Pratik P. Dholabhai

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.

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.

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.

Predicting the optimal dopant concentration in gadolinium doped ceria: a kinetic lattice Monte Carlo approach

Gadolinium doped ceria (GDC) is a promising alternative electrolyte material for solid oxide fuel cells that offers the possibility of operation in the intermediate temperature range (773?1073?K). To determine the optimal dopant concentration in GDC, we have employed a systematic approach of applying a 3D kinetic lattice Monte Carlo (KLMC) model of vacancy diffusion in conjunction with previously calculated activation energies for vacancy migration in GDC as inputs. KLMC simulations were performed including the vacancy repelling effects in GDC. Increasing the dopant concentration increases the vacancy concentration, which increases the ionic conductivity. However, at higher concentrations, vacancy?vacancy repulsion impedes vacancy diffusion, and together with vacancy trapping by dopants decreases the ionic conductivity. The maximum ionic conductivity is predicted to occur at ?20% to 25% mole fraction of Gd dopant. Placing Gd dopants in pairs, instead of randomly, was found to decrease the conductivity by ?50%. Overall, the trends in ionic conductivity results obtained using the KLMC model developed in this work are in reasonable agreement with the available experimental data. This KLMC model can be applied to a variety of ceria-based electrolyte materials for predicting the optimum dopant concentration.

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.

In search of enhanced electrolyte materials: a case study of doubly doped ceria

Various compositions of gadolinium-praseodymium doubly doped ceria (GPDC) have been studied to elucidate the effect of two co-dopants in enhancing the ionic conductivity. A Kinetic Lattice Monte Carlo (KLMC) model of vacancy diffusion in GPDC has been developed, which uses activation energies obtained from DFT-calculations for vacancy migration in gadolinium-doped ceria (GDC) and praseodymium-doped ceria (PDC) as input. In order to identify the optimal composition of electrolyte materials for solid oxide fuel cells, three different classes of GPDC were studied; (i) Gd rich, (ii) Pr rich and (iii) equal Gd-Pr content. It is assumed that the Gd and Pr are 100% ionized to Gd3+ and Pr3+. KLMC simulations showed that GPDC compositions with ≈0.20 mole fraction to 0.25 mole fraction of total dopant content exhibited the maximum ionic conductivity. Among the three classes studied, Gd-rich GPDC is found to have the highest conductivity for temperatures ranging from 873 K to 1073 K. The optimal co-doped compositions were found to be slightly temperature dependent. Analysis of vacancy migration pathways for millions of jump events show that GPDC has a slightly higher number of next neighbor jumps, which seems to explain most of the reason why GPDC has a higher ionic conductivity than PDC or GDC. The current KLMC calculations present a novel approach to study doubly doped ceria, as so far the theoretical results for ceria-based materials have been limited to mono-doped ceria.

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.