Unconventional Properties of Nominally Charged Domain Walls in Uniaxial Ferroelectric Lead Germanate

Application of the scanning probe microscopy techniques, such as Piezoresponse Force Microscopy (PFM), opens a possibility to re-visit the ferroelectrics previously studied by the macroscopic electrical testing methods and establish a link between their local nanoscale characteristics and integral response. Here, we report the nanoscale PFM studies and phase field modeling of the static and dynamic behavior of the domain structure in the well-known ferroelectric material - lead germanate, Pb₅Ge₃O₁₁ (PGO). Several unusual phenomena were revealed: (1) domain formation during the paraelectric-to-ferroelectric phase transition, which exhibits an atypical cooling rate dependence; (2) unexpected electrically-induced formation of the oblate domains due to the preferential domain walls motion in the directions perpendicular to the polar axis, contrary to the typical domain growth behavior observed so far; (3) absence of the bound charges at the head-to-head (H-H) and tail-to-tail (T-T) domain walls, which typically exhibit a significant charge density in other ferroelectrics due to the polarization discontinuity. This strikingly different behavior is rationalized by the phase field modeling of the dynamics of uncharged H-H and T-T domain walls. Our results provide a new insight into the emergent physics of the ferroelectric domain boundaries, revealing unusual properties not exhibited by conventional Ising-type walls.