Metal phosphonate-carboxylate compounds repre-sent a promising class of materials for proton conductionapplications. This study investigates the structural, thermal, andproton conduction properties of three groups of lanthanide-basedcompounds derived from 5-(dihydroxyphosphoryl)isophthalic acid(PiPhtA). The crystal structures, solved ab initio from X-raypowder 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 alayered structure with unbonded carboxylic groups orientedtoward the interlayer region. All compounds feature carboxylicgroups and coordinating water molecules. Impedance measurements demonstrate that these materials exhibit water-mediated protonconductivity, 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 conductivityconsistent with a Grotthuss-type proton-transfer mechanism. Notably, group II of the studied compounds undergoes the formationof 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
