Lithium ion conducting glass and glass ceramic of the composition Li-1.4[Al0.4Ge16.(PO4)(3)], have been synthesized. The monolithic glass pieces on thermal treatment resulted in single-phase glass ceramic with the Nasicon structure. Experiments with different electrodes proved that the lithium electrodes provide accurate values for the ionic conductivity using impedance spectroscopy. sigma(ionic) of the glass ceramic was found to be 3.8 x 10(-5) S cm(-1) at 40degreesC with an activation energy (Ea) of 0.52 eV The corresponding values for the glass are 2.7 x 10(-9) S cm(-1) and 0.95 eV, respectively. The Arrhenius dependence of sigma(ionic) with temperature in glass and glass ceramic is interpreted with a hopping mechanism from which the microscopic characteristics of the lithium cation motion are deduced. (C) 2002 Elsevier Science Ltd. All rights reserved.