Crystal structure and oxide ion conductivity in cubic (disordered) and tetragonal (ordered) phases of Bi(25)Ln(3)Re(2)O(49) (Ln = La, Pr)

Two bismuth oxide materials containing large lanthanide and rhenium cations, Bi25La3Re2O49 and Bi25Pr3Re2O49, have been synthesised in the stabilised face-centred cubic structure. They exhibit very high oxide ion conductivity but prolonged annealing at 600 degrees C causes a decrease in conductivity, which has been shown to relate to an order-disorder phase transformation. The details of this transition have been determined using neutron powder diffraction on annealed samples. Upon annealing, the samples undergo cation ordering to form a tetragonal phase (I4/m, a approximate to 8.7 angstrom, c approximate to 17.4 angstrom), which is structurally related to the unsubstituted phase, Bi28Re2O49. Neutron powder diffraction refinement reveals that the lanthanide cations enter only one of the three Bi sites in the ordered structure.