Impedance and modulus studies of Pb(Fe0.5Nb0.5)O3–Pb(Co0.33Nb0.67)O3 solid solutions

Highlights: • The formation of a single-phase without any secondary phase and the monoclinic structure with Cm symmetry were confirmed. • The Z' vs. frequency shows the creation of space charges by the participating ionized OVs in the conduction process. • The Nyquist plot (Z′′ vs Z′) shows the depressed semicircles suggesting the deviation from Debye type of relaxation behaviour. • Decrease in the grain resistance for all compositions with rise in temperature, correlates the NTCR nature. • Fitted Kohlrausch-Williams-Watts (KWW) function to M′′ vs. frequency demonstrates the non-Debye type relaxation behavior. -- Abstract: The high quality polycrystalline (1-x)Pb(Fe_0.5Nb_0.5)O₃(PFN)–(x)Pb(Co_0.33Nb_0.67)O₃(PCN) (abbreviated as PFCN) ceramic solid solutions (x = 0.1, 0.2, 0.3, 0.4 and 0.5) were synthesized via single-step solid-state reaction technique. The formation of a single-phase, monoclinic structure with Cm symmetry and without any secondary phase was confirmed through the room temperature (RT) X-ray diffraction (XRD). The microstructure of sintered ceramics with well-grown grains having average grain size of 1–2 µm were observed through scanning electron microscopy (SEM). To understand the electrical response of the material impedance and modulus spectroscopy studies were carried out. A detailed temperature (301–481 K) and frequency (100 Hz to 6 MHz) dependent impedance and modulus of PFCN ceramic solid solutions were reported. The impedance spectroscopy reveals the increase of conductivity in the system by the addition of Pb(Co_0.33Nb_0.67)O₃ due to increased number of oxygen vacancies (OVs)/defects created at the grain boundary interface during the synthesis. The Z′ vs. frequency shows the creation of space charges by the participating ionized OVs in the conduction process and Z′′ vs. frequency shows the presence of temperature-dependent relaxation processes in the PFCN solid solutions. The Nyquist plot (Z′′ vs Z′) shows the depressed semicircles suggesting the deviation from Debye type of relaxation behaviour and a decrease in the grain resistance for all compositions with rise in temperature, correlates the NTCR (negative temperature coefficient of resistance) nature. The M′ vs. frequency display the step-like behavior due to dipole contribution to the modulus present in the grain and grain boundary region. Fitted Kohlrausch-Williams-Watts (KWW) function to M′′ vs. frequency demonstrates the non-Debye type relaxation behavior in the PFCN solid solutions. The mismatch of f_max in the Z′′, M′′ vs. frequency indicates the short-range mobility of localized charge carriers.