Fig. 1: Schematics of the dynamics of the anharmonic oscillator pairs under electric field.

Anharmonic oscillator pair of ionic crystals illustrates the electrostrictive effect with a > 0. The dashed box represents the primitive cell of the one-dimensional lattice. δr(t) and δl(t) are the deformations of the right and left dipole moments solved by Eq. (1) with a plus and minus sign, respectively. And δ(t) = δr(t) + δl(t) is the deformation of a unit cell.