Spontaneous polarization (Ps) in ferroelectrics has provided the impetus to develop piezoelectric devices such as sensors, actuators and diagnostic imaging transducers. Widely used lead-based perovskites exhibit a composition-driven phase diagram involving a transition region, known as a morphotropic phase boundary, where the ferroelectric structure changes dramatically and the piezoelectric activity is maximal. In some perovskites, ferroic polarization coexists with nonpolar rotations of octahedra, suggesting an unprecedented phase diagram. Here, we show morphotropic phase boundaries, where 'ferrielectric' appears as a bridging phase between ferroelectrics with rhombohedral and tetragonal symmetries in Bi1/2Na1/2TiO3-based perovskites. Neutron diffraction analysis demonstrates that the intermediate ferrielectric displays a small Ps resulting from up and down polarizations coupled with an in-phase TiO6 rotation. Our ab initio calculations indicate that a staggered Bi-O conformation at an appropriate chemical pressure delivers the ferrielectric-mediated phase boundaries, which provides a promising platform for (multi)ferroic materials with enhanced physical properties.