Electrostriction: it is just a phase

The discovery of "giant" (or non-classical) electrostrictors has reignited interest in electrostriction, a second-order electromechanical coupling phenomenon. While the relationship between the generated strain and the square of the electric field may seem straightforward, determining the sign of the electrostrictive coefficients proves challenging, as evidenced by the opposite signs reported by various research groups. We show that electrostrictive coefficients must be treated as complex values with their sign dictated by their phases rather than by the overall shape of the strain versus electric field curve. Moreover, our analysis of the electrostrictive properties of La₂Mo₂O₉ ceramics reveals that both the electrostrictive coefficients and the induced strains may undergo sign changes when the frequency varies, with each sign change occurring at its own critical frequency. Finally, we introduce a model to explain the differing frequency response of the electrostrictive coefficients.