Reversible amorphization under ac electric field and character of conductivity of TlInTe2 crystal at ionic conductivity temperature

Abstract

Temperature dependence of x-ray diffraction (XRD) investigation of crystal over a wide temperature interval (300–570 K) reveals a sequence of phase transitions, including ferroelectric (300–370 K) and ionic conductivity (485–500 K) phase transitions (FPT and ICPT), which are caused by shifting and thermal activation of ions inside of Thomson cube cages of unit cells. The results of XRD versus temperature studies confirm our previous investigations of this crystal (Alekperov O Z et al 2018 J. Appl. Phys. 123 135701) by means of the impedance spectroscopy (IS) method. Temperature dependence of almost all the reflexes’ intensity in XRD data was investigated up to a few tens of Kelvin higher than the so called ionic conductivity (IC) temperature. Intensities of almost all reflexes decrease with increasing the temperature up to K, and reach a minimum at K, corresponding to the maximum of at FPT. The same strong decrease of intensity takes place for reflex 110 at K, which corresponds to Tl+ -ion activation along the chain direction of crystal. A small decrease of the 221 reflex intensity at IC temperature is also observed. The reflexes, intensities of which were almost not changed, are 220 and 440. Room temperature XRD measurements, which were performed in wide temperature interval under alternate electric field with amplitude and frequency 0.5 MHz applied along c-direction of the crystal . Our measurements show a partial amorphization of structure which relaxes back during a few days gradually to more perfect tetragonal structure with XRD reflex intensities higher than the initial reflexes. By analyzing the results of IS and conductivity measurements in wide- and narrow-gap semiconductors on the example of rubidium nitrate () and thallium indium telluride () crystals correspondingly, it was shown that the definition of super ionic conductivity is completely inapplicable, since a high conductivity in these semiconductors originates from increase of free electrons concentration.