Proton Conduction Study on Water Confined in Channel or Layer Networks of La^IIIM^III(ox)₃·10H₂O (M = Cr, Co, Ru, La)

Abstract

Proton conduction of the La^IIIM^III compounds, LaM­(ox)₃·10H₂O (abbreviated to <b>LaM</b>; M = Cr, Co, Ru, La; ox^2– = oxalate) is studied in view of their networks. <b>LaCr</b> and <b>LaCo</b> have a ladder structure, and the ladders are woven to form a channel network. <b>LaRu</b> and <b>LaLa</b> have a honeycomb sheet structure, and the sheets are combined to form a layer network. The occurrence of these structures is explained by the rigidness versus flexibility of [M­(ox)₃]^3– in the framework with large La^III. The channel networks of <b>LaCr</b> and <b>LaCo</b> show a remarkably high proton conductivity, in the range from 1 × 10^–6 to 1 × 10^–5 S cm^–1 over 40–95% relative humidity (RH) at 298 K, whereas the layer networks of <b>LaCr</b> and <b>LaCo</b> show a lower proton conductivity, ∼3 × 10^–8 S cm^–1 (40–95% RH, 298 K). Activation energy measurements demonstrate that the channels filled with water molecules serve as efficient pathways for proton transport. <b>LaCo</b> was gradually converted to La^IIICo^II(ox)_2.5·4H₂O, which had no channel structure and exhibited a low proton conductivity of less than 1 × 10^–10 S cm^–1. The conduction–network correlation of LaCo­(ox)_2.5·4H₂O is reported