IONIC CONDUCTIVITY IN SINGLE CRYSTALS OF IODINE - IONS IN IODINE

Doping of iodine single crystals with p-phenylenediamine (PPD) leads to paramagnetism and a considerable increase in the electrical conductivity of these crystals. From electron spin resonance (ESR) measurements on doped single crystals it follows that the paramagnetism results from the ionization of the PPD molecules. The concentration and temperature dependence of the ESR signal can be interpreted employing the theory of chemical equilibrium PPD+I 2⇄PPD++I2-. The electrons donated to the lattice form Is~ ions, which may move as ions through the lattice. The donor ionization energy of PPD is found to be 0.50 eV. The motion of the I2- ions does not lead to considerable conductivity. Two of these ions can cooperate, however, to form an I 3- ion and an I- ion: 2I2 -⇄. The I3- ion can be rapidly transported in a manner similar to the protons in ice I3 -+I2→I2+I3-, while the I- ion formed stays behind in the hole left by the dissociated I2- ion. The rapid transfer process of the I 3- ion leads to the conductivity in the iodine single crystals. The concentration and temperature dependence of this conductivity can be quantitatively understood on the basis of this mechanism. Plating and current-efficiency experiments have been carried out on polycrystalline samples of PPD-doped iodine to prove the ionic nature of the conductivity.