Dielectric properties of (1−x)KNbO3−xNiO two-phase composites

Abstract

We report a detailed study on structural and dielectric properties, Raman scattering, and ac-conductivity of ferroelectric KNbO3 and antiferromagnetic NiO two-phase composites [() KNbO3 + x NiO]. Two major transitions were observed in the temperature dependent relative dielectric permittivity (T)—the first at C () and the second at C (T1)—respectively associated with the structural transitions from tetragonal to cubic and orthorhombic to tetragonal phases. In addition, a new anomaly across T2 ∼ C has been observed in (T), and the loss-tangent (Tanδ(T)), which may be associated with the combined effect of antiferromagnetic ordering of NiO and ferroelectric coupling of KNbO3. The dispersive nature of , and the Vogel–Fulcher temperature dependence of its characteristic relaxation frequency reveals the existence of relaxor-like behavior, which is further supported by Uchino–Nomura criteria (for x ⩽ ) but departs from relaxor behavior for x . Temperature dependent ac-resistivity analysis (T) provides strong evidence for a Mott’s Variable range hopping process of charge carriers between localized states, with average hopping length of nm and hopping energy eV. The variation of , T1 and T2 as a function of x, in consonance with the crystal structure, has been discussed. Raman spectroscopic studies were carried out to study the local structure of the () + x NiO two-phase composites, which shows significant increase of tilt-angle between adjacent octahedra as the divalent Ni occupies octahedral sites of the matrix. All the Raman-active modes of these composites are indexed as lattice translations, specifically with the motion of (190 ), the core vibrations of the octahedra (220 ⩽ ν ⩽ 320 ) and their collective modes (50 ⩽ ν ⩽ 150 ), and Magnon modes (2M ∼ 1500 ) associated with the antiferromagnetic ordering of NiO.