.
After 600 s, the voltage was switched back to the initial value of U = 0 V. While maintaining the electric field, the Bragg peak continuously moves to lower angles with time, indicating a lattice expansion.
This lattice expansion may originate from the Joule effect heating up the sample. Compared to the piezoelectric strain induced by the electric field, the thermal lattice expansion is a factor of 5 less over a time period of 10 min.
The studied sample contains Au and Pt electrodes whose thermal expansion coefficients are much higher than those for piezo ceramics [36] making them ideal as a reference for detecting heating effects.
Hence, the evolution of the Pt 111 Bragg peak was studied as a function of the applied potential (not shown here) revealing a variation of ~10−3 degrees in its 2θ-position, which is close to the resolution limit of this experimental approach.
This change in diffraction angle corresponds to a thermal strain of b10−4 and, thus to a temperature increase of b10 °C at U = 9 V.
Thus, the thermal expansion by Joule heating was neglected for the further studies.