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 the 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 the high frequency dynamic mode due to the several orders of magnitude difference in the relaxation times of ferroelectric polarization and surface ion charge density. These studies provide an insight into the dynamic behavior of nanoscale ferroelectrics with an open surface exposed to different kinds of the electrochemically active gaseous surrounding.

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