Effects of preparation method on the microstructure and transport properties of La2Mo2O9 based materials

Different synthesis methods based in precursor routes such as citrate (CITR), precipitation (PREC), acetylacetonate (Ac_Ac) and freeze-dried (FD) have been used to prepare polycrystalline powders of the ionic conductor α-La2Mo2O9 and β-La2Mo1.5W0.5O9 polymorphs. The phase formation, phase transition, sinterability and microstructure of the different polycrystalline powders were studied by XRD, thermal and thermomechanical analysis (TMA) and scanning electron microscopy (SEM). The microstructure of the polycrystalline powders depends on the synthesis method used. The sinterability of the powders enhances as the starting crystallite sizes decrease, allowing to obtain dense pellets at relatively low temperature. Impedance spectroscopy measurements were carried out to determine the total conductivity and to study the influence of the synthesis method on the transport properties. Bulk conductivity is independent on the preparation method. On the other hand, the grain boundary contribution is negatively influenced by the microstructure or small impurities introduced during the synthesis. The ionic transport numbers were determined by a modified electromotive force method between 923 and 1123 K and they remain higher than 0.98 under O2/air gradient.