Complex impedance studies of sodium pyrotungstate -: Na2W2O7

Polycrystalline Na2W2O7 (anhydrous) was prepared by a solid-state reaction technique. The formation of the compound has been confirmed by X-ray powder diffraction studies and Raman spectroscopy. The unit cell structure of the material has been found to be orthorhombic of the Cmca space group. Complex impedance analysis of the material indicates the electrical conduction to be due only to the grain interior up to a temperature of 300 degreesC. Above 300 degreesC the contribution of the grain boundary to electrical conduction is more evident. The grain boundary resistance is found to decrease with increasing temperature. Impedance results also indicate that the material exhibits a negative temperature coefficient of resistance. The electrical processes in the material show temperature-dependent relaxation phenomena. The electrical conduction in the material is a thermally activated process governed by the release of space charge in close agreement with the observations made from impedance spectrum analysis. The do conductivity of the material as evaluated from the complex impedance spectrum is observed to attain a maximum value of 1.16 x 10(-6) S cm(-1) at 425 degreesC. This is a very large increase, nearly four orders of magnitude, when compared with the room temperature conductivity of the material (3.99 x 10(-10) S cm(-1)). (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.