Two-Dimensional Room-Temperature Giant Antiferrodistortive ${\mathrm{SrTiO}}_3$ at a Grain Boundary

The broken symmetry at structural defects such as grain boundaries (GBs) discontinues chemical bonds, leading to the emergence of new properties that are absent in the bulk owing to the couplings between the lattice and other parameters. Here, we create a two-dimensional antiferrodistortive (AFD) strontium titanate (${\mathrm{SrTiO}}_3$) phase at a $\mathrm{\Sigma }13(510)/[001]$ ${\mathrm{SrTiO}}_3$ tilt GB at room temperature. We find that such an anomalous room-temperature AFD phase with the thickness of approximate six unit cells is stabilized by the charge doping from oxygen vacancies. The localized AFD originated from the strong lattice-charge couplings at a ${\mathrm{SrTiO}}_3$ GB is expected to play important roles in the electrical and optical activity of GBs and can explain past experiments such as the transport properties of electroceramic ${\mathrm{SrTiO}}_3$. Our study also provides new strategies to create low-dimensional anomalous elements for future nanoelectronics via grain boundary engineering.

Figure 1

Atomic structure of a $\mathrm{\Sigma }13(510)/[001]{\mathrm{SrTiO}}_3$ GB. (a) A HAADF image of the ${\mathrm{SrTiO}}_3$ $\mathrm{\Sigma }13(510)/[001]$ tilt GB. The polygon highlights the structure unit of GB. (b) An EDS map of Sr (red) and Ti (green) showing cationic distribution of the GB. (c) An IDPC image of GB overlapped with an atomistic structure model. (d) An atomistic schematic of the GB. Edge sharing ${\mathrm{TiO}}_6$ octahedrons with rocksaltlike TiO structure is observed in the GB core.

Figure 2

The room-temperature AFD phase in the vicinity of the ${\mathrm{SrTiO}}_3\mathrm{GB}$. (a) Reconstructed atom positions of Ti and O atoms on the basis of the IDPC image. The Sr atoms and core structure are hidden for clarity. The white lines highlight the O-Ti-O atomic chains. The curved arrows highlight the ${\mathrm{TiO}}_6$ rotation direction. (b) The schematic diagrams of AFD phase in the vicinity of the GB. The adjacent Ti-O octahedrons tilt to opposite directions, viewing along the [001] zone axis. Size difference between SrO squares ($L$) and TiO squares ($L^{\prime }$) lead to the ${\mathrm{TiO}}_6$ octahedron rotation. The rotation angles of ${\mathrm{TiO}}_6$ octahedron are illustrated as “vertical” component ${\theta }_v$ (along the [010]) and “horizontal” component ${\theta }_h$ (along the [100]). (c) The ${\theta }_v$ map of ${\mathrm{TiO}}_6$ octahedrons in the vicinity of $\mathrm{\Sigma }13(510)/[001]$ GB.

Figure 3

Electronic structures of the ${\mathrm{SrTiO}}_3$ near the GB. (a) EELS of $\mathrm{Ti-}L$ edge within a range of 8 nm across a GB. (b) A comparison in EELS between bulk matrix and GB indicated by blue and orange doted lines, respectively. (c) The energy-loss values of peaks in $\mathrm{Ti-}L$ edge across the GB. (d) The corresponding O-K edge across the GB. (e) The comparison between bulk matrix and GB. Green and purple shades label the windows used to calculate (f) the $\alpha /\beta$ intensity ratio. Reseda green and indigo shades indicate the GB core and AFD phase regions, respectively. (g) The fitting coefficient of ${\mathrm{Ti}}^{4+}$ across the GB. (h) The intensity of $\mathrm{Ti-}L$ and $\mathrm{O-}K$ and (i) their relative ratios after background subtracted. The error bars present the standard deviations among ten groups of line-scan measurements from a single EELS spectrum image containing ten rows datasets across the GB ($10\times 50\mathrm{pixels}$ mapping).