Electrical properties and phase transformations of graphite nitrates

Abstract

Methods are described for converting pieces of well alined graphite into nitrates of the first and second sequences, using vapour-phase nitration. In general, the more highly pinned starting materials require a higher threshold vapour pressure of nitric acid, before onset of conversion to the nitrates. Measurements of electrical resistivity and of thermo-electric power in the a -axis direction have been made over a range of temperatures, including temperature cycles around the λ point. Electronically, the solid nitrates of both first and second sequences behave as good p -conductors, in accordance with a structural model in which carbon hexagon networks act as macro-cations, with intercalations of nitrate anions, and of molecules of nitric acid. Resistivity measurements down to about 64 °K show that the well ordered graphite nitrates below the λ point have electrical conductivities and temperature coefficients of conductivity comparable with natural elemental conductors such as silver or copper. Resistivity data and thermo-electric power (t. e. p.) measurements both reveal novel features about the order–disorder transformation taking place around –20 °C. Definite hysteresis is found, and details of the hysteresis loops depend on the original texture of the specimen of near-ideal graphite used, as well as on nitration procedures. Furthermore, a large positive excess t. e. p. is observed at the λ peak, flanked by premonitory effects extending on either side of it. Both the hysteresis and the excess t. e. p. at the λ point are interpreted in terms of a general theory of phase transformations, which involve intermediate hybrid structures, with domains of the two forms coexistent in the λ region of temperatures.