Ionic conductivity of apatite-type rare-earth silicates prepared by mechanical milling

Apatite-type silicates, RE10-x(SiO4),O'(2+y) (RE = La, Nd, Gd and Dy), were prepared by mechanical milling (MM) starting from stoichiometric mixtures of the constituent oxides, RE2O3 and SiO2. XRD patterns collected after grinding the starting mixtures in a planetary ball mill for 9 h contain only the characteristic reflections of the target materials. The electrical properties were analyzed on sintered pellets by using impedance spectroscopy and the isothermal conductivity data were successfully fitted to a Jonscher-type empirical expression sigma'(omega)alpha omega '' with a decreasing fractional exponent n as the RE3* cation size increases. Activation energies for oxygen migration were found to decrease and conductivity to increase as the size of the RE3+ cation increases with the highest conductivity values obtained for the apatite-type lanthanum silicate. In addition, we also show that the electrical properties of the as-prepared materials are influenced by the silicon source used in their synthesis. (c) 2008 Elsevier B.V. All rights reserved.