Nanodomain Patterns in Ultra-Tetragonal Lead Titanate (PbTiO3)

Very recently, the discovery of ultra-tetragonal PbTiO3 thin films was reported [L. Zhang et al., Science, 361, 494 (2018)], in which the switchable out-of-plane polarization was seen to be almost twice that of any previously known ferroelectric. To understand more about this system and to explore features that might contribute to these remarkable functional properties, we have mapped the polarisation microstructure on the nanoscale using piezoresponse force microscopy (PFM) (scan sizes less than 700 nm). Our study reveals an extremely fine-scale pattern, with individual domains typically 20-50nm in size, all of which exhibit both out-of-plane and in-plane polarisation components. In-plane vector PFM polarisation maps show strongly charged quadrant-lattice domain arrays with four-fold domain wall junctions (vertices). The existence of four-fold vertices in ferroelectrics is rare but can be explained via the ‘Ashkin-Teller’ model, in which very large interfacial lattice mismatch strains and local electric fields play a role. This ultra-tetragonal ferroelectric system was expected to contain straightforward c-axis out-of-plane 180o domains, but instead shows an extremely rich nanoscale domain tiling pattern; these unusual nanodomains may be important in facilitating the extremely high switchable polarisation values previously reported.