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

• Published in: Crystal growth & design 2024-10, Vol.24 (19), p.7910-7918
• Main Authors: Salcedo, Inés R., Bazaga-García, Montse, Pérez Colodrero, Rosario M., Vílchez-Cózar, Álvaro, Cañamero-Cebrián, Fernando, Olivera Pastor, Pascual, Zaręba, Jan K., Cabeza, Aurelio
• Format: Article
• Language: English
• ISSN: 1528-7483; 1528-7505
• DOI: 10.1021/acs.cgd.4c00786

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.