Fig. 4: Stacking orders in SnSe.

a Simulated intensity of {110} and {200} diffraction spots for different stacking orders. b Plot of normalized intensity ratio (details in Methods), which changes as the stacking order varies in our simulated lattice model. The plot is annotated with experimental measurements from (c) with two representative experimental diffraction patterns of AC-dominate (FE) ___domain (green star) and AB-dominate (AFE) ___domain (orange star) in the insets. c Normalized intensity ratio map from region contains both FE and AFE domains, forming periodic FE-AFE ___domain walls. (Scale bar: 500 nm) The averaged diffraction patterns from FE rich (green arrow) and AFE rich (orange arrow) domains are shown in the insets of (b) with the quantitative stacking information labeled as star shapes. The intensity variance in the AFE rich domains is from the slightly lattice tilt (Fig. S26). d Atomic resolution ADF-STEM image of one FE-AFE ___domain wall. (Scale bar: 2 nm) Insets: Zoomed-in ADF-STEM images overlayed with simulations showing the AFE “dumbbell” lattices and FE square lattices. (Scale bar: 0.5 nm) e DFT-calculated of the ferroelastic and ferroelectric potential energy surface with atomic structures superimposed at local minima and unstrained maxima. f Relaxed atomistic structure of FE-AFE twin boundary in bilayer SnSe. The twin boundary is indicated by a dashed line. Red and blue arrows indicate the electric polarization direction in the x–y plane for top and bottom layer, respectively.