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

We report a detailed study on structural and dielectric properties, Raman scattering, and ac-conductivity of ferroelectric KNbO3 and antiferromagnetic NiO two-phase composites (1-x) KNbO3 + x NiO]. Two major transitions were observed in the temperature dependent relative dielectric permittivity is an element of(r)(T)-the first at 445 degrees C (T*) and the second at 220 degrees C (T-1)-respectively associated with the structural transitions from tetragonal to cubic and orthorhombic to tetragonal phases. In addition, a new anomaly across T-2 similar to 320 degrees C has been observed in is an element of(r)(T), and the loss-tangent (Tand delta(T)), which may be associated with the combined effect of antiferromagnetic ordering of NiO and ferroelectric coupling of KNbO3. The dispersive nature of T*, 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 0.10. Temperature dependent ac-resistivity analysis rho(ac)(T) provides strong evidence for a Mott's Variable range hopping process of charge carriers between localized states, with average hopping length of 4.2 nm and hopping energy 0.72 eV. The variation of T*, T-1 and T-2 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 (1-x) KNbO3 + x NiO two-phase composites, which shows significant increase of tilt-angle between adjacent NbO6 octahedra as the divalent Ni occupies octahedral sites of the KNbO3 matrix. All the Raman-active modes of these composites are indexed as lattice translations, specifically with the motion of K+ (190 cm(-1)), the core vibrations of the NbO6 octahedra (220 cm(-1) <= nu <= 320 cm(-1)) and their collective modes (50 cm(-1) <= nu <= 150 cm(-1)), and Magnon modes (2M similar to 1500 cm(-1)) associated with the antiferromagnetic ordering of NiO.