Unveiling the impact of stress: a dynamic twist on properties in halide-perovskites

Halide perovskites are a promising candidate for next generation energy-harvesting technologies owing to their excellent optoelectronic properties and low-cost solution processability. A striking difference between halide perovskites and conventional semiconductors (e.g., silicon) is the dual ionic-covalent bond nature within the anionic inorganic framework. This bond nature results in a mechanically soft and dynamically disordered lattice whose alteration affects the optoelectronic properties and the stability of these solids. Thus, metal-halide perovskites are particularly sensitive to variations in composition, fabrication and external stimuli that can induce strain in the material. The high magnitude of strain in halide perovskites is remarkable as they are one of the most fragile semiconductors, yet their resilience to adapt to stress is their most fascinating property. Understanding their crucial elastic properties for synthesis and device operation remains limited. We performed temperature- and pressure-dependent synchrotron-based powder X-ray diffraction of several lead-halide (3D) and double perovskites at low pressures (ambient to 0.06 GPa, similar to those experienced during manufacturing) to investigate their elastic properties in their ambient-pressure crystal structure. In this talk, I will show that we found common trends in bulk modulus and thermal expansivity, with an increased halide ionic radius (Cl to Br to I) resulting in greater softness, higher compressibility and thermal expansivity in both class of materials. For non-cubic systems, in which the elastic properties are anisotropic, we obtained axis-dependent compressibility. The A cation has a minor effect, and mixed-halide compositions show intermediate properties. Notably, thermal phase transitions in MAPbI₃ and CsPbCl₃ induced lattice softening and negative expansivity for specific crystal axes, even at temperatures far from the transition point. These results emphasize the significance of considering temperature-dependent elastic properties, which can significantly impact device stability and performance during manufacturing or temperature sweeps.