Correlated barrier charge hopping and non-Debye relaxation in columbite MnNb2O6

A comprehensive study on the mechanism of ac-charge transport and dielectric relaxation in the MnNb2O6 columbite system has been reported. Thermally driven, Arrhenius-like behaviour of the ac-conductivity (sigma(omega,T)) is predominant at temperatures above 300 K in the frequency range 50 Hz-5 MHz with activation energies (Eac) lying between 0.45 and 0.38 eV. Besides, a quadratically decreasing trend in the activation energy is evident as given by: E-ac(f) = A + Bf + Cf-2 with the constants A (0.413 eV), B (0.023 eV Hz-1) and C (-0.004 eV Hz-2). The measured sigma(omega,T) data is analysed using the Double power law to explain the dispersive behaviour of electrical conductivity. These studies also provide evidence for the correlated-barrier hopping (CBH) conduction mechanism of charge carriers for temperatures in the range 173-473 K. Further, the temperature dependence of frequency exponent, s(T) displays two distinct regions both of which are associated with the CBH mechanism with slopes theta s(1)/& part;T = -1.9 x 10(-3) K-1 and theta s(2)/theta T= -1.3 x 10(-3) K-1 in Region-I (173-300 K) and the slope theta s1/theta T = -1.7 x 10(-3) K-1 with non-linear variation of s(2) in Region-II (300-473 K). However, the second region is more dominated by thermally activated Arrhenius-like behaviour. The dynamical response of complex electric modulus spectra (M*(omega,T)) and the corresponding analysis using Kohlrausch-Williams-Watts method reveals the presence of a non-Debye type relaxation process with decay function exponent beta lying between 0.794 and 0.840. This inference of the non-Debye type relaxation process is further supported by depressed semicircles in Nyquist plots and the higher magnitude of full width half maximum (FWHM) (asymptotic to 1.44 decades) of the normalized master cusp (M ''/M ''(max) vs omega/omega(max)) compared to the FWHM ( = 1.14 decades) for ideal Debye behaviour. Both short-range and long-range conductivity regions are ubiquitous in (M*(omega,T)) with a pronounced distribution of relaxation times (tau asymptotic to 935.8 - 0.36 mu s) with temperature. This study further leads to the estimation of activation energy of charge carriers, E-M = 0.44 eV which is in consonance with E-ac(f,T).