Changes in structure and electrical conductivity of rare-earth titanate pyrochlores under highly reducing atmosphere

M2Ti2O7 (M=Y, Gd, Sm) pyrochlore materials are synthesized by a Pechini method. The pyrochlore phase evolution with temperature under air and under highly reducing conditions (2% H2 in Ar) is investigated using high-temperature X-ray diffraction. The decomposition of pyrochlore oxides into their component single oxides is evidenced when they are exposed to a hydrogen atmosphere. Sm2Ti2O7 pyrochlore is the most stable compound, while Gd2Ti2O7 undergoes a strong reduction even at RT. The bulk electrical conductivity of the samples is evaluated by electrochemical impedance spectroscopy. The greatest increase in conductivity is evidenced for Gd2Ti2O7, while Sm2Ti2O7 is the most stable among the investigated compounds. Besides oxygen ions and electrons, the contribution of hydrogen defects to the bulk conductivity must be considered in such reducing atmosphere.