Structural matching of ferroelectric domains and associated distortion in potassium titanyl phosphate crystals

The surface deformation and atomic-level distortions associated with crystal structural matching at ferroelectric inversion domain walls are investigated in periodically poled potassium titanyl phosphate (KTP) crystals. A deformation, of the order of 10 −8 m in scale and having the periodicity of the domains, is observed at the surfaces by optical interferometry. It is discussed in terms of the piezoelectric effect. The matching of the crystal structures at the domain walls is studied by combining the hard x-ray Fresnel phase-imaging technique with Bragg diffraction imaging methods ('Bragg–Fresnel imaging') and using synchrotron radiation. Quantitative analysis of the contrast of the Bragg–Fresnel images recorded as a function of the propagation distance is demonstrated to allow the determination of how the domains are matched at the atomic (unit cell) level, even though the spatial resolution of the images is on the scale of micrometres. The atom P(1) is determined as the linking atom for connecting the inversion domains across the wall in KTP crystals for domain walls forced by electric-field processing to be parallel to (100). In addition to this, it is shown that a shift of 1/2( a + b ) between atoms in the original and inverted structures is introduced as a result of the domain inversion operation.