Determination of the Space Group of Ceramic BaO.Pr₂O₃.4TiO₂ by Electron Diffraction

Ceramic specimens of BaO^.Pr₂0₃^.4TiO₂ (Ba_4.5Pr₉Ti₁₈O₅₄) were prepared by the mixed-oxide route. Electron diffraction indicated that the compound has orthorhombic symmetry with cell parameters a ≃ 22.2, b ≃ 12.2 and c ≃ 7.6 Å. The space group was identified as Pnam, one of the two possible space groups previously identified for Ba_3.75Ln_9.5Ti₁₈O₅₄, where Ln = Nd, La or Sm. The fact that Pnam is a subgroup of the 2c superlattice of Pbam and the existence of antiphase domains with a displacement vector c/2 in BaO^.Pr₂O₃^.4TiO₂ suggests that it underwent a phase transition during cooling from Pbam to Pnam in which the c axis doubled.