shear deformation within the inorganic layer during the phase transition, as illustrated in Figs. and, where the shear displacements are visualized on the Pb-I octahedra. Such shear deformation was previously hypothesized to explain the widening of line widths in powder diffractograms in experiments. Our findings not only support this hypothesis but also offer a detailed quantitative analysis of the deformation. We found that this shear deformation aligns with the shear of BA cations but with a reduced magnitude, and is facilitated by significant distortion of the terminal Pb-I bonds. Meanwhile, the shear is accompanied by a slight expansion of Pb-I skeleton in the Z direction, as shown in Fig. .