Effects of O2 and N2 sintering atmospheres on electric properties of 0.9SrTiO3–0.1NiFe2O4 composite ceramics

Highlights: • The N₂-sintered 0.9SrTiO₃–0.1NiFe₂O₄ (STO/NFON) possessed a larger microstrain (0.092(47)%) than the O₂-sintered samples. • Ti³⁺ ions in STO/NFO_N formed dipoles of [Ti^4+·e−$V_0^{\mathrm{..}}$−Ti⁴⁺·e], fixing oxygen vacancies and showing the low dielectric loss. • Different conductivity mechanisms arising from oxygen vacancies were responsible for the anomaly permittivity peaks. -- Abstract: The 0.9SrTiO₃–0.1NiFe₂O₄ (STO/NFO) composite ceramics sintered at O₂/N₂ atmospheres were investigated. The N₂-sintered STO/NFO (STO/NFO_N) had a larger microstrain (0.092(47)%) than the O₂-sintered STO/NFO (STO/NFO_O) revealed by the Rietveld refinement and the Williamson-Hall analysis. The maximum polarization of STO/NFO_N increased to ~7.1 μC/cm², but the larger leakage current in STO/NFO_O resulted in a 20% larger false remanent polarization value. Moreover, STO/NFO_N presented a lower activation energy (E_a, 1.22 eV), which implied more oxygen vacancies ($V_O^{\cdot \cdot }$). The amount of Ti³⁺ ions (60%) in STO/NFO_N increased as well, which formed defect dipoles [Ti⁴⁺·e−$V_O^{\cdot \cdot }$−Ti⁴⁺·e] and fixed $V_O^{\cdot \cdot }$. The composite of STO/NFO_N also showed decreased $V_O^{\cdot \cdot }$ content (~53%) owing to the fixing effect and presented low leakage current, increased impedances, and the improved ferroelectric hysteresis loop. The anomaly dielectric constants peaks indicated different conductivity mechanisms related to $V_O^{\cdot \cdot }$, and the STO/NFO_N ceramic possessed a low rate of dielectric loss.