Investigation of oxygen defects in Eu doped NKBT by XPS & conductivity: evidence of amphoteric nature

This study investigates the role of oxygen defects in europium (Eu)-doped (Na0.41K0.09Bi0.5)TiO3 (NKBT) perovskite ceramics, focussing on their impact on piezoelectric properties. Using X-ray photoelectron spectroscopy (XPS) and temperature-dependent Ac-conductivity measurements, this study presents evidence of the amphoteric nature of Eu ions in the NKBT host matrix. The XPS analysis shows the presence of oxygen vacancies, with O-1 s core level spectra showing distinct peaks corresponding to lattice oxygen (binding energy: ~ 529 eV) and oxygen vacancies (binding energy: ~ 532 eV). The concentration of oxygen vacancies reduces with low Eu doping concentrations (≤ 1.0 at%), corresponding to Eu ions absorbing the A-site (Na/K/Bi), where Eu act as donor dopants which lowers the oxygen vacancy concentration and enhancing piezoelectric properties. Conversely, at higher doping levels (> 1.0 at%), Eu ions favourably substitute the B-site (Ti4⁺), behaving as acceptor dopants, which increases the oxygen vacancy concentration and shows a deterioration in piezoelectric performance. Conductivity measurements further verify these findings and verifying their foremost role in conduction mechanisms for the reason that the activation energy values aligning with oxygen vacancy migration. This study not only clarifies the amphoteric behaviour of Eu ions in NKBT but also highlights the critical role of oxygen defects in tuning the material’s electromechanical properties.