Obvious Phase Transition Status Induced by He +-Ions Implantation in KTN Crystal

We report on the formation of a critical ferroelectric state induced by the He+ ion implantation in potassium tantalate niobate crystal. An obvious phase change has been observed in the ion irradiated region, which is mostly related to the stable polarized nanometric regions formed during the ion implantation process. Under the irradiation of 2 MeV He+ ions, two distinguishable layers corresponding to different energy transfer modes (elastic nuclear collision and inelastic electronic collision, respectively) between the incident He + ions and the intrinsic lattices have been formed beneath the irradiated surface. Lattice dynamics before and after the ion implantation process are investigated by a confocal μ-Raman system. And the variations of typical Raman-active vibrational modes demonstrate the presence of lattice distortion in the irradiated region. X-ray diffraction experiments further suggest the uniform lattice elongation in this region. Piezo-response force characteristic measurements reveal the existence of stable polarized nanometric regions with more intense polarization and verify that the crystal with such a phase status possesses extraordinary microscopic disorders, which is different from the traditional ferroelectric or paraelectric phase. The ion implantation method provides a new approach to form a temperature-stable critical ferroelectric state in relaxor ferroelectric materials. Also, analyses of the modification on the lattice dynamics of the irradiated region can help us build a clear awareness of the physical essence of this critical state and the relaxor ferroelectricity.