Recently, an ultrahigh piezoelectricity (longitudinal piezoelectric coefficient d33 ~ 1500 pC/N) was reported in polycrystalline specimens of Sm-modified relaxor ferroelectric system Pb(Mg1/3Nb2/3)O3–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 d33 ~ 380 pC/N) and its Sm modified counterpart (d33 ~ 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.

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We numerically investigate the phase diagram of the giant-piezoelectric (1 − x)Ba(Zr 0.2 Ti 0.8)O 3x(Ba 0.7 Ca 0.3)TiO 3 system, treating the ions either as classical objects (via classical Monte-Carlo or CMC simulations) or quantum mechanically (via Path-integral Quantum Monte-Carlo or PI-QMC simulations). It is found that PI-QMC not only provides a better agreement with available experimental data for the temperature-composition phase diagram but also leads to the existence of an orthorhombic ground state in a narrow range of composition, unlike CMC that "only" yields ground states of rhombohedral or tetragonal symmetry. X-ray powder diffraction experiments are further conducted at 20 K. They confirm the occurrence of a quantum-fluctuation-induced orthorhombic state for some compositions and therefore validate the PI-QMC prediction. The role of quantum effects on the local structure, such as the annihilation of a homogeneous rhombohedral system in favor of an inhomogeneous mixing of orthorhombic and rhombohedral clusters, is also documented and discussed.

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Polycrystalline Bi1∕2Sr1∕2FeO3 was synthesized by solid state reaction method. Powder x-ray diffraction pattern of the sample was fitted using a noncentrosymmetric rhombohedral crystal system of space group R3c. The sample was found to be free from any secondary phases. This single phase material exhibits simultaneously ferrimagnetism and ferroelectricity with magnetic and electrical Curie transitions at TCM=745K and TCE=465K, respectively. Origin of this spontaneous magnetic order is discussed using the superexchange interaction between Fe ions. The observed ferroelectric polarization is explained based on the spontaneous ordering of 6s2 lone pair electrons of Bi3+ ions and a possible coexistence of charge-ordering of site- and bond-centered types.