We report first principles studies of the lattice instabilities of as yet unsynthesized perovskites based on alloys in the (Th,Pb,Ba,Bi,Y)ScO3system. These systems are characterized by strong structural instabilies of the cubic perovskite lattice and A‐site driven ferroelectricity. Tetragonal ferroelectric structures in these systems are found to show rather large values of thec/aratio, which when present near a morphotropic phase boundary are thought to be associated with large piezoelectric actuation. The relationship between octahedral rotational and ferroelectric instabilities is discussed as are the conditions for obtaining a morphotropic phase boundary. © 2003 American Institute of Physics