DETERMINATION OF THE SPACE GROUP OF CERAMIC BAO-CENTER-DOT-PR2O3-CENTER-DOT-4TIO(2) BY ELECTRON-DIFFRACTION

Ceramic specimens of BaO . Pr2O3 . 4TiO(2) (Ba4.5Pr9Ti18O54) were prepared by the mixed-oxide route. Electron diffraction indicated that the compound has orthorhombic symmetry with cell parameters a similar or equal to 22.2, b similar or equal to 12.2 and c similar or equal to 7.6 Angstrom. 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 . Pr2O3 . 4TiO(2) suggests that it underwent a phase transition during cooling from Pbam to Pnam in which the c axis doubled.