First Principles Study Of Scandium-Nitrogen Compounds And Inert Elements Compounds Under High Pressure

The acquisition of new materials by means of high pressure has been one of the hot research topics in recent years.Pressure can change the atomic spacing and affect the interaction between electrons,resulting in changes in the properties of substances and the resulting emergence of new high-pressure phases that exhibit excellent physical properties.In recent years,a number of new functional materials with excellent properties have been synthesized using dynamic and static high-pressure techniques,for example,single-bonded forms of cg-N have been synthesized using high-pressure means,thus opening up a wave of research on polymeric nitrogen in high-energy density materials,and people have started to explore nitrogen-containing and multi-nitrogen compounds,and a large number of nitrogen-containing compounds have been intensively studied.In addition,inert elements have a full shell electron arrangement,making it more difficult to gain or lose electrons,a limitation that can be broken using high pressure techniques.High pressures can lower the chemical reaction potential to facilitate the synthesis of compounds of inert elements.As the high synthetic pressures required for most materials are difficult to achieve under experimental conditions,therefore theoretical simulations have been used to find high-pressure phases with new structures in recent years and properties to provide theoretical support for experimental studies.In this paper,the crystal structures of compounds with different chemical ratios at different pressures were predicted and their electronic properties were calculated by using CALYPSO crystal structure prediction software combined with first-principles calculation method.The details of the study are as follows:（1）First principles study of transition metal scandium-nitrogen compounds under high pressure.We have systematically investigated Nitrogen-rich compounds Sc N_x（x=5-7）in the pressure range of 0-100 GPa.Theoretically predicted the monoclinic P2₁/c-Sc N₅structure is predicted to be an energy stable phase at 62 GPa,the synthetic pressure for P^_<sub>1-Sc N₆is 80 GPa and the polymeric nitrogen form of both structures is a 3D extended folded multi-nitrogen network with potentially high energy storage properties.P_<sub></sup>1-Sc N₇is thermodynamically stable in the 30-90 GPa range and contains an N₅five-membered ring and a bent N₄molecular unit.In addition,the structure of P2₁/c-Sc N₅and P_<sub></sup>1-Sc N₇are mechanically stable at ambient pressure,indicating that both compounds can be recovered to ambient pressure and exist stably after high-pressure synthesis.Structural analysis shows that P2₁/c-Sc N₅,P_<sub></sup>1-Sc N₆and P_<sub></sup>1-Sc N₇exhibit excellent energy storage properties due to the large number of N-N single bonds and N=N double bonds,and the energy calculation shows that the energy density released by decomposition of P2₁/c-Sc N₅,P_<sub></sup>1-Sc N₆and P_<sub></sup>1-Sc N₇decompose into Sc N and N₂at atmospheric pressure with energy densities of 4.19 k J·g-1,3.97 k J·g-1and 3.12 k J·g-1respectively,which is higher than that found in many known inorganic poly-nitrogen compounds.Excellent energy storage properties show that the three high-pressure structures can be used as potential high-energy materials.The study of Nitrogen-rich compounds Sc N_x（x=5-7）will help to obtain more polymeric forms of nitrogen and provide theoretical guidance for experimentalists to synthesize energy-containing materials with promising applications.（2）First principles study of calcium-argon compounds stabilised under high pressure.We used first-principles calculations combined with the CALYPSO structure prediction method to obtain a stable structure（P6₃/mmc-Ca Ar）and six metastable structures(R_<sub></sup>3m-Ca Ar₂,P4/mmm-Ca Ar₂,P_<sub></sup>3m1-Ca Ar₃,P4/mmm-Ca Ar₃,P2₁/m-Ca Ar₄and P_<sub></sup>3m1-Ca Ar₅).All Ca-Ar compounds exhibit metallic properties and are dominated by Ca-d orbital contributions at the Fermi energy level,which play an important role in the stability of the crystal structure.In these structures,as an electron acceptor,Ar can acquire additional electrons and form ionic compounds with Ca at pressures above 90 GPa.In addition we have found that the interaction between Ca and Ar increases with increasing pressure and that this unusual bonding mechanism may be a result of pressure-induced reordering of atomic orbital energies.It is worth noting that the only stable P6₃/mmc-Ca Ar structure remains structurally stable at 1000 K,exhibiting remarkable thermodynamic stability.The high-pressure structure and electronic behavior of the Ca-Ar system is expected to expand the understanding of the high-pressure chemical reactivity of inert element-containing compounds and provide important theoretical support for the search for novel anomalous alkaline-earth metal inert element compounds.（3）The study of theoretical prediction and properties of strontium argon system under high pressure.This section mainly uses CALYPSO crystal structure prediction method to search the structure of Sr Ar_x（x=1-4）system within the pressure range of 0-300 GPa and studies the properties of stable structures.Two thermodynamically stable structures（Cmcm-Sr Ar and I4/mmm-Sr Ar₃）and two metastable structures（P6₃/mmc-Sr Ar and P2₁/m-Sr Ar₄）are found at 150 GPa.The stability of Cmcm-Sr Ar changes at a pressure of 210GPa,while the Sr Ar₃compound remains structurally stable in the pressure range of 190-300GPa,while the other two metastable structures are dynamically stable above 282 GPa and 280GPa respectively.The predicted Sr-Ar compounds all exhibit metallic properties and the conducting states are mainly derived from Sr-d orbitals near the Fermi energy level.It was found that in the alkaline earth metal inert element Sr-Ar compounds,the inert element exhibits anionic properties and acts as an oxidizer.In this study,the reaction mechanism of alkaline earth metals and inert elements Ar under high pressure has been thoroughly explored,broadening the horizon of structural and electronic property studies for anionic inert element compounds.</sup>