Large Electromechanical Coupling Due to Cooperative Effects in Zr-doped Ceria

We have investigated electrostriction (ES) in Zr-doped ceria (Zr_xCe_1-xO_2-δ) ceramics. ES is a second order electromechanical response, i.e., strain, u, is proportional to M×E², where E is the applied electric field and M is the longitudinal electrostriction strain coefficient. Large ES in ceria has generally been associated with oxygen vacancies (V_o) which provide charge compensation for aliovalent dopants or for cerium reduction (Ce³⁺). ES induced by V_o is restricted to frequency <1 Hz and low saturation strain (|u_sat| <15 ppm). Doping CeO₂ with Zr results in a large ES strain coefficient, reaching |M|=10^-16 m²/V² for x=0.1, without formation of V_o. This effect persists to frequency ≥3 kHz with |u_sat| ≥220 ppm, making Zr_0.1Ce_0.9O₂ competitive with the best commercial electrostrictors (PMN-PT15), but with ~100 times lower dielectric permittivity and three-fold higher elastic modulus. |M| of Zr-doped ceria increases exponentially with Zr content for x=0-0.1, suggesting that the contribution of Zr-ions to electrostrictive strain is collective. However, such collective motion does not lead to a phase transition even at 123 K, implying that interaction between Zr-ions is neither sufficiently strong nor sufficiently long-range to produce freezing of the displacement, an effect that has been observed for perovskite relaxors. XAS data, DFT modelling and ab initio molecular dynamics (AIMD) calculations demonstrate that elastic dipoles formed by Zr-doping are dynamic. In the absence of an E-field, [ZrO₈]-local bonding units remain, on average, centered with respect to the second (cation) coordination shell. Due to bond anharmonicity, displacement of Zr by an E-field requires less energy than displacement of the host cations, resulting in a large dynamic elastic dipole. This polarizable elastic dipole gives rise to large electrostrictive strain and constitutes the first example of non-classical electrostrictors (NCES) relying solely on substitutional point defects.