The exploration of the systems Fe-H-P-O, Li-Fe-H-P-O, and K-Fe-H-P-O using soft chemistry methods has allowed three new hydrogenophosphates to be synthesized, whose structures have been determined by ab initio calculations. The structures of two of them-FeH2P2O7 and LiH3Fe(P2O7)(2)-exhibit close relationships: their 3D framework consists of [FeO4](infinity) and [LiFe2O12](infinity) Chains of edge-sharing octahedra, respectively, interconnected through diphosphate groups. These two diphosphates exhibit a paramagnetic to antiferromagnetic transition at low temperatures. The former phase exhibits intrachain ferromagnetic interactions (theta(P) > 0) in competition with the antiferromagnetic interchain ordering, whereas for the second one, the ferromagnetic interactions have disappeared due to the presence of Li in the chains. Differently, the third phosphate, K2Fe(HP2O7)(H2PO4)(2), exhibits a chain structure, involving isolated FeO6 octahedra, and consequently is paramagnetic in the whole temperature range (4-300 K). Among these three phosphates, only the latter exhibits ionic conductivity, which may originate from the proton mobility.