Dielectric properties of hexagonal perovskite ceramics prepared by different routes

In this work, four different methods, i.e. solid state reaction, citrate sal gel process, Pechini and microwave are employed to synthesize Ba4YMn3O11.5 +/-delta ceramics. The phase structure of the powders can be well indexed as a 12R hexagonal perovskite crystallizing in space group R (3) over barm. The density and morphology (average grain size) of sintered samples vary with the synthesis processes. The dielectric permittivity and loss tangent of the samples have been measured in the frequency range 1 kHz-1 MHz. The results show that they are very sensitive to the synthesis process. The best properties are obtained for the sample synthetized by citrate process. This compound has a high dielectric permittivity (epsilon'(r)= 10(4)), which is almost frequency independent over the 100 Hz-100 kHz range from room temperature to 150 degrees C. This has been attributed to the IBLC mechanism. By impedance spectroscopy analysis, all the compounds were found to be electrically heterogeneous, exhibiting semiconducting grains and insulating grain boundaries. Finally, we show that the IBLC model is well adapted to the materials obtained by the modified citrate synthesis route, but not to the ones from the other routes. (C) 2012 Elsevier Ltd. All rights reserved.