Studies of layered uranium(VI) compounds. I. High proton conductivity in polycrystalline hydrogen uranyl phosphate tetrahydrate

We have found that hydrogen uranyl phosphate tetrahydrate HUO₂PO₄·4H₂O has a high proton conductivity. The ac conductivity was 0.4 ohm^?1 m^?1 at 290°K measured parallel to the faces of sintered disks of the compound. The activation energy was found to be 31 ± 3 kJ mole^?1. The values of conductivity were between 3 and 10 times lower when measured perpendicular to the disk faces due to preferred orientation of the plate-like crystals. Both the powder and sintered disks are stable in air and insoluble in phosphoric acid solution of pH 2.5. Experiments are described which enable possible grain boundary contributions to the conductivity to be determined in such hydrates. The extrinsic grain boundary contribution to the conductivity was found to be small from experiments in which the pH in a solution cell was varied. The abnormally high bulk H⁺ conductivity thus inferred is attributed primarily to the high concentration of H⁺, which exists as H₃O⁺ in the interlamellar hydrogen-bonded network. A Grotthus-type mechanism of conduction is proposed which involves intermolecular transfer steps (hopping) and intramolecular transfer steps, in comparable numbers, the former facilitated by the high concentration of H₃O⁺ ions in the structure, and the latter most likely facilitated by the high concentration of H-bond vacancies.