Effect of simultaneous substitution of Li+ and Ti4+ in ceramics of Pb2KNb5O15 on structure, dielectric, modulus, impedance and conductivity properties

Lead potassium lithium titanium niobate, Pb0.8K0.2Li0.2Ti0.25Nb1.8O6 (PKLTN) is prepared through a solid-state reaction route. The XRD analysis at room temperature indexed as single phase with orthorhombic tungsten bronze (TB) with completely filled (A(6)C(4)B(10)O(30)-type) structure. Partial substitution of Ti4+ in Nb5+ increased the B sites ordering, and is associated in charge compensation, creating oxygen vacancies entering in the crystal structure described by Kroger-Vink notation. Simultaneous substitution of Li+ and Ti4+ in Pb2KNb5O15 (PKN) resulted in an increase in the lattice parameters (a = 18.0131 angstrom, b = 18.1520 angstrom and c = 3.9376 angstrom) and a decreased T-C to 445 degrees C. Space-charge polarization, relaxation phenomenon and conduction mechanism due to charge carriers have been analyzed using dielectric, impedance and modulus characterization. Cole-Cole plots showed non-Debye (polydispersive) type relaxation. DC conductivity activation energies calculated from R-b, M-II peak frequencies and thermal behavior of AC and DC conductivity at 1 kHz in the ferro region are comparable to 0.32 eV, reveals the ionic conduction mechanism in PKLTN. Conduction via a hopping mechanism has been established from Jonscher's power law, sigma(omega) = sigma(DC) + A omega ''. The stretched exponential parameter, beta is a measure of the distribution of relaxation times and was computed as a function of temperature (360-595 degrees C) using an impedance formalism, and has a minimum value of 0.74 at T-C. (C) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.