A novel high-temperature commensurate superstructure in a natural bariopyrochlore: A structural study by means of a multiphase crystal structure refinement

Additional X-ray diffraction effects yielding an eightfold commensurate superstructure [a=20.974(5)Å] of the ideal pyrochlore structure were observed after annealing at 873 K of a thallium-doped bariopyrochlore single crystal. Electron diffraction indicated the coexistence of two cubic phases, the pyrochlore structure and a new F-centred, cubic phase. The superstructure was solved and refined in the space group F4̄3m. The two phases were combined together and refined as independently diffracting to R=0.0628. The resulting unit-cell content is (A,□)₂₀Nb₁₆Ti₂O ₅₃ (Z=8), with A=Ba, Tl, Ce, Th. For some atomic positions of the superstructure, third- and fourth-order anharmonic ADP's were used to account for the specific density shape having a continuous character as typical for ionic conductors. There are three distinct clusters in the superstructure, leading to a new structure type no longer strictly of pyrochlore-structure type.