Piezoelectric actuators used in nano-positioning devices exhibit highly non-linear behavior and strong hysteresis, which limits the efficiency of conventional non-model-based controllers. This paper presents the first results of a free energy model based on the theory of thermal activation for single crystal piezoceramics that couples mechanical stress and electric field. It is capable of predicting the hysteretic behavior along with the frequency-dependence present in these materials. The model is then coupled with a spring as a first step toward a SDOF model of a commercial nano-positioning stage and is the basis for future control applications. Quasi-static simulations are presented to illustrate the effects of spring loading on the actuator behavior.