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 formamidinium-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, reducing the energy barrier for thermal conversion.