5 - Structural studies of proton conductors

Summary

The structural situation

Let us here focus on the special structural situations we are likely to meet in studying crystalline proton conductors, and on how these can best be treated by conventional crystallographic refinement techniques. Reasonably, the same types of local situation occur in amorphous solids and in liquids, but their lack of translational symmetry renders them inaccessible to accurate study. We shall see that access to high quality single-crystal data (X-ray or neutron) gives no guarantee of a satisfactory result; careful thought must also be given to the method of refinement. Since many structural examples of proton conducting materials can be found elsewhere in this book, we shall here approach the problem from a general standpoint. Examples will subsequently be taken from the author's own work to illustrate the structural situations discussed.

What then is the structural feature most characteristic of a proton conductor system? It is generally believed that a proton is transferred through a solid in one of two distinct ways: by a vehicular mechanism, whereby the proton rides on a carrier molecule of type NH₄⁺ or H₃O⁺ ion, or by a Grotthuss mechanism, in which the proton jumps from a donor to a suitably placed acceptor molecule (typically, from H₃O⁺ to H₂O, or from H₂O to OH^–). How then is such a process sensed in a conventional diffraction experiment?