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.

Structural and dielectric properties of the fluorite-type LaxCe1−xO2−δceramics

“…In order to understand the nature of this hopping mechanism we fitted the experimental data to the equation ρ = ρ o e (To/T) n , where ρ o (pre-exponent factor) and T o is the characteristic temperature coefficient. Both of these constants were extracted from the slope and intercepts of figure 7 [39][40][41]. Usually the exponential value n should be either 0.25 or 0.33 for 3D and 2D systems, respectively.…”

Thermal hysteresis and vibrational excitations in NiO containing NaNbO3

“…Soni et al recently reported a new anomaly across 593 K in ε r (T) along with the two known transitions at 718 K and 493 K in the FE KNbO 3 and antiferromagnetic NiO composites [21]. These authors also studied the role of NiO in the vibrational spectra of KNbO 3 , and reported the emergence of a two-magnon mode at 1500 cm −1 [21]. Moreover, a very recent study reported structural distortion from orthorhombic to monoclinic phase at room temperature in NNO + NiO twophase composites [14].…”

“…These authors also pointed out that the phase transition from the P-phase to the R-phase (T ∼ 643 K) in NNO crystals may be due to dipole relaxation around the offcentre Nb ions, resulting in an order-disorder character of the AFE phase [13]. Soni et al recently reported a new anomaly across 593 K in ε r (T) along with the two known transitions at 718 K and 493 K in the FE KNbO 3 and antiferromagnetic NiO composites [21]. These authors also studied the role of NiO in the vibrational spectra of KNbO 3 , and reported the emergence of a two-magnon mode at 1500 cm −1 [21].…”

“…Moreover, a very recent study reported structural distortion from orthorhombic to monoclinic phase at room temperature in NNO + NiO twophase composites [14]. Also, the incorporation of NiO inside the KNbO 3 and NNO compounds leads to some major changes in their global dielectric behaviour and ac-conductivity [14,21]. Although there have been numerous studies of NNO and its composites, detailed Raman spectroscopy down to a temper ature of 80 K in NiO-substituted NNO is still lacking in the literature; that is the subject of this paper.…”