Atomic-scale structure of misfit dislocations in CeO2/MgO heterostructures and thermodynamic stability of dopant-defect complexes at the heterointerface.

Complex oxide heterostructures and thin films have found applications across the board in some of the most advanced technologies, wherein the interfaces between the two mismatched oxides influence novel functionalities. It is imperative to comprehend the atomic-scale structure of misfit dislocations, which are ubiquitous in semi-coherent oxide heterostructures, and obtain a fundamental understanding of their interaction with point defects and dopants to predict and control their interface-governed properties. Here, we report atomistic simulations elucidating the atomic-scale structure of misfit dislocations in CeO2/MgO heterostructures. Our results demonstrate that the 45° rotation of CeO2 thin film is one of the potential fundamental mechanisms responsible for eliminating the surface dipole, leading to the experimentally observed mixed epitaxial relationship. We further report the thermodynamic stability of diverse dopant-defect complexes near misfit dislocations, wherein various scenarios for nearest neighbor bonding environments within the complexes are explored. Complex misfit dislocation structure, asymmetry, strain, and the availability of diverse nearest neighbor bonding environments between dopants and oxygen defects at the interface are accountable for a wide dispersion in energies within a given dopant-defect arrangement. As opposed to the bulk, the thermodynamic stability of oxygen vacancies is found to be sensitive to the dopant arrangement at the heterointerface. Extended stabilities of dopant-defect complexes at misfit dislocations reveal that they would influence ionic transport at heterointerfaces of fluorite-structured thin film electrolytes. Notably, the results herein offer a fundamental atomic-scale perspective of the intricate interplay between dopants, defects, and misfit dislocations at the heterointerfaces in mismatched oxide heterostructures.

On the mobility of carriers at semi-coherent oxide heterointerfaces.

In the quest to develop new materials with enhanced ionic conductivity for battery and fuel cell applications, nano-structured oxides have attracted attention. Experimental reports indicate that oxide heterointerfaces can lead to enhanced ionic conductivity, but these same reports cannot elucidate the origin of this enhancement, often vaguely referring to pipe diffusion at misfit dislocations as a potential explanation. However, this highlights the need to understand the role of misfit dislocation structure at semi-coherent oxide heterointerfaces in modifying carrier mobilities. Here, we use atomistic and kinetic Monte Carlo (KMC) simulations to develop a model of oxygen vacancy migration at SrTiO3/MgO interfaces, chosen because the misfit dislocation structure can be modified by changing the termination chemistry. We use atomistic simulations to determine the energetics of oxygen vacancies at both SrO and TiO2 terminated interfaces, which are then used as the basis of the KMC simulations. While this model is approximate (as revealed by select nudged elastic band calculations), it highlights the role of the misfit dislocation structure in modifying the oxygen vacancy dynamics. We find that oxygen vacancy mobility is significantly reduced at either interface, with slight differences at each interface due to the differing misfit dislocation structure. We conclude that if such semi-coherent oxide heterointerfaces induce enhanced ionic conductivity, it is not a consequence of higher carrier mobility.

Disorder-induced transition from grain boundary to bulk dominated ionic diffusion in pyrochlores.

We use molecular dynamics simulations to investigate the role of grain boundaries (GBs) on ionic diffusion in pyrochlores, as a function of the GB type, chemistry of the compound, and level of cation disorder. We observe that the presence of GBs promotes oxygen transport in ordered and low-disordered systems, as the GBs are found to have a higher concentration of mobile carriers with higher mobilities than in the bulk. Thus, in ordered samples, the ionic diffusion is 2D, localized along the grain boundary. When cation disorder is introduced, bulk carriers begin to contribute to the overall diffusion, while the GB contribution is only slightly enhanced. In highly disordered samples, the diffusive behavior at the GBs is bulk-like, and the two contributions (bulk vs. GB) can no longer be distinguished. There is thus a transition from 2D/GB dominated oxygen diffusivity to 3D/bulk dominated diffusivity versus disorder in pyrochlores. These results provide new insights into the possibility of using internal interfaces to enhance ionic conductivity in nanostructured complex oxides.

The role of surfaces, chemical interfaces, and disorder on plutonium incorporation in pyrochlores.

Pyrochlores, a class of complex oxides with formula A2B2O7, are one of the candidates for nuclear waste encapsulation, due to the natural occurrence of actinide-bearing pyrochlore minerals and laboratory observations of high radiation tolerance. In this work, we use atomistic simulations to determine the role of surfaces, chemical interfaces, and cation disorder on the plutonium immobilization properties of pyrochlores as a function of pyrochlore chemistry. We find that both Pu(3+) and Pu(4+) segregate to the surface for the four low-index pyrochlore surfaces considered, and that the segregation energy varies with the chemistry of the compound. We also find that pyrochlore/pyrochlore bicrystals A2B2O7/A2'B2'O7 can be used to immobilize Pu(3+) and Pu(4+) either in the same or separate phases of the compound, depending on the chemistry of the material. Finally, we find that Pu(4+) segregates to the disordered phase of an order/disorder bicrystal, driven by the occurrence of local oxygen-rich environments. However, Pu(3+) is weakly sensitive to the oxygen environment, and therefore only slightly favors the disordered phase. This behavior suggests that, at some concentration, Pu incorporation can destabilize the pyrochlore structure. Together, these results provide new insight into the ability of pyrochlore compounds to encapsulate Pu and suggest new considerations in the development of waste forms based on pyrochlores. In particular, the phase structure of a multi-phase pyrochlore composite can be used to independently getter decay products based on their valence and size.

Structure and segregation of dopant-defect complexes at grain boundaries in nanocrystalline doped ceria.

Grain boundaries (GBs) dictate vital properties of nanocrystalline doped ceria. Thus, to understand and predict its properties, knowledge of the interaction between dopant-defect complexes and GBs is crucial. Here, we report atomistic simulations, corroborated with first principles calculations, elucidating the fundamental dopant-defect interactions at model GBs in gadolinium-doped and manganese-doped ceria. Gadolinium and manganese are aliovalent dopants, accommodated in ceria via a dopant-defect complex. While the behavior of isolated dopants and vacancies is expected to depend on the local atomic structure at GBs, the added structural complexity associated with dopant-defect complexes is found to have key implications on GB segregation. Compared to the grain interior, energies of different dopant-defect arrangements vary significantly at the GBs. As opposed to bulk, the stability of oxygen vacancy is found to be sensitive to the dopant arrangement at GBs. Manganese exhibits a stronger propensity for segregation to GBs than gadolinium, revealing that accommodation of dopant-defect clusters depends on the nature of dopants. Segregation strength is found to depend on the GB character, a result qualitatively supported by our experimental observations based on scanning transmission electron microscopy. The present results indicate that segregation energies, availability of favorable sites, and overall stronger binding of dopant-defect complexes would influence ionic conductivity across GBs in nanocrystalline doped ceria. Our comprehensive investigation emphasizes the critical role of dopant-defect interactions at GBs in governing functional properties in fluorite-structured ionic conductors.

Termination chemistry-driven dislocation structure at SrTiO3/MgO heterointerfaces.

Exploiting the promise of nanocomposite oxides necessitates a detailed understanding of the dislocation structure at the interfaces, which governs diverse and technologically relevant properties. Here we report atomistic simulations demonstrating a strong dependence of the dislocation structure on the termination chemistry at the SrTiO3/MgO heterointerface. The SrO- and TiO2-terminated interfaces exhibit distinct nearest neighbour arrangements between cations and anions, leading to variations in local electrostatic interactions across the interface that ultimately dictate the dislocation structure. Networks of dislocations with different Burgers vectors and dislocation spacing characterize the two interfaces. These networks in turn influence the overall stability of and the behaviour of oxygen vacancies at the heterointerface, which will dictate vital properties such as mass transport at the interface. To date, the observed correlation between the dislocation structure and the termination chemistry at the interface has not been recognized, and offers novel avenues for fine-tuning oxide nanocomposites with enhanced functionalities.

Defect interactions with stepped CeO2/SrTiO3 interfaces: implications for radiation damage evolution and fast ion conduction.

Due to reduced dimensions and increased interfacial content, nanocomposite oxides offer improved functionalities in a wide variety of advanced technological applications, including their potential use as radiation tolerant materials. To better understand the role of interface structures in influencing the radiation damage tolerance of oxides, we have conducted atomistic calculations to elucidate the behavior of radiation-induced point defects (vacancies and interstitials) at interface steps in a model CeO2/SrTiO3 system. We find that atomic-scale steps at the interface have substantial influence on the defect behavior, which ultimately dictate the material performance in hostile irradiation environments. Distinctive steps react dissimilarly to cation and anion defects, effectively becoming biased sinks for different types of defects. Steps also attract cation interstitials, leaving behind an excess of immobile vacancies. Further, defects introduce significant structural and chemical distortions primarily at the steps. These two factors are plausible origins for the enhanced amorphization at steps seen in our recent experiments. The present work indicates that comprehensive examination of the interaction of radiation-induced point defects with the atomic-scale topology and defect structure of heterointerfaces is essential to evaluate the radiation tolerance of nanocomposites. Finally, our results have implications for other applications, such as fast ion conduction.

Point defect-grain boundary interactions in MgO: an atomistic study.

The way in which point defects interact with grain boundaries in oxides is important for understanding radiation damage evolution, sintering, and many other technologically important applications. Here, we examine how vacancies interact with three different grain boundaries in MgO, chosen as a model oxide ceramic. Further, we compare the vacancy interaction with both pristine (as constructed) and 'damaged' boundaries, in which excess interstitials are placed in the boundary plane to mimic the structure after a damage event. We find that the excess interstitials significantly change the interaction of the vacancies with the boundaries and that this change is sensitive to the atomic structure of the boundary. We further observe that complex electrostatic effects arise that can dominate the interaction. These results show that, as boundaries absorb defects, their interaction with other defects will change dramatically.

Electronic structure and quantum dynamics of photoinitiated dissociation of O2 on rutile TiO2 nanocluster.

The adsorption and photoinitiated dissociation of molecular oxygen on reduced rutile TiO2 nanocluster have been studied using a hybrid density functional theory (DFT)/time-dependent DFT approach and a time-dependent wavepacket dynamics method. Results show that the most favorable state for O2 at the bridging row O-vacancy site of TiO2 is O2(2-) with an orientation parallel to the surface. We find that its dissociation in the electronic ground state involves a spin forbidden intersystem crossing, and therefore has a large barrier along the reaction pathway. However, time-dependent wavepacket calculations reveal that the photoinitiated O2 dissociation on TiO2 is very fast via a direct mechanism on the excited states. The lifetime of excited O2 molecules is predicted to be about 266 fs. Non-adiabatic effects among the singlet electronic states are found to play an important role in the O2 dissociation whereas the spin-orbit effect is negligible. In addition, adsorption of two O2 molecules at an O-vacancy site shows that the second O2 molecule can stabilize the system by about 0.22 eV.

Exploring the ring-opening pathways in the reaction of morpholinyl radicals with oxygen molecule.

Quantum chemistry calculations using hybrid density functional theory and the coupled-cluster method have been performed to investigate the ring-opening pathways in the oxidation of morpholine (1-oxa-4-aza-cyclohexane). Hydrogen abstraction can form two different carbon-centered radicals, morpholin-2-yl or morpholin-3-yl, or the nitrogen-centered radical, morpholin-4-yl, none of which are found to have low-energy pathways to ring-opening. Extensive exploration of multiple reaction pathways following molecular oxygen addition to these three radicals revealed two competitive low energy pathways to ring-opening. Addition of O(2) to either carbon-centered radical, followed by a 1,4-H shifting mechanism can yield a long-lived cyclic epoxy intermediate, susceptible to ring-opening, following further radical attack. In particular, the second pathway begins with O(2) attack on morpholin-2-yl, followed by a 1,5-H shift and a unimolecular ring-opening without having to overcome a high barrier, releasing a significant amount of heat in the overall ring-opening reaction. The calculations provide valuable context for the development of mechanisms for the low temperature combustion chemistry of nitrogen and oxygen-containing fuels.

A density functional study of defect migration in gadolinium doped ceria.

Oxygen ion conductivity of doped ceria is observed to be two-three orders of magnitude higher than yttria stabilized zirconia, the most widely used solid electrolyte material at temperatures below 600 degrees C. Gadolinium doped ceria (GDC) is known to be one of the most promising solid electrolyte materials for operation of solid oxide fuel cells below 600 degrees C. To understand the atomic defect migration in GDC, we have used total energy calculations within the framework of density functional theory to follow oxygen vacancy migration in GDC. We report activation energies for various oxygen vacancy migration pathways in GDC. Oxygen vacancy formation and migration were evaluated for first, second, and third nearest neighbor positions to a Gd(3+) ion. Due to the comparable ionic radii of Gd(3+) and host Ce(4+) ions, the first nearest neighbor site with respect to the dopant cation is found to be the most favorable oxygen vacancy formation site. The migration pathway where the vacancy migrates from a second to first nearest neighbor is found to be most favorable. The calculated activation energies for oxygen vacancy migration in GDC are compared against the reported measured and calculated values from the literature. This work will provide a foundation for the development of a kinetic lattice Monte Carlo model for vacancy diffusion in GDC, which will improve the understanding of oxygen ion conductivity in doped ceria.

Oxygen vacancy migration in ceria and Pr-doped ceria: a DFT+U study.

Oxygen vacancy formation and migration in ceria (CeO(2)) is central to its performance as an ionic conductor. It has been observed that ceria doped with suitable aliovalent cationic dopants improves its ionic conductivity. To investigate this phenomenon, we present total energy calculations within the framework of density functional theory to study oxygen vacancy migration in ceria and Pr-doped ceria (PDC). We report activation energies for oxygen vacancy formation and migration in undoped ceria and for different migration pathways in PDC. The activation energy value for oxygen vacancy migration in undoped ceria was found to be in reasonable agreement with the available experimental and theoretical results. Conductivity values for reduced undoped ceria calculated using theoretical activation energy and attempt frequency were found in reasonably good agreement with the experimental data. For PDC, oxygen vacancy formation and migration were investigated at first, second, and third nearest neighbor positions to a Pr ion. The second nearest neighbor site is found to be the most favorable vacancy formation site. Vacancy migration between first, second, and third nearest neighbors was calculated (nine possible jumps), with activation energies ranging from 0.41 to 0.78 eV for first-nearest-neighbor jumps. Overall, the presence of Pr significantly affects vacancy formation and migration, in a complex manner requiring the investigation of many different migration events. We propose a relationship illuminating the role of additional dopants toward lowering the activation energy for vacancy migration in PDC.