Extended Data Fig. 9: Piezoresponse force microscopy results and schematics.
From: Electron ptychography reveals a ferroelectricity dominated by anion displacements
Piezoresponse force microscopy (PFM) images showing the amplitude (a, b) and phase (c, d) in the vertical and lateral directions, respectively. The lateral direction displays a much clearer domain pattern with stronger PFM signal when compared to the vertical direction, which matches the ptychography result with polarization predominantly oriented along the [110]pc direction. We can also extract an average domain periodicity of 44 nm along [100]pc and 52 nm along [010]pc, which are in rough agreement with the results from X-ray diffraction in Extended Data Fig. 5. Schematics of the domain configurations for the (e) lateral and (f) vertical measurements are also provided. For these measurements, the sample is rotated by 45° with respect to the tip, as polarizations orthogonal to the tip provide the strongest lateral signal and the polarization is known to be mostly along [110]pc. There are four superdomain variants described in these schematics which have been labelled in (a-d). For lateral PFM measurement, the polarization of individual domains will oscillate within a superdomain between pointing along the tip (providing lower amplitude in b and shown in pastel colors in e) and orthogonal to the tip (providing higher amplitude in b and shown in darker colors in e). Superdomains 1 and 3 show the same lateral phase because the PFM tip is torqued in the same direction by the orthogonal domains for these regions (the same is also true for superdomains 2 and 4). For the vertical PFM images, we can see that two superdomain variants (3 and 4) have roughly no net polarization since the polarization of neighboring domains will cancel, and thus the vertical PFM amplitude is low in a. We note that even in these low amplitude regions, oscillating phases can be observed in c as expected from f. However, in regions 1 and 2 the vertical polarizations of neighboring domains are aligned, and so the vertical PFM amplitude is higher in a, and those regions have a single phase in c.
