Phase Diagram and Dielectric Properties of MA_1–xFA_xPbI₃

With the help of 10 different compositions (x) between 0.0 and 0.4 for MA1–xFAxPbI3 and about 20 temperature points between 15 and 310 K for each composition, we establish a rich temperature–composition phase diagram in terms of structural variations determined by powder and single-crystal X-ray diffraction. Four crystallographic phases are found to exist for this solid solution series, namely, cubic (Pm-3m), tetragonal (I4/mcm), orthorhombic (Pnma), and large-cell cubic (Im-3). Variable-temperature dielectric measurement reveals qualitative changes in dielectric properties of these materials driven by the observed structural phase transitions; it establishes that with an increasing FA content the high-temperature-dominant Curie-like behavior in dielectric constant of MAPbI3 gets suppressed and an unusually prominent glassy behavior evolves in direct correlation with the structural evolution. This reveals a strong structure–property correlation existing in these solid solutions, qualitatively modifying properties of MAPbI3 with FA substitution.