Phase transitions of BaTi0.9Rh0.1O3±δ perovskite-type oxides under reducing environments

Perovskite-type oxides of composition BaTi0.9Rh0.1O3±δ were prepared following a new chemical route that avoids the formation of hydroxyl species and precipitation, and allows the homogeneous distribution of Rh in the final mixed metal oxide. The high dispersion of Rh and the formation of the solid solution between Rh and the BaTiO3 perovskite is confirmed by means of X-ray diffraction (XRD) and extended X-ray absorption fine structure spectroscopy (EXAFS). The presence of Rh stabilized the hexagonal BaTi0.9Rh0.1O3±δ phase, which decomposes into barium orthotitanate (BaTi2O4) and metallic Rh° in reducing environment. This phase transition starts already at 700 °C and is only partially completed at 900 °C suggesting that part of the Rh present in the perovskite lattice might not be easily reduced by hydrogen. These aspects and further open questions are discussed.