Impedance spectroscopy of ionic conduction in hydrogen-bonded TlH ₂PO ₄

TlH ₂PO ₄, a hydrogen-bonded ferroelectric, was investigated by AC impedance measurements at various temperatures. Both the real and the imaginary impedance values showed anomalous behaviors at the ferroelastic-paraelastic transition. On the complex impedance plane, the Cole-Cole plots evolved into two regions with increasing temperature. The low-frequency region shows Warburg impedance (n ∼ 0.5), as in the case of surface conduction in solid electrolytes. The high-frequency conduction mechanism appears to be proton ionic transport in the bulk, The proton conduction for TlH ₂PO ₄ consists of two-phase elements, corresponding to a glass-like surface region and a normal bulk region.