First-principles Calculations On The Electronic Energy Band And Phonon Structure Of NaBH₄ And NaBH₆ Under High Pressure

Hydrogen energy has the advantages of green and clean,abundant resources,high calorific value,good thermal conductivity,and a variety of storage methods,and can meet the various needs of human life.Therefore,each country,in terms of energy security or sustainable development,has increased exploitation and utilization.Hydrogen is very likely to become the dominant energy source in the future.In the current research on hydrogen energy,the biggest problem is the storage of hydrogen.Metal hydride hydrogen storage materials are currently the most common and have better hydrogen storage effects.NaBH₄and NaBH₆are metal hydride hydrogen storage materials.In this paper,the new and stable crystal structures of NaBH₄and NaBH₆at 100 GPa are found under the calculation of USPEX based on the evolutionary algorithm,and the electronic structure and phonon are analyzed.The analysis of the electronic structure has resulted in some theoretical research results:NaBH₄under 100 GPa is an orthorhombic structure with space group Pnma.Each structural unit contains 1 Na atom,1 B atom,and 4 H atoms,but there are only 3 unequal H atoms.Under this condition,NaBH₄does not conduct,and the Na,B,and H atoms in different orbitals are hybridized to different degrees.The conduction band mainly relies on the B and Na atoms,and the valence band mainly relies on the B and H atoms.There is a gap between the top of the conduction band and the bottom of the valence band,with a width of 5 e V which is close to the bandgap of the diamond.In addition,the new structure of NaBH₄can exist stably under 100 GPa,and there is no imaginary frequency in the phonon dispersion curve.There is an obvious phonon bandgap in the frequency range of 2000 cm^-1to 3100 cm^-1.Na atoms,B atoms,and H atoms only vibrate together in the lower frequency range of 0 cm^-1to 1000 cm^-1,but as the frequency increases,they all mainly rely on H atoms’vibration.NaBH₆under 100 GPa is a trigonal crystal system with space group R3m.Each structural unit contains 1 Na atom,1 B atom,and 6 H atoms,but there are only 4 unequal H atoms.NaBH₆at 100GPa has conductive properties,but NaBH₄at 100 GPa is an insulator with no conductive properties,indicating that the insulator-metal transition has been achieved under the condition that the pressure is unchanged and only the H element ratio is increased.And in NaBH₆,the Na,B,and H atoms are hybridized differently.In the valence band,Na atoms,B atoms,and H atoms all make contributions.The electronic state at the Fermi level mainly depends on the B and H atoms.The new structure of NaBH₆at 100 GPa is also stable,and there is no imaginary frequency in the phonon dispersion curve.In the phonon dispersion curve,there is a phonon bandgap with a width of about 1100 cm^-1between the middle frequency region and the high-frequency region,and the high-frequency region.The phonon densities of states in the low-frequency region are contributed by the vibration of Na,B,and H atoms,but as the frequency increases,the vibrations of H atoms are mainly contributed in the middle and high-frequency regions.Due to the electronic and phononic bandgaps of Pnma-NaBH₄and R3m-NaBH₆,they may realize superconducting applications in certain extreme environments or become phononic crystal materials for acoustic stealth.