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.