The First-principles Study Of Electride States Of Intermetallic Compound BaMg₂ Under High Pressure

As a basic physical parameter independent of chemical composition and temperature,pressure can promote the rearrangement of atoms or molecules,resulting in the generation of new compounds,and provides a new way for the development and design of novel functional materials.Previous studies have shown that high pressure plays an irreplaceable role in the search for novel intermetallic compounds.High pressure can effectively reduce the chemical reaction barrier of metals,driving them to form high-pressure intermetallic compounds that are not stable under ambient conditions.Due to the presence of unique crystal structures,mixed chemical bonds（e.g.,ionic bond,covalent bond,and metallic bond）,and diversified stoichiometric ratios,intermetallic compounds exhibit unique physical and chemical properties,which gives them a wide range of applications in the fields of catalysts,superconducting materials,superhard materials,hydrogen storage alloys and so on.Therefore,using high-pressure technology to develop and design novel intermetallic compounds has important research prospects and scientific significance.Metal elements exhibit unique electron transition behavior under high pressure,which is different from that under ambient conditions.For example,as pressure increases,metals often undergo anomalous s-p,s-d,and p-d orbital electron transition.The electrons of some metals will enter the quantized orbitals of non-nuclear electride states.For example,under high pressure,the electrons of sodium will enter the interstitial voids,which are surrounded by the nucleus,resulting in the transformation from metal to semiconductor.Such electrons that enter the quantized orbital of non-nuclear electride states are called“electride-state electrons”,and the corresponding compounds are called“electride”.The appearance of electride states leads to the unique physical and chemical properties of intermetallic compounds,which have an important effect on the properties of intermetallic compounds.Magnesium and barium are important alkaline earth metal elements with high electropositivity.Under ambient conditions,they are easy to lose their s-shell electrons and show a+2 oxidation state in the compound.But under high pressure,their electron behavior must be quite different from that under ambient conditions.At present,there are some studies on intermetallic compounds composed of magnesium and barium under ambient conditions.Under ambient conditions,magnesium and barium can form the MgZn₂-Laves phase stable binary intermetallic compound BaMg₂.However,the crystal structure and physical properties of intermetallic compounds composed of magnesium and barium under high pressure are rarely reported.Understanding the physical properties of Ba-Mg intermetallic compounds under high pressure is of great scientific value for the practical application of these compounds.Therefore,in this paper,the first-principles calculation method combined with the crystal structure prediction program CALYPSO based on the swarm optimization（PSO）algorithm was used to conduct a global search study on the crystal structure of BaMg₂under high pressure.Based on the stable crystal structure of MgZn₂-Laves BaMg₂under ambient pressure,the phase transition sequence and structural evolution of BaMg₂under high pressure are predicted,and the physical properties of BaMg₂compounds under high pressure,especially the electride-state behavior,are further studied.Through systematic theoretical research,the following innovative research results are obtained:1.The high-pressure phase transition sequence of BaMg₂ intermetallic compound was determined and the high-pressure phase structure of BaMg₂ with metallic properties was predicted.2.BaMg₂ was identified as a typical intermetallic electride under high pressure.It is found that with the increase of pressure,the interstitial electrons gradually decrease,and BaMg₂ gradually transforms from 2D electride to 0D electride accompanied by the high-pressure phase transitions.3.Anomalous Barium 6s-5d electron transition was found in BaMg₂under high pressure.Besides,barium has strong electronegativity under high pressure and exhibits-2 oxidation state.4.It is determined that the primary reason for the formation of electride states is that the energy level of magnesium 3s-3p hybrid orbital is higher than that of the interstitial nucleus-free electride states under high pressure.