HIGH TEMPERATURE Conductivity MEASUREMENT
AND MODELING OF THE PROTON MOBILITY IN NH4H2PO4
H. Feki1*, H. Khemekhem1, Y. Abid1, M. Bahri2, N. Jaidane2, Z. Ben Lakhdar2
1(L.E.S) Laboratoire de Physique moléculaire et cristalline
Faculté des Sciences, 3018 Sfax, TUNISIA
2Laboratoire de Physique atomique et moléculaire – Faculté des Sciences de Tunis, TUNISIA
* Correspondant author: Habib.Feki@ fss.rnu.tn
Receveid : 08 May 2001; revised version accepted 02 October 2001
The antiferro-paraelectric phase transition at 148 K in NH4H2PO4 (ADP for short) has been characterized by typical drastic anomaly in the curve of vs temperature. In addition, the thermal evolution of the real and imaginary parts of dielectric constant (and) reveals a low frequency dispersion phenomena for T>250K, this behavior was interpreted as a result of the proton mobility at high temperatures. In the same temperature interval, a conductivity measurement has been carried out and shows an Arrhenius behavior. The level of the conductivity in this material is comparable with those of typical superprotonic conductors AHSO4 (A = Cs, Rb and Li). The associated activation energy was deduced from the log (T) = f (1000/T) plot, Ea = 0.52 eV. In addition, we have undertaken a theoretical calculation and molecular modeling of hydrogen bonded (…H…) dimers using a Self Consistent Field (SCF) semi-empirical method. A two-potential well model has been adopted and the transition state of the proton has been found. The activation energy associated to the oscillation of the proton between two equivalent ions has been determined. Ea = 0.5 eV. The study of the Raman spectrum at ambient temperature confirms the validity of this model.
Keywords : Protonic conductors; Molecular modeling; NH4H2PO4.