Structural Landscape and Proton Conduction of Lanthanide 5-(Dihydroxyphosphoryl)isophthalates

Metal phosphonate-carboxylate compounds represent a promising class of materials for proton conduction applications. This study investigates the structural, thermal, and proton conduction properties of three groups of lanthanide-based compounds derived from 5-(dihydroxyphosphoryl)isophthalic acid (PiPhtA). The crystal structures, solved ab initio from X-ray powder diffraction data, reveal that groups Ln-I, Ln[O₃P–C₆H₃(COO)(COOH)(H₂O)₂] (Ln = La, Pr), and Ln-II, Ln₂{[O₃P–C₆H₃(COO)(COOH)]₂(H₂O)₄}·2H₂O (Ln = La, Pr, Eu), exhibit three-dimensional frameworks, while group Ln-III, Ln[O₃P–C₆H₃(COO)(COOH)(H₂O)] (Ln = Yb), adopts a layered structure with unbonded carboxylic groups oriented toward the interlayer region. All compounds feature carboxylic groups and coordinating water molecules. Impedance measurements demonstrate that these materials exhibit water-mediated proton conductivity, initially following a vehicle-type proton-transfer mechanism. Upon exposure to ammonia vapors from a 14 or 28% aqueous solution, compounds from groups II and III adsorb ammonia and water, leading to an enhancement in proton conductivity consistent with a Grotthuss-type proton-transfer mechanism. Notably, group II of the studied compounds undergoes the formation of a new expanded phase through the internal reaction of carboxylic groups with ammonia, coexisting with the as-synthesized phase. This postsynthetic modification results in a significant increase in proton conductivity, from approximately ∼5 × 10^–6 to ∼10^–4 S·cm^–1 at 80 °C and 95% relative humidity (RH), attributed to a mixed intrinsic/extrinsic contribution. Remarkably, the NH₃(28%)-exposed Yb-III compound achieves an enhancement in proton conductivity, reaching ∼ 5 × 10^–3 S·cm^–1 at 80 °C and 95% RH, primarily through an extrinsic contribution.