A three-dimensional metal-organic framework (MOF), poly[[μ-5'-pentahydrogen [1,1'-biphenyl]-3,3',5,5'-tetrayltetrakis(phosphonato)]erbium(III)] 2.5-hydrate], formulated as [Er(CHOP)]·2.5HO or [Er(Hbtp)]·2.5HO (I) and isotypical with a Y-based MOF reported previously by our research group [Firmino et al. (2017b). Inorg. Chem. 56, 1193-1208], was constructed based solely on Er and on the polyphosphonic organic linker [1,1'-biphenyl]-3,3',5,5'-tetrakis(phosphonic acid) (Hbtp). The present work describes our efforts to introduce lanthanide cations into the flexible network, demonstrating that, on the one hand, the compound can be obtained using three distinct experimental methods, i.e. hydro(solvo)thermal (Hy), microwave-assisted (MW) and one-pot (Op), and, on the other hand, that crystallite size can be approximately fine-tuned according to the method employed. MOF I contains hexacoordinated Er cations which are distributed in a zigzag inorganic chain running parallel to the [100] direction of the unit cell. The chains are, in turn, bridged by the anionic organic linker to form a three-dimensional 6,6-connected binodal network. This connectivity leads to the existence of one-dimensional channels (also running parallel to the [100] direction) filled with disordered and partially occupied water molecules of crystalization which are engaged in O-H...O hydrogen-bonding interactions with the [Er(Hbtp)] framework. Additional weak π-π interactions [intercentroid distance = 3.957 (7) Å] exist between aromatic rings, which help to maintain the structural integrity of the network.