Ferroelastic domains and phase transitions in organic–inorganic hybrid perovskite CH3NH3PbBr3. Issue 9 (24th February 2021) - Bari, Maryam
Abstract : Polarized light microscopy reveals twin domains and symmetry of the phases in CH3 NH3 PbBr3 crystal; domain structure remains unresponsive to electric field but changes under external stress, confirming ferroelasticity while ruling out ferroelectricity. Abstract : The high performance of CH3 NH3 PbX3 (X = I, Br, and Cl) perovskites in the photovoltaics and optoelectronics has been suggested to be associated with phase symmetry and the possible existence of ferroelectricity. The accurate characterization of their crystal structures has been hampered due to the structural complexities, soft chemical bonding, and dense twinning in crystals. Here, we precisely determine the crystal symmetries and phase transitions via in situ observations of the MAPbBr3 crystals between 80 K and 500 K using polarized light microscopy and temperature-dependent measurements of dielectric properties. The phase transitions are found to occur at 237 K, 152 K and 148 K, from cubic (α) → tetragonal (β) → orthorhombic (β′) → orthorhombic (γ) phases, respectively. This new sequence of phases is different from the one previously published based on diffraction methods. Characteristic twin domains are discovered in the β, β′, and γ phases. Application of an external electric field does not change the domain structure, which rules out ferroelectricity. However, Joule heating can raise the temperature of the crystal by dozens of degrees, induce a phase transition and change irreversibly the domain structure and properties. The domain structure changes also under external stress, proving that the twins are ferroelastic domains. Our results suggest that elastic stress-mediated domain engineering may serve as a useful tool for tuning the microstructure by eliminating/modifying the domain walls, which may ultimately improve the stability and optoelectronic properties of halide perovskites.