Alumina (Al₂O₃) is one of the most common materials used in various industrial sectors. Despite such importance, atomistic-level details of the dislocations, which are responsible for plastic deformation of the material, have been identified only recently. Shibata et al. have reported the High-Resolution TEM (HRTEM) measurements on atomic-level structure of dissociated dislocations and their associated stacking faults[1]. We have performed a large-scale classical Molecular-Dynamics (MD) and hybrid Density-Functional (DF)/classical MD analysis on the electronic structure and the stability of the basal edge dislocation in α-alumina[2]. We have obtained the qualitatively consistent results that support the experimental observation of stable core-core distance of dissociated partials in the dislocation. In addition, effects of segregation of doped atom, such as Si and Ti, into dislocation core have been also investigated via the hybrid DF/MD method. Localized mid-gap states emerge at the doped cores as signatures of the dopant&’s electronic structure. As compared with previous researches on the edge dislocations, screw dislocations in α-alumina have not been studied very extensively. This is mainly due to the difficulty in finding the screw dislocations generated intrinsically in experiment. Recently, Tochigi et al. have reported observations of basal screw dislocation networks formed in an alumina low-angle twist boundary[3]. On the contrary to the basal edge dislocations, no dissociated partials have been observed in the case of basal screw dislocations. To elucidate causes on this observation, we performed a large-scale molecular dynamics computer simulation on the system with the screw basal dislocations in α-alumina, involving four dislocation cores with 288,000 atoms. We introduce partial screw dislocations dissociated along [1-100] and [0001] directions from a perfect dislocation, accompanied by {0001} and {1-100} stacking faults, respectively. Comparisons are made on the energies of these partials with that of perfect dislocation. The result reveals that the partial dislocations dissociated along both directions are unstable at 0 K. This is consistent with the fact that only perfect dislocation has been observed experimentally. We also analyze stability of the perfect screw dislocation with {0001} stacking fault at finite temperatures. The stability trend below 1000K is similar to that at 0K, while there is little difference in energy between perfect and partial dislocations above 1000K. Detailed analyses on the finite-temperature behaviors and a charge-distribution analysis via a variable-charge interatomic model will also be discussed. [1] N. Shibata et al., Science 316 (2007) 82; E. Tochigi et al., Acta Matter. 58 (2010) 208. [2] K. Tsuruta et al., Mater. Trans. 50 (2009) 1015. [3] E. Tochigi et al., Acta Mater. 60 (2012) 1293.

Nanocrystalline metals promise unique and improved properties, such as high wear strength, corrosion resistance, and enhanced thermoelectric performance. These gains are unfortunately mitigated by the inherent instability associated with the high fraction of atoms located at grain boundaries. Solute segregation to the grain boundaries can lower this energy penalty, and this approach has been successfully applied in a handful of systems to achieve some measure of stabilization. To better understand the conditions required for complete stability of a nanocrystalline structure we use a thermodynamic model for a regular nanocrystalline solution to construct nanocrystalline stability maps for use in alloy design. A modified ratio of the enthalpies of mixing and segregation is found to be the descriptive criteria for nanocrystalline stability. We adapt the Miedema surface segregation model to the grain boundary environment to provide the segregation enthalpy necessary to predict nanocrystalline stability, and use available materials data to predict the relative stabilities of hundreds of binary alloys.

Dopant atoms in a crystal are known to segregate to dislocations and form so-called Cottrell atmospheres in order to accommodate strain fields around dislocations. In addition, dislocations act as rapid diffusion paths of dopant atoms, which is known as the pipe-diffusion phenomenon. It has been shown that these characteristic properties of dislocations can be used to form novel nano-scale structures inside oxide materials. For instance, conductive Ti-rich nanowires in insulating sapphire single crystals have been fabricated by dislocations [1]. However, fundamental mechanisms of dopant segregation at dislocations are not well understood yet, especially in oxide materials. One successful method for introducing dislocations into crystals is the fabrication of low-angle tilt grain boundaries by bicrystal method. With this method, several dislocation structures can be introduced in nanometer order by controlling crystallographic orientations of two adjacent crystals; therefore, these model dislocation structures are advantageous for studying the interaction between dislocations and dopant atoms systematically. In a {11-20}/<1-100> 2° low-angle tilt grain boundary of alumina (α-Al₂O₃), a periodic array of climb-dissociated basal edge dislocations is introduced [2]. The core structures of those dissociated basal edge dislocations have been observed by scanning transmission electron microscopy (STEM). It was found that the upper partial dislocations and the lower partial dislocations are terminated by -Al-Al columns and -Al-O columns respectively, and being locally nonstoichiometric [3]. In the present study, several kinds of metal ions (Ni²⁺, Sr²⁺, Er³⁺, Zr⁴⁺ and Ti⁴⁺) are doped into the low-angle tilt grain boundaries of alumina in order to understand the dopant segregation behaviors around the partial dislocation cores. All of the metal doped grain boundaries consist of periodic arrays of b=1/3<10-10> and b=1/3<01-10> partial dislocation pairs with stacking faults in between. Z-contrast STEM images revealed that the dopant atoms segregate in the vicinity of the dislocation cores but not to the stacking fault regions. The ratio of the segregation amount to the upper/lower partials is likely to depend on the valence states of dopant atoms, but not on the ionic radii. More explicitly, we found that the upper partials tend to attract divalent cations, while the lower partials tend to attract tetravalent cations. These results suggest that the partial dislocation cores may be locally electrically charged; and thus, electrical interactions between dislocation cores and dopant atoms must be the key to understanding the dislocation segregation mechanisms in alumina. Referrences [1] A. Nakamura et al., Nat. Mater., 2, 453-456 (2003). [2] A. Nakamura et al., Phil. Mag., 86, 4657-4666 (2006). [3] N. Shibata et al., Science, 316, 82-85 (2007).

Grain boundary sliding is an important deformation mode in polycrystals, and it has been extensively investigated, for example, there are many studies on influences of the atomic geometry in the grain boundary region. However, it is important to investigate grain boundary sliding from the electronic structure of grain boundary for deeper understandings of the sliding mechanisms. In the present work, we investigated the grain boundaries sliding in pure and segregated bicrystals with classical molecular dynamics (MD) simulations and first-principles calculations. It is accepted that the sliding rate is affected by the grain boundary energy. However, there was no correlation between the sliding rate and the grain boundary energy in either the pure or the segregated bicrystals. First-principles calculations revealed that the sliding rate calculated by the MD simulations increases with decreasing minimum charge density at the bond critical point in the grain boundary. This held in both the pure and segregated bicrystals. It seems that the sliding rate depends on atomic movement at the minimum charge density sites.

UO2 pellets, used to power light water reactors, contain micron-sized grains separated by a network of grain boundaries. As the pellets go through the fission process, the resulting fission products must be accommodated within the UO2 lattice and grain boundaries provide important space to accommodate them. Therefore quantifying the way in which fission products interact with grain boundaries is essential for understanding, predicting, and ultimately controlling fuel performance. Here, atomic-level simulations using empirical potentials are used to examine the segregation of various fission products to one specific grain boundary - Σ5 tilt - in UO2, which is chosen as a model representative boundary in this material. The calculated segregation energies of fission products are typically in the range of several eVs. We find that two factors - the ionic charge and ionic radius of a given fission product - are dominant determiners of the segregation behavior and at times compete to determine the final segregation tendencies. In particular, larger aliovalent ions segregate to a specific uranium site that is characterized by smaller oxygen coordination and greater free volume, compared to other sites at the grain boundary. This work is supported by the Nuclear Energy Advanced Modeling and Simulation (NEAMS) program.

The deformation behaviors at the grain boundary and the grain interior were investigated in magnesium and its alloys using nanoindentation technique. The grain interior had a low strain rate sensitivity exponent, because its matrix was too large to be influenced by the grain boundary. The deformation mechanism in the grain interior was the dislocation slip. On the other hands, the grain boundary showed a high strain rate sensitivity exponent and was dominated by the grain boundary sliding due to the high diffusion rate at the grain boundary. The grain boundary affected the deformation behavior into the grain interior at the distance of 2 mu;m. In addition, the occurrence of grain boundary sliding was closely related to the grain boundary energy; grain boundary with high energy showed high strain rate sensitivity. Furthermore, the addition of solute atoms into magnesium tended to prevent the grain boundary sliding due to the decrease in grain boundary energy.

Tungsten and tantalum are neighbors in the Periodic Table of the Elements and they have many properties in common such as high melting point, high cohesive energy, high mass density etc. Their mechanical properties such as ductility or ductile-to-brittle transition temperature (DBTT) can be affected by the presence of impurities. However, the extents of the impurity effect on the two metals are rather different. Due to the presence of impurities, the increase of DBTT is more than 400K for tungsten, but is only about 20K for tantalum. Despite the ample experimental evidence, the nature behind this preferentially embrittling effect of impurities on the ductility of tungsten still remains elusive. In this work, based on the understanding that ductility is the competition between grain boundary (GB) separation and dislocation activities, we used density functional theory to systematically calculate the pristine and contaminated GB separation energy, the GB and dislocation core segregation energy of various impurities for tungsten and tantalum. The results show that for each impurity species, the GB and core segregation energies in tungsten are always significantly higher than those in tantalum, indicating that impurities in tungsten are more likely to segregate to GB regions and the vicinity of dislocation core to influence them. The binding energy difference between GB and free surface site for each impurity species in tungsten is always higher than that in tantalum, indicating that the presence of impurities, if deemed undesirable, will cause a greater reduction in GB separation energy for tungsten. The analyses of the chemical and mechanical effects of impurities suggest that tungsten is more sensitive to impurities because of its low lattice constant and high bulk modulus despite other possible causes.

Atomistic simulation of the uniaxial straining of nanocrystalline palladium along different loading directions at room temperature demonstrated that both the elastic and plastic behavior displayed a pronounced anisotropy after some deformation. Surprisingly, even the Young&’s modulus can significantly change with the deformation history of the sample. Some of these changes are found to be reversible. They either relax or can be removed by applying the opposite deformation. A mechanism based on excess-free-volume migration in the grain boundaries is proposed to explain such behavior. To explore this further we have introduced significant amounts of porosity into nanocrystalline palladium structures and found that a significant part of the void volume fraction reduces during the sample preparation. The Young&’s modulus and the flow stress decrease considerably with increasing porosity. Similar general trends, decreasing elastic stiffness and decreasing flow stress with increasing free volume content (or decreasing density), is found in amorphous materials as for example diamond-like carbon. Mechanical deformation can modify the free volume content and thereby the properties significantly. It is thus possible to create grain boundary-like regions with a larger free volume between amorphous regions of higher density. Such structures will be investigated.

In the plane-wave density-functional theory (DFT) methods, physical quantities such as energy and stress are given as an integral or average throughout the supercell. If such quantities can be given in each local region in the supercell, nature of defective systems such as grain boundaries (GBs) and interfaces between dissimilar materials can be effectively analyzed. Historically, energy-density [1] and stress-density [2] methods to calculate local energy and local stress were proposed within the plane-wave pseudopotential methods. However, practical applications have not been performed enough, due to the difficulty in the gauge-dependent problem in these methods. Recently, we have clarified the formulation of the energy density and stress density in the projector augmented wave (PAW) method for the first time [3], and proposed that local energy and local stress can be given as unique physical quantities if the energy and stress densities are integrated within proper local regions where the gauge-dependent terms are integrated to be zero. This strategy has been successfully applied to atomic-layer resolved stresses on Al surfaces [3] using our package software QMAS (Quantum MAterials Simulator) for the PAW method [4]. In this paper, we present our recent applications of local-energy and local-stress analysis to GBs and interfaces in metals and ceramics. For defective systems, it is necessary to define local regions to satisfy the condition for the gauge-dependent problem effectively. We adopt atom-centered regions via fuzzy-Voronoi or Bader integration schemes. We deal with tilt and twist GBs in Al and Cu, tilt GBs in Fe and SiC, and metal-carbide interfaces. Results of metallic GBs are compared with those by the embedded-atom method (EAM) potentials. We show that the local-energy and local-stress calculations are quite effective to investigate the stability, mechanical behavior and impurity effects of various GBs and interfaces [5]. Acknowledgement: The present study was supported by the Grant-in-Aid for Scientific Research on Innovative Area, “Bulk Nanostructured Metals”, by JST Industry-Academia Collaborative R&D Programs, and by the Strategic Programs for Innovative Research, MEXT, and the Computational Materials Science Initiative, Japan. References: [1] N. Chetty and R.M. Martin, Phys. Rev. B 45, 6074 (1992), [2] A. Filippetti and V. Fiorentini, Phys. Rev. B 61, 8433 (2000), [3] Y. Shiihara, M. Kohyama and S. Ishibashi, Phys. Rev. B 81, 075441 (2010), [4] S. Ishibashi, T. Tamura, S. Tanaka, M. Kohyama and K. Terakura, http://qmas.jp, [5] S. Ogata, Y. Umeno and M. Kohyama, Modell. Simul. Mater. Sci. Eng. 17, 013001 (2009).

Models of crystal plasticity should cover many length scales and overlap each other with effective scale transitions. Using Molecular Dynamics (MD) techniques we study the impacts of an edge dislocation on a {(914),(19-4),0} grain boundary (GB) in Al under load. This GB minimizes the anisotropy effect caused by the application of a shear stress on a bicrystal with fcc structure and hence allows a comparison between 3-dimensional MD and 2-dimensional Dislocation Dynamics (DD) modeling. Several thousands of GBs having the same misorientation were constructed, different only in very small microtranslations of one of the two crystals parallel to the GB plane prior to final relaxation by MD. The lowest-energy GB of these was selected for further study. The resulting GB structure has such a complexity that there are 15 different impact points for an approaching dislocation. Applying iterative MD and energy/stress minimization techniques we force a dislocation to move towards the GB on each of the 15 glide planes. After absorption of the dislocation the 15 atomic displacement fields are analyzed in terms of dislocation spreading, yielding values that can be subsequently used in DD simulations.

The practice of adding impurities to control the evolution of crystalline interfacial microstructures is based on rough estimates on the drag force exerted by the resultant impurity cloud on the moving interfaces. However, in spite of several existing theoretical frameworks, there is no reliable estimate on the extent of the drag due to challenges are in extracting two key ingredients, i) intrinsic mobilities of grain boundaries and ii) the impurity-grain boundary interaction energies, thereby limiting efforts to control microstructural evolution during processing of commercial purity alloy systems. In this talk, we present results of atomic-scale simulations on extracting the variation in these two quantities for a series of high-angle grain boundaries in the Al-Mg system. We find dramatic variations in the impurity-grain boundary interactions with boundary type and the interaction strength renders the anisotropy in intrinsic mobilities inconsequential under typical processing conditions. Extraction of the effective (extrinsic) mobility using existing theoretical frameworks confirms the effect, and we find excellent agreement between our predictions and mobilities extracted via prior in-situ experiments. A conclusion, we discuss the implications of the results on microstructural evolution during annealing phenomena.

The evolution of grains in nanocrystalline materials is examined using the phase field crystal (PFC) method. The strength of the PFC method lies in its ability to follow the atomic scale motion of a process that occurs on diffusive timescales. Due to the atomic scale resolution of the PFC method, the evolution of the dislocation structure of a grain boundary and the local atomic displacements of atoms near the boundary during grain growth can be determined. For example, we find that grain boundary dislocation interactions give rise to a repeating faceting-defaceting transition during grain shrinkage that leads to a different relationship between the grain area and time than that seen in classical curvature-driven grain growth. The evolution of the dislocation structure during grain growth in nanocrystalline materials can also induce grain rotation and, in particular, translation. The results of the simulations will be presented and discussed.

Strontium titanate based materials have great interest for a wide range of applications in microelectronics namely in tunable microwave devices, due to the high electric-field dependence of the permittivity and more recently as a possible thermoelectric oxide material. A grain growth anomaly in strontium titanate (SrTiO3) was recently reported [1]. Between 1200 to 1600 degree C grain growth kinetics did not follow the classical Arrhenius-type temperature dependence. Changes in the faceting behavior of the grain boundaries at high temperatures were pointed as the possible explanation for the anomalous grain growth behavior observed in SrTiO3 [1, 2]. In this work the anomaly in the dependence of grain growth kinetics on the temperature of strontium titanate is revisited. New data is presented which lead to a reinterpretation of the phenomenon. Discontinuities on the evolution of grain growth with the sintering temperature were observed, defining four grain growth regimens with transitions at temperatures around 1500, 1550 and 1605 degree C. These transitions correspond to grain size decreases, despite the increasing sintering temperature. These discontinuities were also observed in the conductivity response of the system (grain and grain boundaries) determined by impedance spectroscopy. By using complementary experimental tools, we proved that this anomalous behavior is directly related to the presence of liquid phase, which amount varies with the temperature and showing trends consistent with a scenario of retrograde solubility, reported for metals and semiconductors, but not reported before for ceramics. These new results have great scientific and technological relevance in tailoring the microstructure and dielectric response of SrTiO3 based materials and using grain boundary behavior for materials design. References [1] M. Baurer, D. Weygand, P. Gumbsch, M.J. Hoffmann, Scripta Mater, 61 (2009) 584-587. [2] M. Baurer, H. Stormer, D. Gerthsen, M.J. Hoffmann, Adv Eng Mater, 12 (2010) 1230-1234.

A classical force-field model with partial ionic charges was applied to study the behaviour of oxygen vacancies in the perovskite oxide strontium titanate (SrTiO₃). The dynamical behaviour of these point defects was investigated as a function of temperature and defect concentration by means of molecular dynamics (MD) simulations. The interaction between oxygen vacancies and an extended defect, here a Σ3(111) grain boundary, was also examined by MD simulations. Analysis of the vacancy distribution revealed considerable accumulation of vacancies in the envelope of the grain boundary. Possible clustering of oxygen vacancies in bulk SrTiO₃ was studied by static lattice calculations within the Mott-Littleton approach. All binary vacancy-vacancy configurations were found to be energetically unfavourable.

It is well established that grain boundaries and interfaces act as sinks for radiation-induced defects, promoting the radiation tolerance of materials. What is less clear is exactly how boundaries separately modify the thermodynamics and kinetics of defects, especially larger defect clusters that might be produced during collision cascades, to provide this enhanced radiation tolerance. Here, we use accelerated molecular dynamics (AMD) and adaptive kinetic Monte Carlo (AKMC) techniques to examine the mobility of defect clusters in a variety of grain boundaries in Cu, with special emphasis on how that mobility depends on the character of the grain boundaries. In particular, we find that different boundaries interact very differently with defect clusters, with some greatly retarding the mobility of such clusters and others having relatively little effect, compared to bulk behavior. These results have implications not only for radiation damage evolution, but for other applications where boundary-mediated mass transport is crucial.

Ferroelectric lead zirconate titanate (PZT) is one of the most important electroceramic materials and applied in actuators, sensors, memory devices, etc. In all these cases, PZT has to be insulating and mass or charge transport along and across interfaces is usually undesirable. Still, high electric fields as well as high temperatures may cause substantial diffusion or migration of ions in bulk and along/across grain boundaries and this can drastically change the overall materials properties. However, information on the ion transport kinetics in PZT bulk and grain boundaries is scarce. In the first part of this contribution it is shown how combination of several experimental tools helps analyzing a number of interface-related ion transport phenomena in donor-doped PZT: i) Very fast 18O tracer diffusion along grain boundaries was observed at temperatures above 600°C. ii) Strongly accelerated oxide ion (tracer) diffusion in PZT was triggered by application of high electric fields; interestingly the fast diffusion was perpendicular to the field direction. iii) At temperatures of about 350-500°C, PZT multilayers with co-sintered Ag-Pd electrodes underwent strong resistance degradation under field load. The highly conducting region was located in a near-surface zone of several 10 micrometer thickness. Conductive atomic force microscopy (c-AFM) and chemical analysis revealed deposition of metallic silver paths in grain boundaries. Their formation started close to the anode and is not explainable by conventional electrochemical metal deposition. In many regards SrTiO3 is similar to PZT or BaTiO3, all being large band gap perovskite-type oxides, and it often acts as a kind of prototype material for such oxides. Detailed knowledge is available on its defect chemistry and also on structure and transport properties of grain boundaries in polycrystalline SrTiO3. However, defect chemistry and electrical properties of very thin SrTiO3 layers are not very well-understood yet; interfacial space charge layers may play an overwhelmingly important role there. In the second part of this contribution, we present the results of a combined 18O tracer diffusion and impedance study on acceptor-doped SrTiO3 layers of about 100 nm thickness, deposited on Al2O3, LaAlO3, undoped SrTiO3 and Nb-doped SrTiO3. Among others, pronounced near-surface space charge zones could be detected in tracer diffusion experiments and a strong dependence of the effective tracer diffusion coefficient on the substrate was found. Anisotropy of mass and charge transport properties are discussed and kinetic results are correlated with structural information from HRTEM and XRD studies.

Alumina (Al₂O₃) is a thermally grown oxide that efficiently enables the alumina-forming alloys to withstand high operating temperatures and oxidizing environments. The growth of alumina scale is closely related to the diffusion process of oxygen and aluminum in the material. To acquire knowledge of diffusion mechanisms and predict diffusion coefficients, we have calculated all the involved energetics about the O and Al diffusion through the grain boundaries of α-Al₂O₃ using first-principles density function theory method. In this study, we examined specifically the diffusions of O and Al via vacancy exchange mechanism along low-energy Σ3 (1 0 -1 0) prismatic grain boundaries (whose energies are in the range of 0.25 to 0.39 J/m²) and high-energy Σ3 (0001) basal grain boundaries (whose energies are in the range of 2.12 to 2.38 J/m²). It was found that both the vacancy formation energy and diffusion activation energy were lower on the grain boundaries than in bulk α-Al₂O₃. Moreover, our results show that the vacancy formation energies are significantly lower on the high energy grain boundaries than on the low energy grain boundaries of Al₂O₃. Furthermore, our results from first-principles climbed image nudged elastic band calculations indicated that the energy barriers were lower by more than 2 eV for atomic O diffusion and lower by about 0.8 eV for atomic Al diffusion on the high energy Σ3 (0001) basal grain boundaries than in the bulk. However, the calculated energy barriers for atomic diffusion on the low energy Σ3 (10-10) prismatic grain boundaries are just lower by a very small amount as compared to those in bulk Al₂O₃. Hence, the atomic diffusion in α-Al₂O₃ was predicted to be much faster along the high-energy grain boundaries than that along the low-energy grain boundaries. In addition, we examined the correlation between the energetics of diffusion and the geometric arrangement of atoms in the bulk and different grain boundaries of α-Al₂O₃.

We use atomistic modeling to investigate the structure and transport of isolated point defects at low-angle twist grain boundaries in MgO, chosen as a simple model oxide. We find that point defects delocalize at misfit dislocations at the grain boundary and the extent of delocalization depends on the length of the misfit dislocation segment. Delocalized point defects migrate from one misfit dislocation segment to another misfit dislocation segment first by localizing at the misfit dislocation intersection and then delocalizing at another of the misfit dislocation segments. Point defect dynamics in these model ceramic grain boundaries are compared with those in metal heterointerfaces and the implications of such point defect dynamics for defect annihilation under extreme conditions, such as under irradiation, are discussed. This material is based upon work supported as part of the Center for Materials at Irradiation and Mechanical Extremes, an Energy Frontier Research Center funded by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number 2008LANL1026

Multiferroic materials of current interest tend to have (anti)ferroelectric and (anti)ferromagnetic properties arising at separate instabilities which may be widely separated in temperature. Although they may be multiferroic, they do not have multiferroic phase transitions and their domain walls are likely to be (anti)ferroelectric or (anti)ferromagnetic but not both simultaneously. Multiferroic materials which involve magnetism and ferroelasticity, on the other hand may undergo genuinely multiferroic transitions at which both properties appear simultaneously. In these cases the domain walls can be either (anti)ferromagnetic (180°) walls or W and Wprime twin walls which are both ferroelastic and (anti)ferromagnetic (90° walls for, e.g. cubic → tetragonal, 60° and 120° walls for e.g. hexagonal → monoclinic). A key property is the strength of coupling of the magnetic order parameter with strain and the dynamic character of the different ferroelastic twin walls can be investigated through their tendency to give rise to acoustic attenuation in measurements of elastic properties. A number of materials have been investigated in this context using resonant ultrasound spectroscopy (RUS). In FexO, magnetic ordering is coupled with a pseudoproper ferroelastic transition and shear strains of up to a few % are observed. Twin walls in the rhombohedral phase are antiferromagnetic/ferroelastic, and give rise to strong attenuation at RUS frequencies. In the weakly ferromagnetic (canted antiferromagnet) phase of Fe2O3 the monoclinic symmetry-breaking strain is sufficiently small as to not yet have been detected with laboratory diffraction techniques but ferroelastic W and Wprime walls appear to be present. From a strain analysis of the orientations of these, an estimate of the symmetry-breaking shear strain at room temperature is ~5permil;. Single crystal samples give strong acoustic attenuation and, even for these low strains, the twin walls behave as if they are normal ferroelastic walls with respect to an applied stress. On the other hand, coupling between magnetic ordering and strain in KMnF3 perovskite appears to be minimal and there is no evidence of acoustic loss due to processes other than motion of twin walls associated with the octahedral tilting (improper ferroelastic) transitions. In the low temperature Pnma structure, which is weakly ferromagnetic (canted antiferromagnetic), the ferroelastic twin walls are immobile. Pinning of ferroelastic twin walls by defects is most likely to occur if the walls are thin, but ferroelastic twin walls which are also magnetic walls will tend to adopt the thicker configuration of the latter if there is significant coupling with strain. Thus they will tend to remain mobile over wider temperature intervals. For systems with multiple order parameters there are theoretical possibilities for controlling the magnetic properties of the twin walls themselves, again depending on the form and strength of strain coupling.

Ferroelectric ceramics with perovskite structures, like Pb(Zr,Ti)O3 (PZT), which is widely used, e.g., for electromechanical actuators and sensors, or (K,Na)NbO3 (KNN), which is a promising lead-free substitute for PZT, have dense polycrystalline microstructures. The crystallites have complex domain structures, and they contain atomic dopants in order to optimize the functional performance. The objective of this work is an investigation of the fundamental behaviour of the ferroelectric polarisation at domain walls, grain boundaries, or atomic dopants in PZT and KNN by means of first-principles supercell calculations using density functional theory (DFT) and classical atomistic simulations with empirical ionic potentials. The focus of this talk will be on isolated Cu substitutionals and defect complexes of these with charge-compensating oxygen vacancies in KNbO3. In this study we found a morphotropic phase transition in Cu-doped KNbO3 similar to one which was reported earlier for Li-doped (K,Na)NbO3. We estimated the energy needed for switching the ferroelectric polarization in a crystal region with a defect complex. This energy determines how defect complexes can pin domain walls and contribute to ferroelectric hardness and/or large piezoelectric strain.

Hexagonal manganites RMnO3 with R=Ho-Lu, Y and Sc exhibit have received much attention because of their intriguing physical properties such as magnetoelectric effect [1]. Hexagonal YMnO3 exhibits two structural phase transitions from a paraelectric P63/mmc structure to an intermediate paraelectric one at 1270 K, accompanying the tilting of the MnO53+ polyhedra and corrugation of the Y3+ layers. Subsequently, additional displacements of the MnO53+ polyhedral and the Y3+ ions at 920 K took place and a spontaneous polarization along the [001] direction appears. At lower temperature of 70 K, the antiferromagnetic transition occurs [2]. In addition, characteristic domain structures consisting of six FE and structural antiphase domains appears in the ferroelectric phase, which can be identified as the “cloverleaf” pattern [3-5]. Recently, the domain wall structures in hexagonal manganites (LuMnO3 and TmMnO3) were investigated at the atomic scale [6]. In this study, in order to elucidate the structural characteristics of the domain walls in the FE phase of hexagonal YMnO3, we have applied high-angle annular-dark-field (HAADF) imaging technique to real-space imaging of the FE and structural antiphase domain walls in hexagonal YMnO3 and mapped the atomic shifts of Y and Mn ions. Two types of 180°domain walls can be identified. One is the transverse and longitudinal domain walls with atomic displacements of a/3 and the other is the transverse domain walls with atomic displacements of 2a/3. In contrast, the displacements related to the MnO5 polyhedral remain intact across the domain walls. These features of the FE and structural antiphase domain walls should be important to understand unusual physical properties in hexagonal manganites. In addition, we have investigated changes of the FE and structural antiphase domain walls by partial substitution of Ti4+ ions for Mn3+ ones in YMnO3. It is revealed that the FE displacement along the [001] direction disappear and correlation length of the Mn3+ trimerization becomes short less than 10 nm. [1] T. Katsufuji et al., Phys. Rev. B 66, 104419 (2001). [2] T. Lonkai et al., Phys. Rev. B 69, 134108 (2001). [3] T. Choi et al., Nat. Matter 9, 253 (2010). [4] M. Lillenblum et al., J. Appl. Phys., 110, 052007 (2011). [5] K. Kobayashi et al., (in press). [6] Q. H. Zhang et al., Phys. Rev. B 85, 020102(R), (2012).

To explore charge-controlled physics of transition metal oxide thin film heterostructures and develop new classes of materials with competing functionalities, understanding the atomic mechanism of structural order parameter changes has remained a goal of active research during last two decades. Plentiful theoretical studies of interface mediated coupling have been reported to decode the atomic mechanisms responsible for coupling phenomenon between polarization and spin exchange. However, direct atomic level observations of polarization and electronic phenomena have not kept pace. An approach combining quantitative aberration corrected scanning transmission electron microscopy (STEM) and advanced EELS techniques, providing atomic scale insight into structural order parameter changes and chemical behaviors, enables us to directly reveal multiple aspects of interface behavior. Here, we report the application of this approach to studies of BiFeO3 (BFO) )/(La,Sr)MnO3 (LSMO) interfaces where a change in local valence state can be detected for different polarization states of BFO. In this study we examined a 50-nm thick BFO film grown on 5-nm of MnO2-terminated LSMO on STO substrate. STEM EELS analysis (Nion UltraSTEM operated at 100 kV and equipped with Gatan Enfina EEL spectrometer) was used to verify the interface termination and evaluate cation intermixing. For opposite polarization orientations in BFO (i.e. downward and upward polarizations with respect to the interface), we discovered different valence states for near-interface Mn cations. Simultaneously conducted unit-cell-by-unit-cell mapping of structural distortions indicated that a local lattice parameter increase at the interface is observed only for downward polarization orientation, implying polarization charge driven localized oxygen vacancy injection. Atomic-scale observations as used in this study are therefore critical for uncovering the interplay of electronic, ferroelectric, and ionic/chemical aspects of interface behavior * This research is sponsored by the Division of Materials Sciences and Engineering (YMK, SJP, SVK, AYB), Office of BES of the U.S. DOE, and by appointment (YMK) to the ORNL Postdoctoral Research Program administered jointly by ORNL and ORISE. Instrument access via ShaRE user facility, which is supported at ORNL by Office of BES of the U.S. DOE, is gratefully acknowledged.

Doping with Al and O atoms in beta-Si3N4 network gives rise to constitute a new type of solid solution material called beta-SiAlON, exhibiting superior outstanding chemical, thermal and mechanical properties. The scanning transmission electron microscopy (STEM) observations of interstitial rare-earth atom (e.g., Eu, Yb and Ce) incorporation into beta(LEDs). On the other hand, recent analytical TEM studies at nanoscopic scale clearly showed the incorporation of transition metal atoms, e.g., Fe, Cr and Ti into beta-SiAlON crystal structure [1-2]. However, a simple question where do these atoms sit in the beta-SiAlON lattice is still not answered. For the sake of the clarity at the atomic-scale, we report the use of various state-of-the-art aberration-corrected STEM/TEM instruments for experimental results and first-principles calculation routes for theoretical studies on the fully densified gas pressure sintered SiAlON samples doped with transition metal atom Interestingly, Z (atomic number)-contrast atomic-resolved STEM images reveal that titanium (Ti) atoms substitute with silicon (Si) and/or aluminum (Al) atomic columns in the basic tetrahedron units of beta-SiAlON lattice. In addition, first principles calculation results strongly suggest the existence of Al and O impurities in beta-Si3N4 network plays a vital role on the doping capabilities of Ti atoms within beta-SiAlON lattice. It is anticipated that these acquired atomic-scale experimental and theoretical results, for the first time, will pave the way to new approaches in the production of transition metal atom-doped beta-SiAlON ceramics for different applications. [1] H. Yurdakul and S. Turan, Cer. Internationals, 37, 1501-1505 [2] H. Yurdakul and S. Turan, http://dx.doi.org/10.1016/j.ceramint.2012.04.028. [3] This research was supported by TUBITAK 2214-coded Ph.D. Scholarship Program (H.Y.) and DOE Basic Energy Sciences, ORNL-Division of Materials Science and Engineering (USA).

The plastic deformation of hexagonal close packed (hcp) metals is normally dominated by single slip (basal or prismatic) and twinning rather than cross-slip which frequently occurs in face-centered cubic (fcc) metals. While most fcc metals shows good ductility, a large range of ductility is found in hcp metals. Here we perform molecular dynamics simulations of hcp Mg and Zr to reveal the key deformation mechanisms of nucleation and interaction of slip and twinning. Mg and Zr have very different c/a ratios (Mg: 1.624 close to the ideal value of 1.633 and Zr: 1.93) and dominant deformation modes (Mg: basal, Zr: prismatic); they thus may be taken to represent typical properties in various hcp metals. Grain-boundary nucleated processes and conventional slip and twinning are compared in textured and random polycrystalline structures. This work of DK and SRP were supported by DOE NERI contract DE-FC07-07ID14833. The work of ZL and MJN was supported by the by the Consortium for Advanced Simulation of Light Water Reactors (www.casl.gov), an Energy Innovation Hub (http://www.energy.gov/hubs) for Modeling and Simulation of Nuclear Reactors under U.S. Department of Energy Contract No. DE-AC05-00OR22725. The work of MVM was sponsored by NSF Award Number DMR-0845868.

This work presents the results of a comparative molecular dynamics study using interatomic potentials describing fcc metals with different stacking fault energies (SFE). Fully periodic polycrystalline samples with an average 40nm grain size and random tilt boundaries were deformed to a 9% strain level. Low SFE materials present highly non-planar grain boundary structures. Grain boundaries with these non-planar structures emit dislocations at lower stresses than their higher SFE counterparts. The implications for the mechanical response of these materials are discussed.

Imaging light atoms requires optimization of electron optics, detector configuration and electron acceleration voltages. Here we report progress in imaging oxygen atoms at the interface of gold Nano Crystals (Au NCs) grown on rutile TiO2, which have attracted considerable interest for their remarkable size-dependent catalytic activities, especially for catalyzing the oxidation of carbon monoxide. Several mechanisms have been proposed to explain such surprising chemical activities and almost all of them involve interactions at the Au NCs and support interface, which renders the study of interfacial structure tremendously important. Scanning transmission electron microscopy (STEM) with annual dark-field (ADF) detector and aberration corrector (AC) is a powerful tool to characterize interfacial structures at atomic resolution, Au and Ti atoms can be resolved at 0.1 nm or better using high angle ADF. Oxygen atoms, however, are difficult to detect using ADF-STEM. Recent reports have shown that Annular Bright Field (ABF) STEM can be used to observe light atoms [1-3]. Here we explore the application of ABF-STEM for imaging interfacial light atoms. To enable high resolution AC-STEM investigation of NC interfaces, we have developed a novel NC synthesis technique for preparing Au NCs on vicinal TiO2 surfaces. Au nanoparticles about 2~5 nm in diameters, were first deposited onto the single crystal TiO2 substrate, pre-thinned and annealed in air, by e-beam evaporation deposition at room temperature. Then the sample was annealed in air at high temperatures to form epitaxial Au NCs. To detect the oxygen atoms, a JEOL JEM 2200FS AC-STEM was modified for ABF-STEM imaging. Optimal imaging conditions were tested by experiment and simulations, including the benefits of using lower acceleration electrons to avoid radiation damage. Oxygen atoms are resolved experimentally and the result was confirmed by multi-slice STEM simulation. To enhance the resolution, we performed spatial averaging during the processing of raw images, which can automatically detect and average the similar areas with a template from the raw image, effectively reduce the S/N ratio and hence improve the contrast. With above techniques, we then explored the possibility to detect oxygen atoms at the interface, in order to measure the interfacial registry of metal-oxides. The results will be presented and discussed in the background of gaining better understanding about interfacial effect in real catalysts. This work is supported by NSF the NSF Grant No. DMR 0449790. Electron microscopy was carried out in the Frederick Seitz Materials Research Laboratory Central Facilities, University of Illinois. References: [1] Findlay, S. D. et al. Ultramicroscopy 110, 903-923 (2010) [2] Findlay, S. D. et al. Ultramicroscopy 111, 285-289 (2011) [3] Ishikawa, R. et al. Nature Mater. 10, 278-281 (2011)

One family of the complex perovskite Ba3BNb2O9 has shown desirable dielectric properties in the field of microwave and satellite communications [1], and some have photocatalytic properties [2]. The cation ordering of these compounds have effect on the dielectric properties, where it was reported that stacking faults and domains are the cause of disordered state [3]. In our work, the physical properties at low temperatures down to 2 K have been measured and shown the materials having multiferroic properties. The cation ordering and disordering and microstructures of these compounds are studied by advanced scanning transmission electron microscope (STEM) techniques at atomic resolution, such as high angle annular dark field imaging (z-contrast imaging), annular bright field imaging, STEM spectrum imaging and electronic structures by electron energy loss spectroscopy. Ref: [1] H. Kagata, et. al., J. Appl. Phys. 33 (1994) 5463. [2] J. Jin, et. al., J. Phys. Chem. B 107 (2003) 4936. [3]Y. Liu et. al., Physica B 385 (2006) 564.

Adhesion of soft-materials onto surfaces of inorganic solids is a key aspect of designing composite material systems with controlled thermal and mechanical properties. To elucidate the role of dispersive coupling between adherent and substrate on resultant bi-material properties, we performed a comprehensive atomistic simulation study of adhesive properties of interfaces between aromatic rigid-rode poly-[(4,4'diphenylene) pyromellitimide] nanometer-size layers and a dimer-reconstructed silicon surface. The effects of interactions across the interface and interface-induced ordering on adhesion are scrutinized in detail via classical molecular dynamics and compared with experimental results, obtained using a novel laser-induced stress-waves technique. Ab initio calculations are employed to investigate the effect of point-like and linear surface defects on adhesive properties. Furthermore, to account for non-planarity of substrate surfaces, we developed theories of adhesion onto rough substrates, thereby quantifying the role of non-planarity in shaping the adhesive behavior. The implications of defects and the non-planar nature of the substrate for energetics of adhesive coupling are discussed in detail, within the frameworks of methods employed.

Direct Atom-resolved Imaging of Ordered Defect Superstructures at Oxide Grain Boundaries Zhongchang Wang,1,* Mitsuhiro Saito,1 Susumu Tsukimoto,1 and Yuichi Ikuhara1,2 1WPI, Advanced Institute for Materials Research, Tohoku University, Sendai, Japan 2Institure of Engineering Innovation, The University of Tokyo, Tokyo, Japan The ability to spatially resolve and chemically identify atoms in complex defect regions would bring atomistic understanding of structure-property interplay closer to reality [1]. This is particular significant for the widely used polycrystalline materials, where their prevalent intercrystalline defects, i.e., grain boundaries (GBs), are universally implicated in affecting remarkably their properties and hence many of their applications. However, this resolving is still extremely challenging [2] because GBs have long been known to serve as effective sinks for atomic defects and impurities, which in turn may drive structural transformation of GBs, resulting in modification of material properties. Regardless of origin of the segregated defects, such intricate interplay associated with the embedded nature of GB complicates our efforts to pinpoint exact site and chemistry of the entities in the defective regions, thereby hindering the development of a deeper understanding of how specific defects mediate property shift. Here, we show that advanced transmission electron microscopy, spectroscopy, and first-principles calculations can provide three-dimensional images of complex multi-component GBs with both atomic resolution and chemical sensitivity. The unprecedented resolution of these techniques allows us to demonstrate that, even for the simple cubic oxide MgO, GBs can accommodate complex ordered defect superstructures that induce pronounced electron trapping states in band gap of MgO [3]. These results point to existence of new impacts associated with defect-GB interactions in ceramics, and demonstrate that atomic scale analysis of complex multi-component structures insides materials is now becoming possible. [1] P. D. Nellist et al., Science, 305, 1741 (2004). [2] C. L. Jian and K. Urban, Science, 303, 2001-2004 (2004). [3] Z.C. Wang, M. Saito, K. P. McKenna, L. Gu, S. Tsukimoto, A. L. Shluger, and Y. Ikuhara, Nature 479, 380 (2011).

In recently discovered intercalated iron-selenide superconductors AFe2-xSe2 (A= K, Li, Na, Rbhellip;), superconductivity with a Tc of 30 K appears to coexist with an exceptionally strong antiferromagnetism and large magnetic moment on the Fe sites. This finding has generated intense experimental efforts in order to understand the relationships between superconductivity, magnetism and structure in these materials. In spite of the large chemical and structural similarity between AFe2-xSe2 and the prototypical iron arsenide superconductors Ba(K)Fe(Co)2As2 (122 Fe-pnictides), it has not been possible to synthesize the former compounds as electronically homogeneous superconducting single crystals. Several experiments have suggested the existence of spatially separated superconducting and antiferromagnetic phases, which might be related to the presence and/or order of Fe vacancies. Based on the investigation of the closely related TlFe1.6Se2 compound, we provide evidence that nanoscale phase separation in ordered and disordered vacancy regions leads to a different magnetic ground state than the ground state of single-phase counterparts, suggesting that magnetoelastic coupling between different phases is important for the emergence of superconductivity in AFe2-xSe2. Here we show how real-space, aberration-corrected scanning transmission electron microscopy can be a critical tool for investigating nanoscale, crystallographically coherent phase separation into ordered and disordered regions. We will show distinctive signatures in the electron energy loss spectroscopy (EELS) fine structure of the Fe-L2,3 edges for these compounds and discuss the results in light of a comparison with 122 Fe -pnictides. Research sponsored by US DOE Office of Science, Materials Science and Engineering Division.

Most oxides, whether bulk or thin film, are generally prepared in polycrystalline form. Therefore, it is not surprising to find that grain boundaries often play a key role in controlling charge and mass transport through these materials. The exact role that they play depends on the nature of the grain boundary stoichiometry, the charge and size of the segregants and impurities, the degree of coverage by second phases and the grain boundary to volume ratio. In this presentation, we focus on semiconducting or mixed conducting (ionic + electronic) oxides in which space charge fields are present in the vicinity of the boundaries. In the case of ZnO, depletion of electrons near the grain boundaries results in space charge barriers and an overall reduction in conductance by as much as 10 orders of magnitude. Here we utilize high cation diffusion along the grain boundaries to transport barrier creating dopants to activate specific boundaries. In the case of nanostructured CeO₂, we attempt instead to create an accumulation layer at the boundaries to enhance ion transport along those boundaries. In the final example, we investigate the impact of segregants on charge transfer across a heterointerface between a mixed conductor and adsorbed gas molecules.

An existing barrier to the development of high performance solid oxide fuel cells is the poor ionic conductivity at room temperature of typical oxide-ion conductors, making high operating temperatures necessary for oxide ion mobility to be sufficiently fast for operational devices. One strategy for improved room temperature ionic conductivities is to exploit the "nanoionic" effects demonstrated by thin films and heterostructures, where enhanced ionic conductivities have been reported compared to corresponding bulk samples. The canonical example when considering heterostructures is that of CaF₂|BaF₂ [1]. As the thickness of the domains is reduced, ionic conductivity increases, attributed to the increasing contribution of space charge regions; with these predicted to form as a consequence of the fluoride ion chemical potential differing between the two component phases, driving a separation of Frenkel pairs across the CaF₂|BaF₂ interfaces [2]. CaF₂ and BaF₂ both adopt the fluorite structure, but have different lattice parameters, giving rise to epitaxial strain or mismatching at interfaces. This strain can be relieved by the formation of dislocations, which may facilitate enhanced conduction of ions [3]. Even in the absence of extended defects, strain can be expected to modify the energies of point defects and their diffusion barriers relative to the corresponding bulk materials, and it is not clear to what extent experimentally observed conductivity enhancements can be attributed to space charge formation or strain effects in this heterostructured material. We have performed atomistic simulations of a model PbF₂|PbF₂ heterostructure as a "strain-free" structural analogue to the CaF₂|BaF₂ system. PbF₂ undergoes a superionic transition at high temperature, and is fluorite structured. By including an adjustable difference in the chemical potential of the fluoride ions, Δmu;(F^-), between neighbouring domains, we have modelled the redistribution of mobile fluoride ions and corresponding change in ionic conductivity produced in response to an interfacial difference in chemical potential in an ideal system, and have assessed how this behaviour varies as a function of the magnitude of Δmu;(F^-). [1] Sata, N., Eberman, K., Eberl, K. & Maier, J. Nature 408, 946-949 (2000). [2] Guo, X. & Maier, J. Adv. Mater. 21, 2619-2631 (2009). [3] Jin-Phillipp, N. Y. & Phillipp, F. Philosoph. Mag. 84, 3509-3516 (2004).

Proton-conducting Y-doped BaZrO3 combines a high chemical stability with good bulk conductivity. Nevertheless, the small grain size (< 1 micrometer) and blocking character of the grain boundaries still impede a broader application, e.g. in intermediate temperature fuel cells. Dense Y- and Sc-doped BaZrO3 ceramics were obtained by "Spark-Plasma-Sintering" at 1600°C (5 min, 50 MPa). Additional long-time annealing (20 h at 1700°C) decreases the grain boundary resistance by 1-3 orders of magnitude, while grain size and bulk conductivity remain constant. TEM-EDXS analysis indicates a pronounced dopant accumulation in the grain boundary region for the annealed samples [1]. The comparable degree of segregation for Sc3+ (negligible size mismatch to Zr) and Y3+ (large mismatch) suggests a positive excess charge in the grain boundary core as main driving force for the dopant segregation. Within the space charge model, this dopant accumulation in the grain boundary core and/or the space charge zone will decrease proton depletion and thus increase total conductivity. Further evidence for the validity of the space charge model is obtained from impedance measurements under large DC bias [2]. An extremely large-grained sample with >100 micrometer grain size and Pt microcontacts allows to measure impedance spectra across individual grain boundaries. DC bias values of up to 1 V per boundary strongly decrease the grain boundary resistance as well as capacitance. Both findings support the model that a space charge depletion zone for protons (and oxygen vacancies) is the main reason for the blocking behavior of the grain boundaries. Grain boundaries in other acceptor-doped perovskites with large band gap (Fe-doped SrTiO3 [3], (La,Sr)GaO3 [4]) also exhibit a positve excess charge in the grain boundary core, which is caused by the segregation of oxygen vacancies to the core. The similar behavior of BaZrO3 grain boundaries suggests that a high excess vacancy concentration in the core, favored by geometric constraints in the distoretd core structure, is a quite common feature for perovskites. Possibilities for decreasing the positive core charge will be discussed. [1] M. Shirpour et al., J. Phys. Chem. C 116 (2012) 2453 [2] M. Shirpour et al., Phys. Chem. Chem. Phys. 14 (2012) 730 [3] R.A. De Souza, J. Fleig, J. Maier, O. Kienzle, Z.L. Zhang, W. Sigle, M. Rühle, J. Am. Ceram. Soc. 86 (2003) 922 [4] H.J. Park, S. Kim, J. Phys. Chem. C 111 (2007) 14903

Experimentally, it is known that small amounts (up to 200 wppm) of thorium added to iridium metal segregates to grain boundaries and forms a precipitate, Ir₅Th, which refines grains, increases inter-grain cohesion and thereby increases the high temperature ductility of resulting alloys. Cerium is similar in many ways: it is chemically similar (in the same column of the periodic table as thorium but one row lower), segregates to grain boundaries, and forms an isomorphic grain pinning precipitate, Ir₅Ce. Despite these similarities, cerium nonetheless fails to provide as much cohesion between grains as thorium. Traditional models of impurity effects at grain boundaries fail to satisfactorily explain these contrasting behaviors of thorium and cerium doped iridium alloys. We develop alternative models which focus on the interfaces between phases in the two phase system, Ir and the intermetallic precipitate. First principles calculations show that there is a structural similarity between the Ir and precipitate phases, which leads to a natural orientation relationship, a small interfacial energy, and a low lattice misfit. Ir₅Th has a strong formation energy, relative to the pure Th and Ir phases; in contrast, Ir₅Ce has a significantly weaker binding. Various models of thin precipitate phases are studied, demonstrating that particularly for Ir₅Th, nanolayers of the precipitate are likely to form, with little driving force to coarsen. In contrast, the Ir₅Ce phases are less likely to form, particularly as nanolayers, due to their energetics. Research supported by the U.S. Department of Energy (DOE), Basic Energy Sciences, Materials Sciences and Engineering Division.

Interfaces play a crucial role in materials behavior and their evolution is of particular interest to understand the kinetics processes taking place during irradiation in the case of structural nuclear materials, or during high temperature exposure in the case of Ni based superalloys or Zr alloys. Atomic scale characterization including transmission electron microscopy and atom probe tomography are used to probe structure and chemistry of specific interfaces in materials under dynamic conditions. Several examples will be discussed, which will include radiation can induce non-equilibrium segregation (RIS) of solutes in ferritic steels, the growth of oxide films on Ni base superalloys and the development of oxides on Zr alloys.

Grain boundaries have long been known as having pervasive influences on various bulk properties, and such effects tend to dominate in most practically important nanocrystalline devices. Grain boundary doping has been demonstrated as a powerful method to alter the atomic and electronic structures,[1-2] therefore enables us to have an effective control over various material properties. As an example in this research, we focus on the rear earth doped titanate, praseodymium (Pr) and Europium (Eu) doped SrTiO3, which have been shown to have luminescence[3-4] and high temperature ferroelectric[5] properties. In this research, a series of undoped, Pr doped and Eu doped SrTiO3 bicrystals containing grain boundaries were fabricated with diffusion bonding technique[6]. Atomic structures of these grain boundaries were characterized using aberration corrected scanning transmission electron microscope (STEM), and the optical and dielectric properties were analyzed from electron energy loss spectroscopy (EELS). Based on these analyses, direct comparison of the atomic structures and optical properties in different dopant systems were obtained. Variations in the grain boundary atomic structure were analyzed statistically using the Z-contrast STEM-HAADF images. In the undoped grain boundary, two distinct atomic arrangements were discovered, one being a symmetric structure and the other being rigid-body shift structure that contains a rigid-body translation along the boundary plane, and statistical analysis of the grain boundary population shows the rigid-body shift structure is the prevalent structure. In the case of Pr doped and Eu doped grain boundaries however, the symmetric structure is the prevalent structure, but with different segregation behaviors. Further discussions on EELS analysis and the doping mechanism from first principles calculations will be shown in the presentation. [7] References: [1] Buban, J.P., et al., Science. 311 (2006) 212. [2] Klie, R.F., et al., Nature. 435 (2005) 475. [3] Jiang, C.G., et al., Appl. Phys. Lett. 94 (2009) [4] Okamoto, S. and H. Yamamoto, J. Lumines. 102 (2003) 586. [5] Duran, A., et al., J. Appl. Phys. 97 (2005) [6] Yamamoto, T., et al., J. Am. Ceram. Soc. 83 (2000) 1527. [7] This work is supported by the United States Department of Energy Grant No. DE-FG02-03ER46057. A portion of the research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory, which is operated by Battelle for the U.S. Department of Energy under contract DE-AC05-76RL01830. Part of this work was also conducted in Research Hub for Advanced NanoCharacterization, The University of Tokyo, supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.