Theoretical Study On Ternary Hypervalent Silicon Hydrides Via Alkali Metal Elements Under High Pressure

With the rapid development of human society,the problem of energy shortage and the environmental problems derived from the massive burning of traditional fossil fuels are becoming increasingly serious.Hydrogen has high energy density,good combustion performance and friendly environment.Due to lack of safe and efficient hydrogen storage materials,hydrogen energy has not been well used.Therefore,the study of new efficient and stable hydrogen storage materials has become an urgent problem in academia and industry,and hydrogen rich hypervalent compounds may be potential hydrogen storage materials.Generally speaking,for hypervalent compounds,hydrogen is rarely observed as a ligand in hypervalent species.However,previous studies have shown that ternary hypervalent compound K₂Si H₆ can be synthesized under high pressure,and there are octahedral-structured hypervalent[SiH₆]^2-anions inside the crystal lattices for this compound.These compounds have high energy density and are considered as potential hydrogen storage materials.We believe that such a type of hypervalent silicon hydrides with similar ratio may also have good hydrogen storage properties and novel structures,and these compounds can be synthesized under high pressure synthesis as well.In this paper,we used first-principles calculation methods combined with random structure search to systematically study the crystal structures,electronic structures,hydrogen storage capacities,and geophysical applications of Li-Si-H and Na-Si-H ternary systems,and innovative research results were obtained as follows:(1)A variety of stable or metastable Li-Si-H compounds with different ratios,which are Li Si H₅,Li₂Si H₆,Li₃SiH₁₀,and Li₂Si H_6+?(?=4,6,8),were predicted and designed by performing crystal structure prediction technique.In addition to the octahedral[Si H₆]^2-anions shown in previous studies,there were also different layer-typed[Si H₅]^-anions and tricapped triangular prismatic[Si H₆]^2-anions emerged.Meanwhile,all of Li-Si-H compounds were insulators except Li₃Si H₁₀,and Li₃Si H₁₀also showed a poor electrical conductivity due to the presence of a large amount of H₂units inside.The hydrogen weight percentage of Li-Si-H compounds is expected to12.50?25.02 wt%,and the volumetric hydrogen percentage of Li-Si-H compounds is expected to 175.04 g/L at ambient pressure,and up to 519.63 g/L at high pressure.We speculated that these compounds have strong hydrogen storage capacity.This study provides some useful guidance for the exploration of new hydrogen storage materials.(2)We predicted a variety of stable or metastable Na-Si-H compounds with different ratios,which are Na Si H₅,Na₂Si H₆,Na₃Si H₁₀,and Na₂Si H_6+?(?=2,4,6,8,10).Beyond the octahedral[Si H₆]^2-anions and layer-typed[Si H₅]^-anions,we also found linear-typed[Si H₆]^2-anions at 190 GPa.We performed ab initio molecular dynamics method to simulate the phase transition of38)1 Na₂Si H₆ at high temperature and pressure combined with the results of geophysical research.The superionic phase caused free diffusion of H^-in crystal lattices in certain ranges of temperature and pressure,which are 1000?2000 K and 5?25 GPa was confirmed combined with the P-T phase diagram.This work provides an important reference for studying the hypervalent hydrides with superionic phase.