Mechanism of the dc conduction in undoped and Sr doped ammonium zinc chloride crystal

The dc conductivity of ammonium zinc chloride (AZC) crystal as functions of temperature, electric field intensity and Sr²⁺-doping concentration has been studied. Anomalous changes at the transition points connecting the antiferroelectric (AF), commensurate (C), incommensurate (IC) and normal (N) phases have been detected. The broadening of the peak anomaly at the C-IC phase transition suggested an important role for impurities assisting creation and annihilation of discommensuration (DC). Considerable shifts of the transition temperatures after doping with different Sr²⁺ concentrations were noticed. Electric conduction in AZC continuously increased with increasing Sr²⁺-concentration. Parameters extracted from the current density-electric field intensity relationship according to the usual Richardson-Schottky (R-S) emission equation were found inconsistent with corresponding experimental values. The results have been treated using the modified R-S equation for dielectrics with small electronic mean-free path. The Pool-Frenkel (P-F) conduction mechanism was also considered and the possibility of conduction by either of R-S or P-F was discussed.