An Operando Study of the Thermal Reduction of BaTiO3 Crystals: The Nature of the Insulator-Metal Transition of the Surface Layer

The insulator-to-metal transition upon the thermal reduction of perovskites is a well-known yet not completely understood phenomenon. By combining different surface-sensitive analysis techniques, we analyze the electronic transport properties, electronic structure, and chemical composition during the annealing and cooling of high-quality BaTiO3 single crystals under ultra-high-vacuum conditions. Our results reveal that dislocations in the surface layer of the crystal play a decisive role as they serve as easy reduction sites. In this way, conducting filaments evolve and allow for turning a macroscopic crystal into a state of metallic conductivity upon reduction, although only an extremely small amount of oxygen is released. After annealing at high temperatures, a valence change of the Ti ions in the surface layer occurs, which becomes pronounced upon the quenching of the crystal. This shows that the reduction-induced insulator-to-metal transition is a highly dynamic non-equilibrium process in which resegregation effects in the surface layer take place. Upon cooling to the ferroelectric phase, the metallicity can be preserved, creating a ferroelectric metal. Through a nanoscale analysis of the local conductivity and piezoelectricity, we submit that this phenomenon is not a bulk effect but originates from the simultaneous existence of dislocation-based metallic filaments and piezoelectrically active areas, which are spatially separated.

Automated summary

The insulator-to-metal transition during thermal reduction of perovskites was studied in high-quality BaTiO3 single crystals under ultra-high-vacuum conditions. Surface layer dislocations played a crucial role as easy reduction sites, allowing for the formation of conducting filaments and the transformation into a state of metallic conductivity with minimal oxygen release. The transition was found to be a highly dynamic, non-equilibrium process involving resegregation effects in the surface layer. The metallicity could be preserved upon cooling to the ferroelectric phase, creating a ferroelectric metal, which was attributed to the coexistence of dislocation-based metallic filaments and piezoelectrically active areas.