Syntheses, crystal structures, and proton conduction properties of two zirconium fluorophosphonates

By introducing hydrogen fluoride (HF) and a competitive ligand into the synthetic system to enhance the crystallinity of products, two two-dimensional (2D) zirconium fluorophosphonate compounds with high crystallinity, namely A₂[Zr(hedp)F₂] (hedpH₂ = 1-hydroxyethylidene-1,1-diphosphonic acid, A⁺ = K⁺ (1), NH₄⁺ (2)), were successfully prepared. Their structures feature the anionic layer of [Zr(hedp)F₂]_<i>n</i>^2<i>n</i>- with A⁺ cations located in interlayer spaces. The unique coexistence of F, -OH, and -PO₃ moieties within the structure generates an extensive hydrogen-bonding network, facilitating efficient proton transport. At the temperature of 85 °C and the relative humidity (RH) of 95%, the proton conductivity (<i>σ</i>) of compounds 1 and 2 is 6.37 × 10^-4 S cm^-1 and 1.27 × 10^-2 S cm^-1, respectively. The proton conductivities of both compounds follow the Grotthuss mechanism. Compound 2 demonstrates better proton conductivity owing to the presence of NH₄⁺ cations, which facilitate the formation of extensive hydrogen-bonding networks that enable efficient proton transport. This study endows the zirconium fluorophosphate family with new members and provides a feasible research strategy for constructing highly crystalline metal fluorophosphonate compounds.