Effective masses, electronic and optical properties of (111)-layered B-site deficient hexagonal perovskite Ba5M4O15 (M = Ta, Nb): a DFT study using HSE06

Abstract

Equilibrium lattice parameters, electronic structures and optical properties of (111)-layered B-site deficient hexagonal perovskite Ba₅M₄O₁₅ (M = Ta, Nb) were studied by first-principles computations on the basis of density functional theory using the norm-conserving-type pseudo-potential technique and screened nonlocal exchange-correlation functional HSE06 as defined by Heyd, Scuseria, and Ernzerhof. The calculated band dispersions showed that Ba₅Ta₄O₁₅ and Ba₅Nb₄O₁₅ are indirect band gap materials (A → G) with band gaps of 3.81 and 3.56 eV, respectively. The effective masses of photogenerated electrons and holes for Ba₅Ta₄O₁₅ and Ba₅Nb₄O₁₅ were evaluated in two principal directions at the G (Gamma) point. The Ta–O and Nb–O bonds in the MO₆ octahedral environments have polar covalent nature due to the p–d hybridization between O-2p and Ta-5d or Nb-4d orbitals. Since the valence and conduction bands of Ba₅Ta₄O₁₅ and Ba₅Nb₄O₁₅ mainly consist of O-2p and Ta-5d or Nb-4d states, changes in the structure of the MO₆ octahedral units can be effective for the band gap energy and consequently photocatalytic activity of Ba₅Ta₄O₁₅ and Ba₅Nb₄O₁₅. The optical analysis revealed that the main peak of the imaginary part of the complex dielectric function of Ba₅Ta₄O₁₅ and Ba₅Nb₄O₁₅ corresponds to the interband electronic transition from O-2p to Ta-5d or Nb-4d. Also, anisotropies in the effective masses of photogenerated charge carriers and static dielectric tensors of Ba₅Ta₄O₁₅ and Ba₅Nb₄O₁₅ in an arbitrary crystallographic direction are presented. High photocatalytic activity of Ba₅Ta₄O₁₅ and Ba₅Nb₄O₁₅ for hydrogen generation from water splitting and photodegradation of organic pollutants and/or dye molecules under UV light is related to the light effective masses of photogenerated charge carriers. For the efficient solar-energy conversion, the electronic band structures, such as band-edge position and band gap, of Ba₅Ta₄O₁₅ and Ba₅Nb₄O₁₅ can be tuned by doping.