Structural and electronic properties of oxygen vacancy in lead-free KTa1−xNbxO3: Comparative first-principles calculations

The structural and electronic properties of (0) and (+2) charged oxygen vacancies ( V o 0 and V o + 2 ) in KTN are investigated through first-principles density functional calculations. We have tested three exchange–correlation functionals local-density approximation (LDA), the Perdew–Burke–Ernzerhof (PBE) generalized-gradient approximation (GGA), and Wu–Cohen (WC) GGA. WC leads to excellent improvements of geometry predications of KTN (with Vo) over LDA and PBE, although they all underestimate the band gap. We find that V o 0 state is metallic, in good agreement with the conclusion of Vo in KNbO3, and there is a defect peak 0.23 eV below the Fermi level which is related to the infrared absorption. The charged Vo state plays an essential role in the displacements of Ta and O ions. We have shown that V o + 2 state of KTN is the most stable in practice, and V o 0 state may be obtained by introducing extra electrons to V o + 2 . The results of our calculations indicate that Vo plays an essential role in the metal–insulator transitions in KTN and hence studying Vo is necessitate and significant for devices design.