Protonic conductivity of solid UO2HPO4·3H2O

The electrical and electrochemical properties of the solid ionic conductor UO₂HPO₄·3H₂O were investigated within the temperature range from room temperature up to 673 K. The conductivity of a nondehydrated sample within the temperature range from 303 K (6.5 × 10^-3 S/m) to 350 K (2.5 × 10^-2 S/m), with the activation energy of 23 kJ/mol, is a consequence of the presence of crystal water, and originates, mostly, from the fast movement of protons across the network of molecules of water in a tunneling mechanism pattern. When heated, the conductor loses crystal water, which leads to a final change in the nature of the conductor and in the conductivity mechanism. The conductivity of the dehydrated salt UO₂HPO₄ (2.5 × 10^-4 S/m at 488 K), with the activation energy 44.04 kJ/mol, is considerably lower than the one mentioned above and can be attributed, to a great extent, to the movement of protons, most likely by a tunneling mechanism, through the less favourable structure formed by phosphate groups of the dehydrated salt. On being heated above 623 K for a certain time, the acid phosphate transforms into pyrophosphate, the conductivity of which is lower than that mentioned above. By means of electrochemical methods, the electrode processes were studied. It has been shown that H⁺ and UO²⁺ ions are reduced at the cathode, while the phosphate groups oxidize at the anode and O₂ is evolved.