Noncentrosymmetric structural transitions in ultrashort ferroelectric AGa O3/A′Ga O3 superlattices

The effect of octahedral tilting on the acentric structural transitions in AGaO3/A′GaO3 [001], [110], and [111] superlattices (A,A′=La,Pr,Nd) is studied using density functional theory. We find the displacive transitions are driven by two octahedral rotations modes (a-a-c0 and a0a0c+ tilting) with amplitudes that depend on the A and A′ chemistry and cation ordering directions. We find the ground-state structures of the [001] and [111] ordered superlattices are polar. The coupling of octahedral tilting modes through a hybrid improper ferroelectric mechanism induces the polar displacements and produces the macroscopic electric polarizations. Our layer decomposed polarization analysis explains the magnitude of the polarization values along the ordering directions.