A Layered Mixed Zirconium Phosphate/Phosphonate with Exposed Carboxylic and Phosphonic Groups: X‑ray Powder Structure and Proton Conductivity Properties

A novel mixed zirconium phosphate/phosphonate based on glyphosine, of formula Zr2(PO4)­H5(L)2·H2O [L = (O3PCH2)2NCH2COO], was synthesized in mild conditions. The compound has a layered structure that was solved ab initio from laboratory PXRD data. It crystallizes in the monoclinic C2/c space group with the following cell parameters: a = 29.925(3), b = 8.4225(5), c = 9.0985(4) Å, and β = 98.474(6)°. Phosphate groups are placed inside the sheets and connect the zirconium atoms in a tetradentate fashion, while uncoordinated carboxylate and P–OH phosphonate groups are exposed on the layer surface. Due to the presence of these acidic groups, the compound showed remarkable proton conductivity properties, which were studied in a wide range of temperature and relative humidity (RH). The conductivity is strongly dependent on RH and reaches 1 × 10–3 S cm–1 at 140 °C and 95% RH. At this RH, the activation energy of conduction is 0.15 eV in the temperature range 80–140 °C. The similarities of this structure with related structures already reported in the literature were also discussed.