into applications ranging from ultrasound imaging technology to precision actuation in micromotors. [ 6 ] The recent discovery of a class of hybrid improper ferroelectrics (HIF), [ 7,8 ] where an electric polarization is driven by two coupled octahedral-rotation lattice modes, offer opportunities for design of ferroic materials with diverse functionalities. Typically, in A -site ordered ultra-short period 1/1 AB O 3 / A ' B O 3 perovskite superlattices exhibiting HIF, the electric dipoles of each cation sublattices are restricted by the symmetry of the orthorhombic a − a − c + B O 6 rotation patterns [ 9 ] in Glazer notation. [ 10 ] Such a rotocrystalline anisotropy stabilizes a ferrielectric (FiE) state, [11][12][13][14] where two of the three cation-sublattice dipoles are directed along the pseudo-cubic [110], while the third is anti-parallel to the others [110], yielding a net non-zero electric polarization. [ 15 ] In such a layered structure, the electric polarizations of each cation-sublattice can be tuned independently-the A and B cation polarizations directly rely on the octahedral connectivity of the system. [ 16,17 ] Is there a suitable way to tune the oxygen lattice topology and the polarization direction of each cation-sublattice? If answered in the affi rmative, then the FiE state can be directly converted into a FE state where all cation-sublattice dipoles are in parallel alignment. Consequently, a new FiE-to-FE ferroic transition would achieve a signifi cant polarization enhancement and a sizable piezoelectric response in cation ordered oxides.In this paper, we use fi rst-principles density functional calculations to demonstrate that the polarization switching between a FiE and FE phases in HIF AB O 3 / A ' B O 3 perovskite superlattices is accessible with substrate-induced epitaxial strain. Specifi cally, we uncover a fi rst-order isosymmetric phase transition and evaluate the feasibility of the FiE-to-FE transition through a minimum energy path (MEP) trajectory. Compared to perovskite oxide solid-solutions, where an enhanced piezoelectric response occurs near the MPB driven by rotation of polarization along an easy rotation path between the rhombohedral and tetragonal FE phases, [ 18,19 ] we predict a piezoelectric response-up to 102 pC/N around the FiE-FE phase boundary-as a result of the easy polarization switching between FiE and FE phases with the same symmetry. As A -site ordered superlattices or heterostructures, such as PbTiO 3 / SrTiO 3 , [ 7,[20][21][22] PbTiO 3 /CaTiO 3 , [ 23 ] and CaTiO 3 /BaTiO 3 , [ 24 ] have been successfully synthesized and well characterized in experiment, our predicted FiE to FE transition in synthetically We identify a fi rst-order, isosymmetric transition between a ferrielectric (FiE) and ferroelectric (FE) state in A -site ordered LaScO 3 /BiScO 3 and LaInO 3 / BiInO 3 superlattices. Such a previously unreported ferroic transition is driven by the easy switching of cation displacements without changing the overall polarization direction or crystallographic sym...