Carrier injection and related phenomena in molecular crystals. Experimental study of proton injection in hydrogen-bonded organic solids

Abstract

The conductivities of single crystals and compressed samples of isocytosine, pentaerythritol, tetrolic acid and cetyl alcohol, all of which, on the basis of their structural properties, are likely to be protonic semi-conductors, have been measured under a wide range of conditions including inter alia variations in field strength, crystallographic direction, crystal thickness and ambient gas. The bulk conductivity of ultra-dry isocytosine is extremely low even at high temperatures: at 473 K σ[010] is 5.6 × 10^–14 and σ[001] less than 8.3 × 10^–14 ohm^–1 cm^–1. The surface conductivity of the ultra-dry solid is, however, significant; for compressed discs the measured conductivity at 473 K is ca. 10^–11 ohm^–1 cm^–1, and there is a reproducible activation energy for conduction of 1.40±0.04 eV. In the presence of water vapour, the conductivity of isocytosine increases markedly, non-ohmic effects being evident and there are strong indications of protonic conductivity.

Pentaerythritol, which displays anisotropy as between (101) and (001) faces in its surface conductivity, does not appear to be a protonic conductor even in the vicinity of its first-order transition temperature. Tetrolic acid shows no signs of protonic migration, and neither does cetyl alcohol at room temperature. The conductivity of cetyl alcohol rises sharply (a thousand-fold increase over a few degrees range) beyond 312 K, when rotation within the (001) sheets of hydroxyl group occurs, and the process is almost certainly protonic.