Effect of strain on the band structure and optical properties of Na2Bi2(SeO3)3F2

The electronic structure and optical properties of Na2Bi2(SeO3)3F2 crystal are calculated under uniaxial, epitaxial, and hydrostatic stress based on the first principles of density functional theory. The crystal structures of the materials are composed of isolated [BiOxFy] polyhedrons and [SeO3] tetrahedrons. The stability of the crystal was confirmed by the cohesive energies and Ab initio molecular dynamics (AIMD). The band gap is 3.93 eV without strain, the maximum band gap is 4.04 eV and the minimum band gap is 2.33 eV under epitaxial stress of −2.61 % and hydrostatic pressure of −9 GPa, respectively. It is found that there is a band gap transition from direct to indirect when uniaxial strain and hydrostatic pressure are applied. This change has been explained by analyzing the near-band-edge orbitals energy. The strain also affects the optical properties of the crystal Na2Bi2(SeO3)3F2. In particular, hydrostatic pressure of −9 GPa causes a minimum increase of transmittance of about 10 % and a maximum increase of about 30 %. The optical properties of the crystal can be effectively improved by applying strain, which can be universal for other crystals.