Abstract
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[O(3)P-C(6)H(3)(COO)(COOH)(H(2)O)(2)] (Ln = La, Pr), and Ln-II, Ln(2){[O(3)P-C(6)H(3)(COO)(COOH)](2)(H(2)O)(4)}·2H(2)O (Ln = La, Pr, Eu), exhibit three-dimensional frameworks, while group Ln-III, Ln[O(3)P-C(6)H(3)(COO)(COOH)(H(2)O)] (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 NH(3)(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.