Electrical conductance mechanisms in solid caesium halides

Electrical d.c. conductances of caesium halides as ordinarily obtained are found to be anomalous compared with the halides of the other alkali metals: (i) In the conventional relationship, log10σ = log10σ0 –E/RT(conductance σ, temperature T° K), the values both of σ0 and of the activation energy E are abnormally low for all the caesium halides, compared with the other alkali halides. (ii) In the neighbourhood of the transition bcc→fcc in caesium chloride at 469° C, no rise of conductance is observed, contrary to what might be expected on structural grounds if the conductance is ionic and the lattice shows premonitory break-up on approaching the transition. E is much the same for both polymorphs, but σ0 for the fcc lattice is about one-fiftieth that for the bcc lattice. (iii) Transport measurements show that much of the current flows through caesium chloride without an accompanying transport of matter. The simplest interpretation of all these anomalies is that an important part of the conductance of the caesium halides as ordinarily obtained is electronic, not ionic. Possible reasons are discussed.