V. Ravikumar

Direct determination of grain boundary atomic structure in SrTiO3

An atomic structure model for a 25� [001] symmetric tilt grain boundary in SrTiO3 has been determined directly from experimental data with the use of high-resolution Z-contrast imaging coupled with electron energy loss spectroscopy. The derived model of the grain boundary was refined by bond-valence sum calculations and reveals candidate sites for dopant atoms in the boundary plane. These results show how the combined techniques can be used to deduce the atomic structure of defects and interfaces without recourse to preconceived structural models or image simulations.

An investigation of acceptor-doped grain boundaries in

Grain boundary (GB) doped exhibits interesting electroceramic phenomena including varistor and barrier layer capacitor behaviour. We present here our investigation of GB acceptor-doped using analytical electron microscopy including electron holography. Mn was diffused into sintered polycrystalline to attain GBs which are rich in Mn. The presence and spatial extent of Mn at the GBs were analysed using x-ray emission spectroscopy (XES) and parallel electron energy loss spectroscopy (PEELS). The valence state of Mn was determined using PEELS to be predominantly +2. Finally, transmission high-energy electron holography was utilized to directly image and quantify the electrostatic potential and associated space-charge across the GBs directly. The holography results reveal a negatively charged GB with positive space-charge, indicating that Mn with a valence of +2 resides as an acceptor dopant on the Ti site at the GB core. The barrier height and local charge density distribution, including the Debye length, of the double Schottky barrier at the GB are derived from these holography results. This investigation demonstrates the usefulness of electron holography as a bulk-sensitive technique to probe the statics and dynamics of electrostatic field distribution and electrical charge across interfaces in technologically useful materials, and the need to employ diverse analytical techniques for such an investigation.

Atomic structure of undoped Σ = 5 symmetrical tilt grain boundary in strontium titanate

The atomic structure of a pristine Σ = 5 grain boundary in SrTiO3 has been investigated using a variety of transmission electron microscopy (TEM) techniques. No cation non-stoichiometry or impurity segregants could be detected at the boundary within the limits of the energy dispersive X-ray microanalysis technique used, while preliminary electron energy loss spectroscopy analysis reflects that changes in the optical/dielectric function or the valence of the cations at the interface are too subtle to be detected with our coarse scale measurements. High-resolution transmission electron microscopy indicates a symmetrical tilt grain boundary with a (130)-type grain boundary plane. The grain boundary has a compact core, with negligible plane-normal rigid body translation (RBT). An in-plane RBT of 12d130 (≈0.62 A) is identified from the high-resolution electron micrographs. A semi-empirical model of the relaxed atomic structure of the grain boundary based on crystal chemistry principles is proposed, which includes the observed RBT and individual atomic relaxations at the boundary core.

Investigation of grain boundary segregation in acceptor and donor doped strontium titanate

Abstract Grain boundary segregation in electronic ceramics is often responsible for dictating the grain boundary properties, which in turn dictate the macroscopic electronic properties of the material. Consequently, it is important to understand the nature of segregation phenomena in these materials. Here we present results from a combination of diverse analytical techniques used to investigate the character of grain boundary segregation in acceptor (Fe, Mn) and donor (Nb) doped strontium titanate. X-ray emission spectroscopy (XES) and electron energy loss spectroscopy (EELS) analysis consistently show segregation of both acceptor (Fe, and Mn) and donor (Nb) dopant species to the grain boundaries. Within the spatial resolution of the techniques, the segregation profiles for these dopants are found to be limited to less than 5 nm about the grain boundaries. Furthermore secondary ion mass spectroscopy shows that the segregation is ubiquitous throughout the samples, and not limited to selected grain boundaries.

Atomic Structure and Properties of the (310) Symmetrical Tilt Grain Boundary (STGB) in SrTiO3. Part I: Atomistic Simulations

The relaxed structure and energy of the (310) symmetrical tilt grain boundary (STGB) in SrTiO3 have been calculated using static lattice energy minimization methods. In principle, the (310) GB plane can either be a cation-rich, positively charged “SrTiO” plane or a negatively charged oxygen plane, and both scenarios have been considered in this report. The effect of point-defect reconstruction at the GB core region, manifested either as completely missing columns or as half-filled columns of ions as suggested by experiments, has been analyzed. The results indicate that while Schottky defects are very strongly preferred energetically at the GB core, there is not significant gain in energy by having half-filled columns, as opposed to fully-dense and fully-empty columns, at the GB core. The simulation results have been analyzed in the context of Paulings rules of crystal chemistry and bicrystallography. The results form the basis for an objective comparison with experimental studies in Part II of the paper.

Atomic Structure and Properties of the (310) Symmetrical Tilt Grain Boundary (STGB) in SrTiO3 Part II: Comparison with Experimental Studies

The atomistic simulation results presented in Part I for SrTiO3 (310) symmetrical tilt grain boundary (STGB, the so-called Σ = 5 GB with 36.8° symmetrical misorientation about [001]) are analyzed in the context of available experimental studies. In particular, atomic imaging studies of SrTiO3 GBs via high resolution TEM and incoherent Z-contrast STEM imaging; and determination of oxygen positions by combining electron energy loss spectroscopy (EELS) and bond-valence-sum rules, are compared with simulation results. The atomistic simulation data on the GB energies are compared with relative experimental estimates obtained via a novel approach of faceting of focused ion beam (FIB) induced microvoids.While there are considerable differences in details of simulation and experimental results, some basic trends seem to emerge about the core structural framework of GBs in SrTiO3. The paper highlights the limitations of both, experimental and simulation techniques, and argues in favor of synergistic use of diverse experimental and simulation approach to determine the atomic structure and properties of GBs.

Atomic scale structure and chemistry of interfaces by Z-contrast imaging and electron energy loss spectroscopy in the STEM

The macroscopic properties of many materials are controlled by the structure and chemistry at grain boundaries. A basic understanding of the structure-property relationship requires a technique which probes both composition and chemical bonding on an atomic scale. High-resolution Z-contrast imaging in the scanning transmission electron microscope (STEM) forms an incoherent image in which changes in atomic structure and composition across an interface can be interpreted directly without the need for preconceived atomic structure models. Since the Z-contrast image is formed by electrons scattered through high angles, parallel detection electron energy loss spectroscopy (PEELS) can be used simultaneously to provide complementary chemical information on an atomic scale. The fine structure in the PEEL spectra can be used to investigate the local electronic structure and the nature of the bonding across the interface. In this paper we use the complimentary techniques of high resolution Z-contrast imaging and PEELS to investigate the atomic structure and chemistry of a 25{degree} symmetric tilt boundary in a bicrystal of the electroceramic SrTiO{sub 3}.

Atomic Structure OF YΣ=5 (130) Symmetrical Tilt Boundary In Strontium Titanate

The atomic structure of a pristine (undoped) boundary in strontium titanate has been investigated using transmission electron microscopy techniques. Results of electron diffraction studies indicate a pure tilt boundary with a common \001] tilt axis, and a tilt angle of 36.8°, which corresponds to a Σ-= 5 grain boundary in the Coincidence Site Lattice (CSL) notation. High Resolution Transmission Electron Microscopy (HRTEM) indicates a symmetric tilt grain boundary with a (130) type grain boundary plane. No cation non-stoichiometry or impurity segregants could be detected at the interface, within the limits of the Energy Dispersive X-ray microanalysis technique used. The grain boundary has a compact core, with negligible planenormal rigid body translation (RBT). An in-plane RBT of (1/2)d 130 (˜ 0.62 A°) was identified from the high resolution electron micrographs. An empirical model of the relaxed atomic structure of the grain boundary is proposed.