Theoretical Study On Stability And Physical Properties Of Binary Li-P Compounds Under High Pressure

Electrides are unique compounds where some electrons reside at interstitial regions of the crystal behaving as anions,which strongly determines its physical properties.Interestingly,the magnitude and distribution of interstitial electrons can be effectively modified either by modulating its chemical composition or external conditions?e.g.,pressure?.Due to strongly electronic location,electrides are usually insulators.However,recent experiments showed that a canonical electride[Ca₂₄Al₂₈O₆₄]⁴⁺?e^-?₄ exhibits superconductivity?T_c⁰.4 K?,opening up the study of superconductivity in electrides.Alkaline and alkaline-earth metals can easily lose s orbital electrons to interstitial sites under pressure,thus forming electrides.Pressure-induced lithium?Li?becomes a metallic electride.Considering that phosphorus?P?has a moderate electronegativity and it is a remarkable superconductor,in Li-rich Li-P compounds,P can capture some of the electrons and the rest can be localized at interstitial regions.Therefore,it is expected to modify the electride states by regulating the Li/P ratio in the compounds,and further obtaining compounds with novel electronic properties.Based on the first-principle calculations and structure search technology,we conducted a systematic and in-depth research in Li-P system under high pressure.The relative stability of Li-P compounds was determined,and the stable phase diagram in0-300 GPa range was given.With the pressure increases,Li-rich compounds tend to be stable,which form electrides with P hypercoordination.In these compounds,excess electrons accumulate in interstitial spaces acting as anions,and there is a strong interaction between them and Li ions,thus enhancing the structural stability.Besides,interstitial electrons form connected electronic channels,so that these electrides present good conductivity.In this thesis,we focus on the properties and applications of Li₅P and Li₆P compounds.Li₅P becomes stable above 10 GPa,which is an unprecedented graphene-like two-dimensional electride and presents good metallicity.As an anode material of lithium-ion batteries,Li5P exhibits favorable performances,such as low open circuit voltage and high theoretical capacity.Besides,its mechanical properties can be improved under increasing pressure,which provides useful information for optimizing the performance of P anode.Superconductivity is commonly obtained in Li-P electrides under high pressure.Notably,Tc of Li6P is as high as 39.3 K,which is the highest among the already known electrides.In addition,interstitial electrons play an important role in superconductivity.As pressure decreases,the localized interstitial electrons enhance,Fermi surface nesting increases,and phonon softening increases,which leads to improve the superconducting transition.These studies provide a new way for exploring high-temperature superconductivity of electrides.