Fig. 4: Large polarizations are created at the interface, leading to the α-to-β phase transition accompanied by a giant elastic expansion of the Se lattice.

a STEM image of the α-β interface, in which the polarizations and spacing of the (001) Se-lattice of α-Ag₂Se are labeled. The scale bar is 1 nm. b STEM image of the interface, in which the structural units are outlined, showing that the α-to-β phase transition is a low-symmetry to high-symmetry and topologically polar to non-polar transition. The scale bar is 1 nm. c, d Strain maps of the interface showing the strain along the (011) plane and the (001) plane, respectively. The scale bar is 1 nm. e Given the [100] α-Ag₂Se transforming into the [111] β-Ag₂Se, a strain of ~5.7% will be generated along the [001] direction (referring to α-Ag₂Se).