First principles study on the effect of Bi³⁺ and Y³⁺ doping on electronic structure, thermal and optical properties of KSr₂Nb₅O₁₅

The electronic structure, thermal and optical properties of KSr₂Nb₅O₁₅ with Bi³⁺ doping (KSN-Bi) and Y³⁺ doping (KSN-Y) are studied using first principles. When Bi³⁺ and Y³⁺ doping in the KSN, the band gap becomes smaller and satisfies to KSN > KSN-Bi > KSN-Y, the original tetragonal structure changes to be orthorhombic due to the great distortion of Nb-O octahedron affected by Bi³⁺ and Y³⁺. The specific heat capacity under constant volume C_v, specific heat capacity under constant pressure C_p, entropy S_vib, thermal expansion coefficient α(T) and volume change decreased after doping. The calculated Gibbs results suggest that KSN-Y is more stable than KSN-Bi. Furthermore, the dielectric constant is increased and the imaginary part of dielectric function decreased, while the refraction property is increased and reflectivity is decreased after Bi³⁺ and Y³⁺ doping. Both the KSN-Bi and KSN-Y show ultraviolet absorption in 100–400 nm, refractivity in 0–5 eV and reflectivity 5–8 eV. KSN-Bi and KSN-Y show optical anisotropy properties and KSN-Bi has better optical properties than that of KSN-Y.