Noncentrosymmetric structural transitions in ultrashort ferroelectric AGaO3/A′GaO3 superlattices

The effect of octahedral tilting on the acentric structural transitions in AGaO(3)/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(-)c(0) and a(0)a(0)c(+) 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.