Effect of Preparation Conditions on the Polymorphism and Transport Properties of La6-xMoO12-δ (0 ≤ x ≤ 0.8)

In this work, La6-xMoO12-delta (0 <= x <= 0.8) materials are investigated as a new family of ceramic proton conductors. Different polymorphic phases with rhombohedral and cubic structure are obtained, depending on the cooling rate applied during the synthesis process. The materials have been thoroughly characterized by different techniques, including X-ray powder diffraction, transmission electron microscopy, and X-ray photospectroscopy in order to fully understand the structural features of the samples. Thermogravimetric analysis and impedance spectroscopy in dry/wet gases (N-2, O-2, and 5% H-2-Ar) confirmed the existence of proton conductivity at low temperature. Under a reducing atmosphere, the materials are mixed ionic-electronic conductors. The sample prepared at the fastest cooling rate exhibits cubic structure and higher n-type electronic conductivity compared to those prepared at a slower cooling rate with rhombohedral structure.