Study On The Structure And Physical Properties Of Alkali Metal/alkaline Earth Metal Borides Under High Pressur

Superhard materials is one of the six fields of modern new materials industry,which is widely used in cutting-edge fields such as geological detective,mechanical manufacturing,electronic information,medical equipment and aerospace.In 2015,the"Made in China 2025"proposed by The State Council listed superhard materials as key materials for national strategic development.At present,there are hundreds of enterprises and research institutions engaged in superhard materials in China,with an annual output value of 10 billion yuan.Diamond and cubic boron nitride are the main superhard production materials.However,such strong and short covalent bonds in traditional superhard matrials such as diamond and cubic boron nitride tend to localize the free electrons,thus causing them to behave as insulators,which limites their practical application.Therefore,improving the conductivity of traditional superhard materials or designing new superhard materials with high transport performance is of great significance for the potential application of next-generation electronics under extreme conditions.Metal borides（MB）have attracted extensive attention due to their fantastic structures and intriguing physical chemical properties.And great efforts have been made in the study of transition-metal borides（TMB）in recent decades.Both high electron density of transition-metal（TM）and strong B-B covalent bonding network of TMB determine their superior mechanical properties.In addition,the existence of conductive channels in the structures makes TMB exhibit excellent electrical conductivity and even low superconductivity.On the other hand,our previous studies on Sr-Ge system have enriched the structure and property diversity of Sr-Ge compounds.These results indicate that alkali and alkaline earth metals（AM）represented by Sr tend to combine with boron to form a wider variety of borides due to their high chemical activity,small radius and small mass,which are expected to exhibit exotic properties.This work focuses on finding and designing hard or superhard multifunctional materials with high conductivity and even superconductivity in AMB compounds,and has achieved innovative results in the following aspects:1.A class of hard superconductor AMB₇（AM=Li,Na,K,Mg,Ca,Sr）has been designed.We conduct an extensive crystal structure search on Ca B_n（n=1-12）compounds under high pressure.A clathrate boride,Ca B₇ with space group of Cmcm was found under high pressure.This structure is constituted by a B₂₃ cage with one centered Ca atom,which is the first clathrate structure in AMB systems.The theoretical prediction shows that the structure can be quenched down to ambient conditions and exhibits superior superconductivity and mechanical properties with a hardness of 20.4GPa and T_c value of 7.7 K.In addition,such structure remains stable after substituting the Ca atoms with other AM atoms,including Li,Na,K,Mg and Sr,forming a new hard AMB₇ superconducting family.Among them,the highest hardness（25.1 GPa）and maximum T_c value（29.3 K）have achieved in Sr B₇ and Mg B₇,respectively,while KB₇exhibits both high hardness（H_v=22.5 GPa）and high superconducting transition temperature（T_c=26.2K）.This work opens up a new idea for searching and designing new superconducting materials with good mechanical properties.2.A waffle-like Ca B₁₂ has been designed,and a series of multifunctional materials,such as superhard metals,hard superconductors and superhard superconductors have been obtained.Besides the known I-42m phase,we found three hitherto unknown Ca B₁₂ structures under 0-200 GPa pressures,Cmmm（0-12 GPa）,I4₁/amd（25-75 GPa）and P6/mmm（75-200 GPa）,completing the study of Ca B₁₂ in high pressure.P6/mmm-Ca B₁₂ phase shows a special waff-like structure,named waffle-Ca B₁₂.The results show that waffle-Ca B₁₂ is a superhard metal material with the Vickers hardness of 50.1 GPa,which is the highest hardness among known metal dodecaborides and AMB compounds.By replacing all or part of the Ca atoms with other AM atoms（AM=Li,Na,K,Mg,Sr）,we have obtained a series of superhard metal materials,hard superconducting materials and superhard superconducting materials.This work provides an important theoretical basis and guidance for the experimental preparation and physical property exploration of superhard and superconducting AMB multifunctional materials.3.Study on the structure of boron-rich K-B compounds under high pressure and superconductivity of several boron allotropes.By searching the structure of boron-rich K-B binary compounds at 0-100 GPa,the phase diagram of boron-rich K-B system is completed.Four metastable potassium borides(Pmm2 KB₅,Pmma KB₇,Immm KB₉ and Pmmm KB₁₀)with open-channels of boron atoms were predicted successfully.Interestingly,removal of K atoms in the center of the channels leads to the formation of four new boron allotropes,named o-B₁₅,o-B₁₄,o-B₃₆,and o-B₁₀,which possess unique structural units and chemical bonding patterns.And the predicted superconducting transition temperatures are 15.9,29.1,28.1,and 22.2 K for o-B₁₅,o-B₁₄,o-B₃₆,and o-B₁₀ at ambient pressure,respectively.This work provides a new way to search new superconducting boron allotropes from high-pressure binary phase.