Phase transitions and electrical conductivity of Bi‐doped La 2 Mo 2 O 9 oxide ion conductors

La 2− x Bi x Mo 2 O y ( x  = 0, 0.02, 0.03, 0.04, 0.05, 0.06, 0.08, 0.1, 0.16, 0.24, 0.3) ceramic samples have been characterized by X‐ray diffraction, differential scanning calorimetry (DSC) and conductivity measurements in order to assess the dynamics of the transitions between the low‐temperature ( α ), high‐temperature ( β ) and metastable ( β ms ) phases. Bi doping to x  ≥ 0.05 fully stabilizes the cubic phase, suppressing the β  →  α phase transition. In addition, DSC results demonstrate that considerable doping levels give rise to a small thermal event between 450 and 500 °C, which has been identified as the β ms  →  β phase transition between cubic phases with static and dynamic oxygen disorder. In this temperature range, the conductivity of the doped materials experiences a transition from Arrhenius to Vogel–Tammann–Fulcher behaviour. Low bismuth concentrations have a significant effect on the conductivity of La 2 Mo 2 O 9 .