Doping‐induced‐effects on conduction mechanisms in incommensurate ammonium zinc chloride crystals

Abstract

AbstractThe dc conductivity (σ) along the polar b‐axis of ammonium zinc chloride (AZC) crystals in its four high‐temperature phases has been measured as a function of temperature. Doping with Mn2+ in different concentrations changed strongly both values of σ at all temperatures and the dependence of ln σdc on 1/T in the phase transition regions. The activation energy of conduction was calculated from the linear portions of this dependence in each phase. The results were discussed in the light of the decomposition of (NH4)2 in the high‐temperature normal phase, the discommensuaration (DC) formation/annihilation in the incommensurate phase and domain wall motion and stripples nucleation in the commensurate‐ and antiferroelectric‐phases. Pinning of DC's in the crystal lattice and/or by the structural defects and the possibility of dislocation formation was also discussed. The bulk‐ and the electrode‐limited conduction mechanisms were also considered. The current density‐voltage gradient relationship according to the usual Richardson‐Schottky (R‐S) equation shows disagreement between extracted parameters and experimentally measured ones. A modified equation was used to solve this difficulty which, in addition, facilitated the calculation of the electronic mobility (μ), the barrier height (φ) at the electrode‐dielectric interface and the R‐S constant (βRS). The effect of Mn2+‐content on values of μ, φ and βRS in different phases of AZC was also considered. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)