Chemical structure of grain-boundary layer in SrTiO₃ and its segregation-induced transition: A continuum interface approach

Grain-boundary (GB) structures are commonly imaged as discrete atomic columns, yet the chemical modifications are gradual and extend into the adjacent lattices, notably the space charge, hence the two-dimensional defects may also be treated as continuum changes to extended interfacial structure. This review presents a spatially-resolved analysis by electron energy-loss spectroscopy of the GB chemical structures in a series of SrTiO₃ bicrystals and a ceramic, using analytical electron microscopy of the pre-C_s-correction era. It has identified and separated a transient layer at the model Σ5 grain-boundaries (GBs) with characteristic chemical bonding, extending the continuum interfacial approach to redefine the GB chemical structure. This GB layer has evolved under segregation of iron dopant, starting from subtle changes in local bonds until a clear transition into a distinctive GB chemistry with substantially increased titanium concentration confined within the GB layer in 3-unit cells, heavily strained, and with less strontium. Similar segregated GB layer turns into a titania-based amorphous film in SrTiO₃ ceramic, hence reaching a more stable chemical structure in equilibrium with the intergranular Ti₂O₃ glass also. Space charge was not found by acceptor doping in both the strained Σ5 and amorphous GBs in SrTiO₃ owing to the native transient nature of the GB layer that facilitates the transitions induced by Fe segregation into novel chemical structures subject to local and global equilibria. These GB transitions may add a new dimension into the structure-property relationship of the electronic materials.