  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.
