Incorporation of γ-butyrolactone (GBL) dramatically lowers the phase transition temperature of formamidinium-based metal halide perovskites

We demonstrate that the δ to α phase transition temperature of FA-based perovskites is reduced by ~50 °C through the incorporation of ~2 wt.% γ-butyrolactone (GBL) into the crystal lattice. The intercalation of GBL is found to expand the unit cell of the δ-phase, thus reducing the energy barrier for thermal conversion.