Structural Landscape\nand Proton Conduction of Lanthanide\n5‑(Dihydroxyphosphoryl)isophthalates

Metal phosphonate-carboxylate compounds represent a promising\nclass\nof materials for proton conduction applications. This study investigates\nthe structural, thermal, and proton conduction properties of three\ngroups of lanthanide-based compounds derived from 5-(dihydroxyphosphoryl)isophthalic\nacid (PiPhtA). The crystal structures, solved <i>ab initio</i> from X-ray powder diffraction data, reveal that groups <b>Ln-I</b>, Ln[O<sub>3</sub>P–C<sub>6</sub>H<sub>3</sub>(COO)(COOH)(H<sub>2</sub>O)<sub>2</sub>] (Ln = La, Pr), and <b>Ln-II</b>, Ln<sub>2</sub>{[O<sub>3</sub>P–C<sub>6</sub>H<sub>3</sub>(COO)(COOH)]<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>}·2H<sub>2</sub>O (Ln = La,\nPr, Eu), exhibit three-dimensional frameworks, while group <b>Ln-III</b>, Ln[O<sub>3</sub>P–C<sub>6</sub>H<sub>3</sub>(COO)(COOH)(H<sub>2</sub>O)] (Ln = Yb), adopts a layered structure with unbonded carboxylic\ngroups oriented toward the interlayer region. All compounds feature\ncarboxylic groups and coordinating water molecules. Impedance measurements\ndemonstrate that these materials exhibit water-mediated proton conductivity,\ninitially following a vehicle-type proton-transfer mechanism. Upon\nexposure to ammonia vapors from a 14 or 28% aqueous solution, compounds\nfrom groups <b>II</b> and <b>III</b> adsorb ammonia and\nwater, leading to an enhancement in proton conductivity consistent\nwith a Grotthuss-type proton-transfer mechanism. Notably, group <b>II</b> of the studied compounds undergoes the formation of a new\nexpanded phase through the internal reaction of carboxylic groups\nwith ammonia, coexisting with the as-synthesized phase. This postsynthetic\nmodification results in a significant increase in proton conductivity,\nfrom approximately ∼5 × 10<sup>–6</sup> to ∼10<sup>–4</sup> S·cm<sup>–1</sup> at 80 °C and 95%\nrelative humidity (RH), attributed to a mixed intrinsic/extrinsic\ncontribution. Remarkably, the NH<sub>3</sub>(28%)-exposed <b>Yb-III</b> compound achieves an enhancement in proton conductivity, reaching\n∼ 5 × 10<sup>–3</sup> S·cm<sup>–1</sup> at 80 °C and 95% RH, primarily through an extrinsic contribution.