Non-adiabatic small polaron hopping conduction in sodium borate tungstate glasses

The dc electrical conductivity of (100-x)Na2B4O7-xWO(3) (x = 5, 15, 20 and 30 mol%) glasses is reported in the temperature range 323-473 K. The density and molar volume for these glasses are consistent with the ionic size, atomic weight and amount of different elements in the glasses. At high temperatures the Mott model of phonon-assisted small polaron hopping between nearest neighbours is consistent with conductivity data, while at intermediate temperatures the Greaves variable-range hopping model is found to be appropriate. The estimated model parameters such as number of ions per unit volume, hopping distance, polaron radius and activation energy are found to be consistent with the formation of localized states in these glasses. The electrical conduction of these glasses is confirmed to be that of non-adiabatic small polaron hopping. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.