Defect chemistry and relaxation processes: effect of an amphoteric substituent in lead-free BCZT ceramics.

The effect of praseodymium (Pr), an amphoteric substituent, on phase transition, dielectric relaxation and electrical conductivity has been studied and analysed in 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (BCZT) ceramics synthesized by a solid state reaction method. Structural investigations showed co-existence of two phases - tetragonal (P4mm) and rhombohedral (R3m) - for compositions with x ≤ 0.05 wt% Pr. Temperature dependent dielectric studies revealed two phase transitions - rhombohedral (R) → tetragonal (T) and T → cubic (C) - that gradually evolved into one T → C transition for x > 0.05 wt% Pr in BCZT. A dielectric relaxation behaviour was observed in the temperature range of 275-500 °C that was attributed to the localized relaxation process (short-range hopping motion of oxygen vacancies) in the bulk of the material. Grain and grain boundary conductivity evaluated from the impedance data revealed that Pr acts as a donor dopant for x ≤ 0.05 wt% while it is an acceptor for higher concentration, in accordance with XRD observations. Defect chemistry analysis for better interpretation of the acquired data is presented. Frequency and temperature dependent ac conductivity studies were also performed and the obtained activation energy values were associated with possible conduction mechanisms.