Studies of layered uranium(VI) compounds. IV. Proton conductivity in single-crystal hydrogen uranyl phosphate tetrahydrate (HUP) and in polycrystalline hydrogen uranyl arsenate tetrahydrate (HUAs)

Single-crystal measurements on hydrogen uranyl phosphate tetrahydrate, HUO2PO4.4H2O (HUP), have confirmed that the high proton conductivity is a bulk characteristic. The conductivity values were in substantial agreement with those previously reported for polycrystalline disks. A conductivity of 0.6 ohm-1 m-1 at 2900K and an activation energy of 30 +- 1 kJ mole-1 were measured parallel to the structural layers of the crystal. The conductivity was at least 100 times lower when measured in the perpendicular direction. A reasonable attempt frequency ω0 of approximately 1015 Hz could be derived from the parallel conductivity on the assumption that the charge carrier concentration was equal to that of the H3O+ ions. This implies a low proton mobility, of the order of 10-9 m2 V-1 sec-1 at 2900K, in support of previous estimates. We have also shown that polycrystalline hydrogen uranyl arsenate tetrahydrate, HUO2AsO4.4H2O (HUAs), has a high conductivity of 0.6 ohm-1 cm-1 at 3100K, with an activation energy of 31 +- 2 kJ mole-1. Below the respective dielectric ordering transition temperatures of HUP and HUAs of 274 and 3010K, the lower conductivity values show a marked frequency dependence, which may be due to dispersion effects caused by water reorientations