Determination of the space group of ceramic BaO.Pr2O3.4TiO2 by electron diffraction

Ceramic specimens of BaO.Pr 2 O 3 .4TiO 2 (Ba 4.5 Pr 9 -Ti 18 O 54 ) 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 A. The space group was identified as Pnam, one of the two possible space groups previously identified for Ba 3.75 Ln 9.5 Ti 18 O 54 , 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.Pr 2 O 3 .4TiO 2 suggests that it underwent a phase transition during cooling from Pbam to Pnam in which the c axis doubled.