Electrical properties and phase transformations of graphite nitrates

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 $\alpha$-axis direction have been made over a range of temperatures, including temperature cycles around the $\lambda$ 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 $^\circ$K show that the well ordered graphite nitrates below the $\lambda$ 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 $^\circ$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 $\lambda$ peak, flanked by premonitory effects extending on either side of it. Both the hysteresis and the excess t.e.p. at the $\lambda$ 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 $\lambda$ region of temperatures.