sentences:  The heteroatom is located centrally and caged by 12 octahedral MO6-units linked to one another by the neighboring oxygen atoms.There are a total of 24 bridging oxygen atoms that link the 12 addenda atoms.The metal centres in the 12 octahedra are arranged on a sphere almost equidistant from each other, in four M3O13 units, giving the complete structure an overall tetrahedral symmetry.