The wide range of fascinating properties observed in complex oxide continue to attract great interest such as ferro-, piezo- and pyroelectricity. Several strategies have been employed to break the lattice symmetry and expand the range of functionalities. Electrostriction is a general property of all dielectrics and occurred when the deformation of a dielectric material is proportional to the square of the applied electric field. Recently, Gd₂O₃-doped CeO₂ (CGO) has emerged as a new family of electrostriction materials with the maximum stress exceeding 500 MPa. To date, various efforts have been made to improve the strain output, such as stimulate by electric field, varying the temperature and aging the defects. However, materials that simultaneously satisfy the requirements of fast response, large strain and high energy density are still very rare. The development of materials with large dielectric breakdown strength, high dielectric constant and low elastic modulus is still challenging.

Heterostructure interfaces plays an important role by contributing to its unique structural and physical properties which result in symmetry breaking, electronic and/or atomic reconstruction(s) as well as strain gradients from which a wealth of new intriguing properties can emerge. Such richness arises from a strong interaction between the charge, orbital, spin, and lattice degrees of freedom.

Here, I will show how symmetry breaking offers extraordinary opportunities such as stabilizing phases which are otherwise not stable using highly coherent interfaces and enhancing the electromechanics with interfaces or inducing piezoelectricity in otherwise centrosymmetric system. In one example I will show the results obtained with a stack of ultrathin defective oxide single-crystal layers with stable phases and electrostriction properties exceeding any known system. Alternating layers of Gd₂O₃-doped CeO₂ (CGO) and Er₂O₃-stabilized δ-Bi₂O₃ (ESB) were deposited on NdGaO₃ substrates. The measured electrostriction coefficients found to follow a linear increase with the decrease of the number of interfaces. We have observed the highest electrostriction coefficient which surpasses any known electrostrictive materials, including the commercial relaxor ferroelectrics, such as PMN-PT. In another examples I will show our recent discovery illustrating the possibility to induce a large piezoelectric response in centrosymmetric oxides, i.e., in cubic fluorite oxides, Gd-doped CeO_2-x. Surprisingly, the results show the generation of large low-frequency piezoelectric responses (d33 ~ 100,000 pm/V). Such a high piezoelectric response from intrinsically nonpiezoelectric materials has not been observed before in centrosymmetric materials.

This collection of possibilities offers unique opportunities for a wide range of rich world and new functionality of complex oxide and their interfaces.