Grain Boundary Chemistry and Space-Charge Potentials in Co-doped BaTiO3

 This research investigates the electrochemical properties of barium titanate (BaTiO₃), focusing on grain boundaries in manganese (Mn) and yttrium (Y) doped BaTiO₃. Impedance spectroscopy revealed that doping BaTiO₃ with Mn and Y decreased electrical conductivity by reducing the hole concentration. The defect chemistry and electrical properties of BaTiO₃ were significantly influenced not only by the presence of dopants but also by the A/B cation ratio.

The brick layer model (BLM) was employed to evaluate the specific grain boundary con- ductivity of BaTiO₃ samples. Interestingly, the calculated specific grain boundary conductiv- ities showed minimal variation compared to bulk conductivities, a phenomenon also observed in Mg-doped BaTiO₃. To gain deeper insights into grain boundary characteristics, particularly regarding space-charge potentials, both Mott-Schottky and Gouy-Chapman models were utilized. These models enabled predictions of space-charge potential barrier heights and widths. Analysis revealed that Mn and Y doping in BaTiO₃ led to decreased space-charge potential barrier heights but increased space-charge widths.

 Differential phase contrast scanning transmission electron microscopy (DPC STEM) was applied to Mn+Y co-doped BaTiO₃ to visualize electric field and potential profiles within space-charge layers. This technique determined a grain boundary space-charge width of approximately 5 nm with a 2V potential barrier. Notably, these values differed significantly from predictions made by Mott-Schottky and Gouy-Chapman models using impedance data. This discrepancy can be attributed to two factors: impedance measurements represent an average of numerous grain boundaries, and the two methods were conducted at different tem- peratures. Moreover, it’s important to note that not all grain boundaries exhibit identical space-charge potential profiles. DPC STEM measurements must also account for additional factors contributing to electron beam phase shifts. Further DPC STEM measurements con- ducted at various sample thicknesses revealed nonlinear maximum beam deflection within the space-charge layer. This nonlinearity is likely due to dynamical scattering effects and differences in mean inner potential (MIP) at the grain boundary core. 

Multislice simulations of BaTiO₃ grain boundaries without potential barriers showed electron beam deflection away from the core due to lower atomic density and electrostatic potential. Introducing a macroscopic potential barrier at the grain boundary to simulate excess positive charges resulted in complex electric field profiles, with overlapping fields from the macroscopic potential barrier and MIP difference. These findings align with previous studies on the grain boundaries of yttria-stabilized zirconia (YSZ). The observed electric field profiles will vary based on grain boundary type and defect segregation, highlighting the need for further research on different grain boundary structures. 

This comprehensive study enhances our understanding of BaTiO₃’s electrochemical prop- erties, particularly at grain boundaries, contributing to ongoing miniaturization trend in ceramic capacitor technology.