Fig. 3: QPlus non-contact (nc) AFM imaging of surface symmetry in ferroelectric and ferroelastic RPP crystals.

a ncAFM image of ferroelastic n = 3 I-RPP of large area I-dimer A′ and I-dimer B′ configurations occasionally separated by a twin-domain boundary indicated by a blue line. b ncAFM image of ferroelectric n = 3 Br-RPP of regular alternating arrangement of 2-unit cell width of twin-domain orientations. c Periodic A-B-A-B double-row arrangement in ferroelectric n = 3 Br-RPP surface. A and B double-row of Br-dimer orientations are indicated by blue and red, separately. d ncAFM image of ferroelectric n = 2 Br-RPP. e Less regular twin-domain arrangement with 2 to 3-unit cell width with local mirror symmetry breaking is also presented in ferroelectric n = 3 Br-RPP. f Schematic of the double-row Br-dimer twin structure alignment of inner MA⁺ dipoles. Each blue or red dot represents a Br atom, while each arrow indicates the orientation of a dipole from the MA⁺. g Vector addition of MA⁺ dipoles resulting in a net polarization direction along the b and c-axes, indicated by the purple arrows in ferroelectric n > 1 Br-RPPs. This contrasts with the behavior observed in non-ferroelectric n = 1 Br-RPP. h Atomic image of non-ferroelectric n = 1 Br-RPP shows a nearly square structure. i A comparison of the atomic arrangement between ferroelectric n = 2 Br-RPP and non-ferroelectric n = 1 Br-RPP based on the lattice constants of each respective unit cell.