Protonic conductivity in copper formate tetrahydrate

Abstract

The conductivities of single crystals of copper formate tetrahydrate (CFT) have been determined along the three principal crystallographic axes with proton-injecting electrodes. The ohmic conductivities at 298 K are: 5.5 ± 1.6 × 10^–7Ω^–1 cm^–1(along the b-axis), 8.5 ± 2.1 × 10^–8Ω^–1 cm^–1(along the a-axis) and 7 ± 2 × 10^–12Ω^–1 cm^–1(along the c axis). Since the H-bonded network lies in the ab-plane, the observed anisotropy of conduction suggests protonic conductivity. The Arrhenius plot of σ exhibits a change of slope and a transition to a lower conductivity at the paraelectric → antiferroelectric phase change (235 K). The energies of activation for conduction within the ab-plane are 0.56 eV above 235 K and 0.65 eV below 235 K. The tetradeuterate has an ohmic conductivity approximately an order of magnitude lower than the tetrahydrate (within the ab-plane). Space-charge-limited currents are observed at high fields. The CFT crystals behave as semipermeable membranes to protons and this supports the view that protons are mobile within CFT. If conduction is indeed protonic, CFT is a very ready hydrate protonic conductor exhibiting a conduction greater than ice.