While thallium forms several clusters in alkali metal thallides, indium clusters in related alkali metal indides are very rare. Here the mixing of the heavy trielides indium and thallium to approximate icosahedral entities present in Na4A6Tr13 (A = K, Rb, Cs; Tr = In, Tl) is reported. Experimental results prove that at least up to 76% indium can be introduced in this structure type. From the moment indium is provided, the central atom of the icosahedral unit changes from thallium to indium, while the vertices of the cluster are mixed occupied by both, thallium and indium. The bonding and energetics of the compounds are investigated with quantum chemical methods.