with rubidium being located on A1, A2, or A3 (see section 5.4). The new compounds (I) & (II) crystallize in the monoclinic space group C2/c. Although there is a group subgroup relation between C2/c and Cmce (KTl) and Fddd (CsTl), respectively, the new compounds are not related in symmetry to the binary materials. Although there is no crystallographic relation between the new monoclinic compounds and KTl or CsTl, the structural relation is given by the type of thallium cluster and the similarity in the first coordination sphere of the different alkali metal positions (see Figure and SI chapters 8 and 9).