Direct Observation of Sr Vacancies inSrTiO3by Quantitative Scanning Transmission Electron Microscopy

Abstract

Unveiling the identity, spatial configuration, and microscopic structure of point defects is one of the key challenges in materials science. Here, we demonstrate that quantitative scanning transmission electron microscopy (STEM) can be used to directly observe Sr vacancies in SrTiO 3 and to determine the atom column relaxations around them. By combining recent advances in quantitative STEM, including variableangle, high-angle annular dark-field imaging and rigid registration methods, with frozen phonon multislice image simulations, we identify which Sr columns contain vacancies and quantify the number of vacancies in them. Picometer precision measurements of the surrounding atom column positions show that the nearest-neighbor Ti atoms are displaced away from the Sr vacancies. The results open up a new methodology for studying the microscopic mechanisms by which point defects control materials properties.

Response of the Lattice across the Filling-Controlled Mott Metal-Insulator Transition of a Rare Earth Titanate

“…The method used in the present study is capable of observing them, see Refs. [27,28]. Neither type of structural nonuniformity are observed here, as will be discussed next.…”

“…To enhance the signal-to-noise ratio and reduce the effect of scan distortions, 20 images (1024 × 1024 pixels, 1 s frame time) were sequentially recorded, rigidly aligned, and then averaged using a crosscorrelation method, from which the position of atomic columns are obtained by a two-dimensional (2D) Gaussian peak fitting. This approach allows for measurements of displacements with picometer precision, as described in detail elsewhere [28].…”

“…Another source of I Sm;Sr variations is the Sr doping. The Sr dopants have a lower Z than the host (Sm); thus, Sr-containing columns show lower image intensity, with the column intensity depending on the number of Sr atoms, their depth locations, the total number of atoms in the column, and the STEM parameters [28,[38][39][40]. The white arrows in the I Sm;Sr maps indicate selected columns that show significantly reduced intensities relative to those of nearest-neighbor Ti-O columns, which have similar thicknesses.…”

“…First, we emphasize that the method employed here can detect atomic-scale variations in local displacements, when present, even in cases where they affect only a few atoms, with picometer precision, despite the fact that along the projection direction through a finite TEM sample thickness not all the atoms in a column may be displaced; see Refs. [27,28]. Therefore, even very small regions (few nm) that have different distortions should be discernable.…”

“…We characterize the local lattice response, using atomic resolution scanning transmission electron microscopy (STEM), which can resolve very small distortions, including those around individual point defects [27][28][29]. This allows us to study the response of the lattice to hole (Sr) doping on length scales that range from extremely local (i.e., the distortion of atomic columns surrounding the dopant) to long range (by analyzing images from different regions).…”