The dielectric behavior of Zn1-xNixO/NiO two-phase composites

The effect of nickel content on the dielectric permittivity 'epsilon(r)' and the ac-electrical conductivity of Zn1-xNixO/NiO (0 <= x <= 0.55) two-phase composites were investigated. The antiferro to the paramagnetic Neel temperature T-N (similar to 523 K) of the NiO associated with the structural phase transition from the rhombohedral to the cubic phase has been exploited to realize a dielectric anomaly across 523-541 K in the Zn1-xNixO/NiO composite system. Also, a giant dielectric peak across 410 degrees C in pure NiO was observed together with an anomaly across T-N. The formation of tiny polar clusters due to the compositional heterogeneity for the samples with x >= 0.16 drove the system to exhibit a weakly coupled relaxor-like behavior with a locally varying maximum temperature of T* (similar to 530 K at 10(6) Hz), obeying the Vogel-Fulcher law and the Uchino-Nomura criteria. The values of the diffuseness-exponent 'gamma' (1.91) and the shape-parameter 'delta' (88 degrees C) were determined by using the empirical scaling relation (epsilon(A)/epsilon(r) = 1 + 0.5 (T - T-A)(2)/delta(2)), which is often used to describe relaxor- like behavior. Our results provide strong evidence for the variable-range-hopping of charge carriers between the localized states. The effects of non-ohmic sample-electrode contact impedance and negative-capacitance on the global dielectric behavior of a Zn1-xNixO/NiO composite system are discussed.