Insights into Twinning Formation in Cubic and Tetragonal Multi-cation Mixed-Halide Perovskite

We use a combination of low-dose electron microscopy studies and atomic structure simulations to identify a common twinning structure in both cubic and tetragonal phases of the multi-cation mixed-halide perovskite. For the first time, we present clear evidence of {111} twins in the cubic phase and the equivalent {011} twins in the tetragonal phase using selected area electron diffraction (SAED) patterns. We develop a unique way of differentiating between the tetragonal and cubic forms of these twins. Our systematic electron diffraction analyses and simulations demonstrate unequivocally that the same twinning structure is present in both phases, notably with a perfectly coherent {111} twin boundary in the cubic phase and a semi-coherent {011} twin boundary in the tetragonal phase. The atomic configuration of the twin boundary is discussed with possibilities of being AX3 centered (with A = Cs, FA, MA and X = I, Br) or B-site centered (Pb) for both crystal structures. We propose that the twin boundary is a key nucleation region for phase segregation, which acts as a potential well or barrier to electrons and holes depending on its composition and hence bandgap.