Phase transitions and transport properties in tetra-n-butylammonium iodide

A remarkable feature of organic salts is easy reorientation of organic groups, especially in high-temperature phases. It would be expected that the reorientational motion of organic groups promotes also ion transport processes within the crystal structure. In this work thermal and transport properties of solid organic salt (n-C ₄ H ₉ ) ₄ NI were investigated by methods of DSC, dilatometry, conductivity measurements and NMR spectroscopy. It was proposed that three low-temperature phases, II, III and IV, along to the high-temperature phase I appear in this salt after heat treatment. The conductivity and temperature stability range of the low-temperature phases depend on conditions of the heat treatment of the salt. Temperature intervals of stability of phases II and III considerably increases after prolonged heating in the melt. In parallel, the conductivity of all low-temperature phases increases in contrast to the conductivity of the phase I. These facts may be explained by the formation of extrinsic iodine vacancies as a result of the dissolution of tributylanine formed at partial thermal decomposition of the salt. According to ¹ H NMR data, in high-temperature phase TBA cations are mobile. No information on anionic diffusion in this phase was obtained due to a low intensity of ¹²⁷ I NMR spectra. The most mobile defects in low-temperature phases of (n-C ₄ H ₉ ) ₄ NI seem to be iodine vacancies.