On the symmetry of Ba3Al2O6 - X-ray diffraction and Raman spectroscopy studies

Single crystals of tribarium aluminate have been synthesised from the melt at 1480 degrees C. The compound adopts space group P2(1)3, with a = 16.4984(5) angstrom and Z = 24 formula units per unit cell. The refinement of the atomic parameters resulted in a residual of R1 = 0.040 (wR = 0.091) for 3788 observed reflections (1 > 2 sigma(1)). The principal building units of Ba3Al2O6 are highly corrugated 12-membered rings of AlO4-tetrahedra which are cross-linked via Ba cations. Alternatively, the compound can be derived from the ABO(3) perovskite-type, representing a defect structure with vacancies on the B- and the O-positions. The relationship with perovskite can be expressed by the following formula: Ba(Al3/4Ba1/8 square(1/8))(O-3/4 square(1/4))(3). The structure shows a pronounced degree of pseudo-symmetry. Apart from one Ba position all atoms in the structure approximately conform to the symmetry requirements of Pa (3) over bar. Nevertheless, the whole structure adopts the cubic sub-group P2(1)3. The present investigation corrects earlier studies describing Ba3Al2O6 in either too low or too high symmetry. The characterization of tribarium aluminate is completed by Raman spectroscopy. (c) 2008 Elsevier Masson SAS. All rights reserved.