Title: Piezoresponse of ferroelectric films in ferroionic states: time and voltage dynamics,
Abstract: The interplay between electrochemical surface charges and bulk
ferroelectricity in thin films gives rise to a continuum of coupled ferro-ionic
states. These states are exquisitely sensitive to chemical and electric
conditions at the surfaces, applied voltage, and oxygen pressure. Using the
analytical approach combining the Ginzburg-Landau-Devonshire description of the
ferroelectricity with Langmuir adsorption isotherm for the ions at the film
surface, we have studied the temperature-, time- and field- dependent
polarization changes and electromechanical response of the ferro-ionic states.
The responses are found to be inseparable in thermodynamic equilibrium and at
low frequencies of applied voltage. The states become separable in high
frequency dynamic mode due to the several orders of magnitude difference in the
relaxation times of ferroelectric polarization and surface ions charge density.
These studies provide an insight into dynamic behavior of nanoscale
ferroelectrics with open surface exposed to different kinds of
electrochemically active gaseous surrounding.