RESUMO
In this work, the dielectric response of polycrystalline Ba3CoSb2O9 was studied as a function of temperature (30 to 900 °C) and frequency (10 Hz to 10 MHz). The triple perovskite Ba3CoSb2O9 was successfully synthesized and characterized for structural and dielectric properties. The Rietveld analysis of the X-ray diffractogram confirms the formation of a hexagonal phase with P63/mmc symmetry. This centrosymmetric 3(BaCo1/3Sb2/3O3) perovskite shows structural similarity to a prototypical non-centrosymmetric relaxor ferroelectric, PbMg1/3Nb2/3O3. The dielectric constant, ε', follows a non-Debye Cole-Cole relation and exhibits anomalous responses such as: (a) a thermally activated colossal dielectric constant (>105) and (b) a highly dispersive peak maximum (523-853 K). The real part of ac conductivity, σ', also shows a change of approximately 6 orders in magnitude (10-8 to 10-2 S m-1). Validation of Jonscher's law and impedance (Nyquist plot) and modulus (M'') analyses indicate that hopping polarization is the predominant thermally activated mechanism. Moreover, the large value of ε' and its dispersion were found to be highly correlated with the underlying crystal structure and were attributed to the local ionic site ordering. The study suggests that the anomalous dielectric dispersion must have an intrinsic origin.
RESUMO
The temperature dependence of the structural and dielectric properties of polycrystalline Ba3NbFe3Si2O14 has been studied using high temperature X-ray diffraction and impedance spectroscopy. In situ X-ray diffraction with temperature (330-873 K) and subsequent Rietveld refinement shows that Fe-langasite crystallizes in a single phase P321 structure, in the measured temperature range. The dielectric constant ε' exhibits low frequency dispersion and large variation (25-104), with temperature and frequency. The real part of the ac conductivity (σ') also shows a change of seven orders of magnitude (10-6 to 10-13). The conductivity was observed to diverge from the 'universal dielectric response' (UDR), σ(ω,T) = σdc + A1ωn. Three frequency (10 Hz-10 MHz) and temperature (123-573 K) dependent regions were observed: (a) a low frequency, frequency independent region, (b) a mid frequency, dispersive region, and (c) a high frequency, dispersive region. This behaviour can be understood by a double power law: σ(ω,T) = σdc + A1ωn2 + A2ωn1, which is similar to the modified Jonscher's law and holds good for other complex dielectric materials as well. The 'sub-linear' variation with frequency for n2 at all temperatures and for n1 above 323 K is attributed to hopping polarization. Remarkably, a 'super-linear' ac conductivity was observed with n1 ≥ 1 below 323 K. This anomalous behaviour is attributed to hopping between non-uniform potential wells. The dielectric relaxation studies in combination with Seebeck measurements (300-573 K) reveal that the colossal dielectric permittivity and deviation from the UDR are predominantly due to the hopping polarization of positively charged species in a distributed potential. It is suggested that this model may be applicable to understand the conductivity mechanism in a broad range of complex materials.
