CRYSTAL-STRUCTURE OF RARE-EARTH ALKALINE ALUMINATES BA3AL2O6 AND BA2.33CA0.67AL2O6 (BA7CA2AL6O18)

Single crystals of Ba3Al2O6 (1) and Ba2.33Ca0.67Al2O6 (2) could be obt ained by reacting the alkaline earth metal carbonates with Al2O3 at 15 00 degrees C and their crystal structures were determined using single -crystal X-ray diffractometry data. Despite the cubic lattice constant s found for both compounds (1, 16.5068(5) Angstrom, 2, 16.180(1) Angst rom), their structure is shown to be orthorhombic. It is closely relat ed to the perovskite structure. The principal structural element is th e Al12O36 ring built up by 12 distorted corner-sharing AlO4 tetrahedra arranged similarly to the sulphur atoms in S-12. The Ca atoms in 2 oc cupy fully four of the 18 alkaline earth metal positions in 1. No anal ogy to the mixed occupation realized in (Sr,Ca)(3)Al2O6 is observed.