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 °C. The compound adopts space group P213, with a = 16.4984(5) Å 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 (I > 2σ(I)). 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 ABO3 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□1/8)(O3/4□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 Pa3¯. Nevertheless, the whole structure adopts the cubic sub-group P213. 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.

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