Ferroelastic Domains and Effects of Spontaneous Strain in Lead Halide Perovskite CsPbBr₃

Halide perovskites have shown great promise for applications in solar cells and other optoelectronic devices. It was recently realized that lattice strain can be considered an effective parameter for the performance of these materials due to their mechanical softness. Various strain-engineering strategies have been considered involving external and internal strains; the latter arises from size mismatch between the cations and halide cage and from crystal defects. Here, we demonstrate the importance of another type of internal strain, namely, a temperature-dependent spontaneous strain originating from ferroelastic phase transitions often observed in halide perovskites. We investigate CsPbBr3 single crystals in the temperature range between 80 and 500 K using polarized light microscopy, X-ray diffraction, and measurements of dielectric properties. This material has excellent optoelectronic performance, but several controversial results related to its crystal structure, phase transitions, and ferroelectricity have been reported. Studies of twin domain structures, birefringence, and permittivity allow us to accurately deduce the phase symmetries and confirm the existence of two phase transitions: between orthorhombic and tetragonal phases at 361 K and between tetragonal and cubic phases at 403 K. The domain response under an external electric field or stress reveals that this crystal is not ferroelectric or antiferroelectric but ferroelastic. We show that the anomalies of some properties previously related to additional phase transitions can be interpreted as artifacts caused by changes in the ferroelastic domain structure and the related inhomogeneous internal strain. This work provides a better understanding of the structure-property relations in ferroelastic halide perovskites.