Factors contributing to the local polar-structural heterogeneity and ultrahigh piezoelectricity in Sm-modified Pb(Mg1/3Nb2/3)O3-PbTiO3

Recently, an ultrahigh piezoelectricity (longitudinal piezoelectric coefficient d(33) similar to 1500 pC/N) was reported in polycrystalline specimens of Sm-modified relaxor ferroelectric system Pb(Mg1/3Nb2/3)O-3-PbTiO3 (PMN-PT) (Li et al 2018 Nat. Mater. 17 349). Here, we examine the factors associated with this anomalous phenomenon by carrying out a comparative structural, dielectric, ferroelectric and impedance spectroscopy on Sm-free 0.71PMN-0.29PT (piezoelectric coefficient d(33) similar to 380 pC/N) and its Sm modified counterpart (d(33) similar to 1160 pC/N). We found that, apart from inducing a Cm -> P4mm inter-ferroelectric instability, Sm modification stabilizes a considerable degree of oxygen vacancies, with Sm exhibiting valence fluctuation. Based on our results, we argue that these defect types are the important source for creating structural-polar heterogeneity on the local scale, the interaction of which with the long-range ferroelectric order makes the system exhibit an ultrahigh piezoelectric response.