Correlation between internal stress and ferroelectric fatigue in Bi4−xLaxTi3O12 thin films

La-substituted bismuth titanate $[{\mathrm{Bi}}_{\text{4−x}}{\mathrm{La}}_x{\mathrm{Ti}}_3{\mathrm{O}}_{12}(\mathrm{BLT}\mathrm{)]}$ films were synthesized using pulsed-laser deposition and ferroelectric fatigue phenomenon was investigated. The internal strain present in the films, which was analyzed through the evaluation of the x-ray diffraction peak, was partially responsible for the fatigue in BLT films. When $\text{x⩾0.75,}$ the change in the internal strain was saturated and there was no significant degradation of switching charge, at least up to ${10}^{10}$ cycles. It revealed that the internal strain, as well as chemical stability of oxygen ions, contributed to the ferroelectric fatigue of the BLT films. The origin of the internal strain is discussed in terms of the lattice mismatch between bulk materials and thin films.