Conduction and relaxation of cations in dehydrated partially copper(II)-exchanged synthetic faujasites

Abstract

The electrical conductivity and dielectric relaxation of dehydrated synthetic faujasites X and Y with various Cu²⁺-contents are measured at temperatures between ambient and 750 K and in the frequency range 200–3 × 10⁶ Hz.

The supercage cations are responsible for the conduction process. At exchange levels below 11–14 Cu²⁺ ions per unit cell these are Na⁺ ions. At higher exchange levels a low temperature and a high temperature conduction process are evident, due to Na⁺ and Cu²⁺ ions, respectively, in the supercages. The activation energies and entropies for Na⁺ conduction decrease with increasing Cu²⁺ in the small cages: 74–32 kJ mol^–1 and –20 to –150 J mol^–1 K^–1, respectively. The activation energy and entropy for Cu²⁺ conduction are 120 ± 10 kJ mol^–1 and –9 ± 13 J mol^–1 K^–1, respectively. They both decrease significantly in the presence of protons.

In the experimental temperature and frequency ranges 3 relaxations were observed. In order of decreasing critical frequency at a given temperature they were assigned to (i) local migration of site III′ cations between two occupied sites II, (ii) cationic jumps confined to the sodalite cages and (iii) electrode polarizations. A model explaining the variation of the intensities of relaxation (i) and (ii) with degree of Cu²⁺-exchange and with the temperature is proposed.

Excess Cu is present in an unidentified hydroxylated form and does not intervene in the ionic processes studied.