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, HUO 2 PO 4 ·4H 2 O (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 290°K 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 ω 0 of approximately 10 15 Hz could be derived from the parallel conductivity on the assumption that the charge carrier concentration was equal to that of the H 3 O + ions. This implies a low proton mobility, of the order of 10 −9 m 2 V −1 sec −1 at 290°K, in support of previous estimates. We have also shown that polycrystalline hydrogen uranyl arsenate tetrahydrate, HUO 2 AsO 4 ·4H 2 O (HUAs), has a high conductivity of 0.6 ohm −1 cm −1 at 310°K, with an activation energy of 31 ± 2 kJ mole −1 . Below the respective dielectric ordering transition temperatures of HUP and HUAs of 274 and 301°K, the lower conductivity values show a marked frequency dependence, which may be due to dispersion effects caused by water reorientations.