UC Berkeley

The goal of this dissertation is to employ computational methods to gain greater insights into the energetics and defect interactions of complex oxides that are relevant for today's energy challenges. To achieve this goal, the development of novel computational methodologies are required to handle complex systems, including systems containing nearly 650 ions and systems with tens of thousands of possible atomic configurations. The systems that are investigated in this dissertation are aliovalently doped lanthanum orthophosphate (LaPO4) due to its potential application as a proton conducting electrolyte for intermediate temperature fuel cells, and aliovalently doped uranium dioxide (UO2) due to its importance in nuclear fuel performance and disposal.

First we undertake density-functional-theory (DFT) calculations on the relative energetics of pyrophosphate defects and protons in LaPO4, including their binding with divalent dopant cations. In particular, for supercell calculations with 1.85 mol% Sr doping, we investigate the dopant-binding energies for pyrophosphate defects to be 0.37 eV, which is comparable to the value of 0.34 eV calculated for proton-dopant binding energies in the same system. These results establish that dopant-defect interactions further stabilize proton incorporation, with the hydration enthalpies when the dopants are nearest and furthest from the protons and pyrophosphate defects being -1.66 eV and -1.37 eV, respectively. Even though our calculations show that dopant binding enhances the enthalpic favorability of proton incorporation, they also suggest that such binding is likely to substantially lower the kinetic rate of hydrolysis of pyrophosphate defects.

We then shift our focus to solid solutions of fluorite-structured UO2 with trivalent rare earth fission product cations (M3+=Y, La) using a combination of ionic pair potential and DFT based methods. Calculated enthalpies of formation with respect to constituent oxides show higher energetic stability for La solid solutions than for Y. Additionally, calculations performed for different atomic configurations show a preference for reduced (increased) oxygen vacancy coordination around La (Y) dopants. The current results are shown to be qualitatively consistent with related calculations and calorimetric measurements of heats of formation in other trivalent doped fluorite oxides, which show a tendency for increasing

stability and increasing preference for higher oxygen coordination with increasing size of the trivalent impurity. We expand this investigation by considering a series of trivalent rare earth fission product cations, specifically, Y3+ (1.02 A, Shannon radius with eightfold coordination), Dy3+ (1.03 A), Gd3+ (1.05 A), Eu3+ (1.07 A), Sm3+ (1.08 A), Pm3+ (1.09 A), Nd3+ (1.11 A), Pr3+ (1.13 A), Ce3+ (1.14 A) and La3+ (1.16 A). Compounds with ionic radius of the M3+ species smaller or larger than 1.09 A are found to have energetically preferred defect ordering arrangements. Systems with preferred defect ordering arrangements are suggestive of defect clustering in short range ordered solid solutions, which is expected to limit oxygen ion mobility and therefore the rate of oxidation of spent nuclear fuel.

Finally, the energetics of rare earth substituted (M3+= La, Y, and Nd) UO2 solid solutions are investigated by employing a combination of calorimetric measurements and DFT based computations. The calorimetric studies are performed by Lei Zhang and Professor Alexandra Navrotsky at the University of Calfornia, Davis, as part of a joint computational/

experimental collaborative effort supported through the Materials Science of Actinides Energy Frontier Research Center. Calculated and measured formation enthalpies agree within 10 kJ/mol for stoichiometric oxygen/metal compositions. To better understand the factors governing the stability and defect binding in rare earth substituted urania solid solutions, systematic trends in the energetics are investigated based on the present results and previous computational and experimental thermochemical studies of rare earth substituted fluorite oxides. A consistent trend towards increased energetic stability with larger size mismatch between the smaller host tetravalent cation and the larger rare earth

trivalent cation is found for both actinide and non-actinide fluorite oxide systems where aliovalent substitution of M cations is compensated by oxygen vacancies. However, the large exothermic oxidation enthalpy in the UO2 based systems favors compositions with higher oxygen-to-metal ratios where charge compensation occurs through the formation of uranium cations with higher oxidation states.