Ferroelectric switching in GeTe through rotation of lone-pair electrons by Electric field-driven phase transition

Germanium telluride (GeTe), which has long been used in phase-change devices, has very unique structural characteristics. While atomic movement and related bond-order switching have been believed to determine the ferroelectric characteristics of this compound, structural changes accompanying ferroelectric switching have not been examined. In this study, using transmission electron microscopy and Raman spectroscopy, we directly observed the atomic movement and the related change in the local structure of GeTe during ferroelectric switching under an applied E-field. The crystal symmetry of GeTe could be alternated between two phases (R3m (rhombohedral) and Cm (monoclinic)) by changing the compound's polarization direction, indicating that the ferroelectric switching behavior of GeTe can be controlled. This observation suggests that ferroelectric switching of GeTe may originate from the phase transition between R3m and Cm through the formation of a morphotropic phase boundary (MPB). Since these two phases can generate MPBs with similar energies, the rotation of lone-pair direction during the phase transition can effectively change the polarization direction. The exotic phenomenon of ferroelectric switching, which involves changes in the chemical bonds, can be predicted on the basis of the role of several dopants through DFT calculations.