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

TL;DR

This study investigates the re-entrant relaxor behavior in lead cobalt niobate ceramics, revealing complex structural, chemical, and magnetic heterogeneities that influence dielectric properties and phase transitions.

Contribution

It provides detailed structural, chemical, and spectroscopic analysis linking heterogeneities to re-entrant relaxor behavior in lead cobalt niobate.

Findings

Re-entrant relaxor behavior observed below 270 K.

Presence of mixed Co oxidation states and chemical ordering.

Structural and chemical heterogeneities linked to relaxor properties.

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 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.