamics and energetics of phase transitions in dense lithium.