STEM HAADF imaging was carried out on JEOL Grand ARM300CF equipped with a cold field-emission gun and double spherical aberration correctors with a spatial resolution of ~0.7 Å operating at 300 keV in Irvine Materials Research Institute at the University of California, Irvine. STEM images were taken with the convergence angle of the incident electrons at 32 mrad and the collection angle at 90 to 165 mrad.
The high-resolution HAADF STEM imaging provides spatial resolution adequate to measure the atomic positions of the A and B site cations of CGB. The high-frequency noise was removed by applying an annular mask in frequency space, and then the initial peak positions were determined by identifying local maxima and refined by fitting Gaussian curves to obtain the atom center positions. Displacements were calculated as the difference between the center of each cation and the center of mass of its adjacent neighbors.
Cs and Ge atomic positions were measured by using 2D Gaussian fitting on their atomic columns to locate centers and record offsets. The polarization vectors on Ge are defined as the displacement of Ge atom from the center of unit cell formed by the four Cs and four Br atoms. The polarization vectors on Cs are defined as the opposite displacement of Cs atom from the center of the unit cell formed by the four Ge and four Br atoms.
The high-resolution HAADF STEM imaging provides spatial resolution adequate to measure the atomic positions of the A and B site cations of CGB. The high-frequency noise was removed by applying an annular mask in frequency space, and then the initial peak positions were determined by identifying local maxima and refined by fitting Gaussian curves to obtain the atom center positions. Displacements were calculated as the difference between the center of each cation and the center of mass of its adjacent neighbors.
Cs and Ge atomic positions were measured by using 2D Gaussian fitting on their atomic columns to locate centers and record offsets. The polarization vectors on Ge are defined as the displacement of Ge atom from the center of unit cell formed by the four Cs and four Br atoms. The polarization vectors on Cs are defined as the opposite displacement of Cs atom from the center of the unit cell formed by the four Ge and four Br atoms.