*2.3.3. Fatigue effects in soft and hard compositions*

A number of studies have been devoted to the understanding of fatigue effects in soft and hard ferroelectric compositions, but at present the differences in bulk materials are not completely understood mainly due to the fragmentary character of the published literature often present‐ ing contradicting results. For PZT bulk ceramics [20], it was found that soft composition samples (sintered from commercial PZT 5A powder obtained from Morgan Electroceramics Inc.) exhibited a faster fatigue degradation compared to that of the hard PZT ceramics (sintered from commercial APC-841 powder obtained from American Piezoceramics Inc.). For BaTiO3 bulk ceramics, it was shown that a partial substitution of Ti4+ ions in the B-site with donor species (e.g., Nb5+ ions) leads to a higher fatigue strength compared to that of the acceptordoped (Ca2+ and Al3+) and of the undoped specimens.[21] This is in agreement with the concept that fatigue characteristics can be improved by reducing the content of oxygen vacancies. On the contrary, the addition of Fe-acceptor in the B-site of [(Na0.5K0.5)0.96Li0.04][(Nb0.86Ta0.1Sb0.04)1 xFex]O3 ceramics has produced improved fatigue characteristics.[22] This effect was attributed to the increased mechanical strength of the grain boundaries upon Fe addition, which had overcome the effect of the expected increase of the oxygen vacancies content due to the presence of the electron acceptor species.
