Investigation of re-entrant relaxor behaviour in lead cobalt niobate ceramic

Abstract The temperature dependent dielectric properties revealed re-entrant relaxor behaviour (Tm ∼130 K and 210 K for 1 kHz) below a high temperature diffused ferroelectric phase transition, Tc ∼270 K in lead cobalt niobate (PCN). Multiple positive/negative magnetodielectric effect and deviation from straight line at ∼130 K is observed in temperature dependence of inverse susceptibility, which might hint at origin of frustration. Microstructure examination depicts closely packed grains with grain size ∼8-10 μm and XRD pattern revealed single phase pseudo cubic crystal structure having Pm3m symmetry with lattice constant ∼4.0496(2) A. Rietveld Refinement on XRD data yields larger value of thermal parameters, implying Pb and O are disordered along and directions, respectively. Observation of A1g (780 cm-1) mode in Raman spectroscopy and F-spot in SAED pattern along unit axis in TEM suggests presence of nano scale 1:1 Co and Nb non-stoichiometric chemical ordering (CORs), akin to lead magnesium niobate (PMN). K-edge XANES spectra reveals the presence of cobalt in two oxidation states (Co2+ and Co3+); whereas, niobium exists in Nb3+ and Nb5+ states. Therefore, these local-average structural properties suggest chemical, structural and spatial heterogeneities. Such multiple heterogeneities are believed to play a crucial role in producing re-entrant relaxor behaviour.