Metastable multiferroic perovskites of the BiFe1-xScxO3 system, stabilised by high-pressure and high-temperature synthesis, demonstrate irreversible phase transformations upon heating/cooling thermal cycling. This allows the compositions in the range of 0.3x1 to be obtained in different polar/antipolar polymorphs providing a unique opportunity to explore a structure-properties relationship. The proposal aims to study the crystal and magnetic structures of two different polymorphs of the compositions with x=0.3 and 0.7. The as-prepared samples crystalize into orthorhombic and monoclinic modifications, respectively, which irreversibly transform to a novel perovskite phases above 500K.