Richard P. Rodrigues

The influence of atomic structure on the formation of electrical barriers at grain boundaries in SrTiO3

An experimental atomic resolution analysis of an undoped Σ5 36° [001] tilt grain boundary in SrTiO3 shows that the structure contains incomplete oxygen octahedra. These incomplete octahedra act as effective oxygen vacancies and lead to a fixed, positive boundary charge. Annealing the boundary in the presence of MnO2 does not change the atomic structure of the boundary plane, and results in a high concentration of Mn3+ (acceptor) enrichment at the specific Ti4+ locations in closest proximity to the effective oxygen vacancies. This result can be explained in terms of standard charge compensation models and indicates that the formation of electrical barriers at oxide grain boundaries may be influenced by the atomic structure of the boundary plane.

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.

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.

4-Probe Micropatterning and Electrical Measurements Across Individual Grain Boundaries in Electroceramics

A novel approach combining conventional contact- and projection-lithography techniques has been devised to implement microelectrodes, down to submicron size, across isolated site-specific features as small as 2–3 micron. For both ex-situ and in-situ electrical characterization, such features of interest are isolated interfaces and grain boundaries in electroceramics and multilayer devices, including those in as-prepared TEM specimen. The procedure has been discussed for implementing 4-probe microelectrodes across several individual isolated grain boundaries of a commercial ZnO varistor containing 2–10 micron size grains. In addition, we discuss the results from dc 4-probe I-V and ac 2-probe impedance measurements across individual grain boundaries, and dc 2-probe I-V and ac 2-probe impedance measurements from grain interiors isolating these grain boundaries. Over and above the generally observed properties of isolated grain boundaries, the measurements reveal (1) inhomogeneity and applied-bias-polarity dependent asymmetry in the nonlinear I-V characteristics of grain boundaries, (2) possible presence of non-ohmic electrode-ceramic contact resistance in 2-probe measurements, and (3) a gradual process of irreversible degradation of the nonlinear I-V behavior with respect to thermal runaways upon application of a dc bias across isolated grain boundaries.

Simulation of electron phase shifts by electrostatic potential across electroceramic interfaces

An expression for the electron-wave phase shifts caused by the potential of an electric dipole is extended to a general equation representing the phase shifts caused by the electrostatic potential across a parallel-plate capacitor. The capacitor is then used to construct a simple model for the phase shifts caused by the potential barriers present across internal interfaces in solids. The phase simulations yields phase shifts for electroceramic interfaces in good agreement with the phase images reconstructed from off-axis electron holograms of acceptor-doped grain boundaries in SrTiO3.

Electronic Structure of Mn Acceptor Impurity Incorporated SrTiO3 Using Embedded Cluster Method

We have performed atomic-level first principle electronic structure calculations on doped grain boundaries (GB) in SrTiO3. This was motivated by the electron holography experiments, which were able to quantify the electrostatic potential and the associated space charge distribution across the Mn-doped GB in this material. The embedded cluster Discrete Variational (DV)-Xα method was used to determine the charge and the densities of states for several idealized models of a single crystal and symmetrical tilt grain boundaries in SrTiO3. Special attention was given to the role of Mn+2 and Mn+3 acceptors substituting for Ti+4 resulting in charge segregation across the grain boundaries, which was shown in the electron holography experiments. We have found that Mn replacing Ti prefers to have valence charge around +2 and this picture agrees with the experimental observation of negative grain boundary charges in the GB core.

Electronic structure of grain boundaries in SrTiO3

Abstract The formation of Schottky potential barriers and their complex interplay with current transport across electrically active grain boundaries (GBs) give rise to many novel properties of electroceramics such as the varistor behavior. The origin of the GB electrical activity lies in the defect chemistry, and associated variations in geometry, chemistry and electronic structure at GBs. Motivated by the recent experimental results of dopant identification and spatially resolved quantification of GB charge and associated space-charge across GBs in SrTiO3, we have performed atomic-level electronic structure calculations on GBs in SrTiO3. The first-principles density-functional embedded-cluster Discrete-Variational (DV) method is used to determine charge densities and densities of states (DOS) for several idealized models of symmetrical tilt GBs in SrTiO3 which have been derived from a combination of atomistic latticestatic simulations and experimental electron microscopy analysis.