We first reported the fcu phase of Zr-EDB, which has formula [Zr6O4(OH)4(EDB)6], in 2015, where the use of L-proline as modulator allowed access to large single crystals (at the same time the material was independently reported by a different group and denoted as BUT-30 ). As part of our ongoing interest in drug delivery from Zr MOFs, we attempted to prepare nanoparticles of this phase by modulated self-assembly, discovering the hcp phase, GUF-12(Zr), when using 100-120 equiv acetic acid and 0.5-2.5% (v/v) water in solvothermal syntheses in N,N-dimethylformamide (DMF) at 423 K. This material, with ideal formula [Zr12O8(OH)14(EDB)9], was found to be defective, but it could not be converted into a 2D material as has been achieved with isoreticular derivatives. Formic acid is also a potent modulator in Zr MOF synthesis: it has a lower pKa than acetic acid and so has greater potential for decreasing particle size 34 and inducing defectivity. Replicating the previous conditions with 105-130 equiv. formic acid as modulator and 0.5% (v/v) water in DMF solvothermal syntheses at 423 K generated a new phase, whose structure we could not determine ab initio by powder X-ray diffraction, but which showed a clearly different morphology by scanning electron microscopy (Figure ).