Low-Frequency Relaxation Phenomena in ALPHA-LiIO3: The Nature and Role of Defects

The lithium iodate ionic conduction in the polar c-axis direction is studied between 35 and 470 K for crystals grown in various conditions. So far, to separate processes induced by the nature of electrodes, the impedance spectroscopy technique had been used at room temperature with both metallic and ideally polarizable insulating electrodes, so that a relaxation of space charges was clearly identified. Here, the temperature dependence of the hopping ionic conductivity exhibits quite different activation energies well related to the growth conditions. Following low-temperature Raman and thermodynamic experiments, a new approach based on a vacancy diffusion mechanism is proposed. Experimental conductivity results are then correlated with the valence, size and concentration of extrinsic impurities incorporated during the growth and analyzed by plasma spectroscopy. Finally, a discussion is made on still not well-understood phenomena such as the strong increase of photorefractivity or the enhancement of some Bragg reflections.