the paraelectric to AFE phase transition, cell quadrupling in PbZrO 3 results in the appearance of TBs. These boundaries are characterized by translation vectors R = [ a / n 1 , b / n 2 , c / n 3 ], 1/ n i ( i = 1, 2, 3), which are fractions of the unit-cell translation vectors. In this work, we report on the relation between formation of polar TBs and the interfacial strain in the system of PbZrO 3 thin fi lms grown on SrTiO 3 substrates. In combination with geometric phase analysis (GPA), [ 28 ] high-angle annulardark-fi eld (HAADF) imaging and Stripe scanning transmission electron microscopy (STEM) techniques based on aberrationcorrected STEM allowed us to determine the structural arrangement and strain distribution near the interfaces.Epitaxial PbZrO 3 thin fi lms were grown on (100) SrTiO 3 substrate and on BaZrO 3 -buffered (100) SrTiO 3 substrate (see the Experimental Section for details). As the lattice parameters of PbZrO 3 are larger than that of SrTiO 3 , the BaZrO 3 buffer layer is used to modulate the misfi t between the PbZrO 3 fi lm and the SrTiO 3 substrate. Figure 1 a shows a plan-view bright-fi eld TEM image of a PbZrO 3 fi lm on BaZrO 3 buffered (100) SrTiO 3 substrate. The dark dot contrast in the image originates from threading dislocations in the fi lm arising from the threading segments associated with the misfi t dislocations due to the lattice mismatch. The HAADF-STEM image of the cross-sectional sample (Figure 1 b) shows morphology of the fi lm system with ≈35 nm thick PbZrO 3 fi lm. In the medium-magnifi ed HAADF image, Figure 1 c, one can clearly see that the PbZrO 3 fi lm is epitaxially grown on the BaZrO 3 buffer layer. Due to antiparallel displacements of Pb atoms and antiphase rotation of the octahedra, [ 29,30 ] the AFE structure characterized by the (010) and (021) refl ections, which are the superstructure refl ections of q Σ = 1/4(110) p and q R = 1/2(111) p with respect to the pseudocubic structure, [ 31 ] can clearly be identifi ed from the fast Fourier transformation (FFT) image illustrated in Figure 1 d. Considering that the image intensities are atomic-number dependent and roughly proportional to Z 2 , [ 32 ] the TBs residing inside the AFE domain areas can be directly observed based on the antiparallel displacements of the heavy Pb atoms along the orthorhombic [100] direction. A typical APB on the atomic scale, type R III-1 = 1/4 [02 n ] ( n = 0 or 2) is shown in Figure 1 e.Based on atomic scale investigation and analysis of GPA on the HAADF images, all types of TBs in the AFE PbZrO 3 thin fi lms were studied and the results are given in Table 1 . Figure 2 a shows the morphology of two types of TBs, the R III-1 and R I-1 = 1/4 [21 n ] type boundaries, in the 35 nm thick fi lm. The characteristic displacements of the Pb atoms are displayed as a function of the distance away from the boundary centers in Figure 2 b. Based on the average displacements (33.1 pm) of the Pb atoms away from their ideal positions, the spontaneous polarization is estimated as P S = 16 and 17 µC cm ...