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

The temperature dependent dielectric properties revealed re-entrant relaxor behaviour (Tm ~130 K and 210 K for 1 kHz) below a high temperature diffused 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 depicts origin of frustration. Microstructure examination depicts closely packed grains with grain size ~8-10 microm and XRD pattern revealed single phase pseudo cubic crystal structure having Pm3m symmetry with lattice constant ~4.0496(2) {\AA}. Rietveld Refinement on XRD data yields larger value of thermal parameters, implying Pb and O are disordered along <111> and <110> directions respectively. Observation of A1g (780 cm-1) mode in Raman spectroscopy and F-spot in SAED pattern along <110> 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+ state. 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.