Sr10Al6O19 is monoclinic, space group C12/c1, a = 34.5823(21) Angstrom, b = 7.8460(6) Angstrom, c = 15.7485(9) Angstrom, beta = 103.68(1)degrees, V = 4151.9(7) Angstrom(3), Z = 8. The structure has been solved from a single crystal diffraction dataset by direct methods and subsequently refined by a full-matrix least-squares process to a residual index of R(\F\) = 0.038 for 2537 observed reflections with I > 2sigma(I). The compound is an oligoaluminate containing highly puckered [Al6O19]-groups of corner-sharing tetrahedra; it is the first purely aluminate cluster of this type, but it resembles the [square(6)O(19)]-group recently found in alpha-Sr(10)square(6)O(19). Linkage between the hexamers is provided by 11 crystallographically different strontium atoms located in planes parallel (100). They are coordinated by six-eight next oxygen neig bours. The structure can be derived from perovskite, ABO(3), by introducing ordered vacancies into the substructure of the oxygen atoms. The A-sites in Sr10Al6O19 are exclusively occupied by Sr atoms, whereas strontium and aluminum atoms reside on the B-positions in the ratio 1:3. The relationship with perovskite can be expressed in the crystal chemical formula Sr(Al3/4Sr1/4)-(O(19/8)square(5/8)). (C) 2002 Elsevier Science Ltd. All rights reserved.