Four seven-nuclearity 3d-4f heterometallic cluster {Fe(6)LnO(28)} substituted polyoxometalates (HPz)(11)K(4)Fe(6)Ln-(mu(3)-O)(2)(B-alpha-GeW9O34)(2)(GeW6O26)center dot xH(2)O (1-Ln, Pz = piperazine, Ln = Tb, Dy, Ho, Er for x = 27, 25, 25, 24, respectively) have been hydrothermally prepared and structurally characterized by single-crystal X-ray diffraction, powder X-ray diffraction, infrared spectrometry, thermogravimetric analyses, elemental analyses, and electrospray ionization mass spectrometry. Single-crystal X-ray diffraction analyses revealed that 1-Ln contain an unprecedented banana-shaped polyanion constructed from an iron-lanthanide heterometallic {Fe(6)LnO(28)} cluster, two trilacunary {B-alpha-GeW9O34} units, and one hexalacunary {GeW6O26} fragment. The magnetic susceptibility surveying proved the presence of antiferromagnetic coupling in 1-Ln.