Tracking Structural Phase Transitions in Lead‐Halide Perovskites by Means of Thermal Expansion

Abstract The extraordinary properties of lead‐halide perovskite materials have spurred intense research, as they have a realistic perspective to play an important role in future photovoltaic devices. It is known that these materials undergo a number of structural phase transitions as a function of temperature that markedly alter their optical and electronic properties. The precise phase transition temperature and exact crystal structure in each phase, however, are controversially discussed in the literature. The linear thermal expansion of single crystals of APbX 3 (A = methylammonium (MA), formamidinium (FA); X = I, Br) below room temperature is measured using a high‐resolution capacitive dilatometer to determine the phase transition temperatures. For δ‐FAPbI 3 , two wide regions of negative thermal expansion below 173 and 54 K, and a cascade of sharp transitions for FAPbBr 3 that have not previously been reported are uncovered. Their respective crystal phases are identified via powder X‐ray diffraction. Moreover, it is demonstrated that transport under steady‐state illumination is considerably altered at the structural phase transition in the MA compounds. The results provide advanced insights into the evolution of the crystal structure with decreasing temperature that are essential to interpret the growing interest in investigating the electronic, optical, and photonic properties of lead‐halide perovskite materials.