First-principles studies of proton-Ba interactions in doped LaPO4

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The interactions between an aliovalent cation dopant, Ba, and protons in LaPO4 are studied with first-principles density functional theory. This work is motivated by the desire to use doped LaPO4 as a proton conducting solid electrolyte or hydrogen separation membrane. In this context, the strength and range of proton-dopant interactions are important factors underlying proton mobilities. Using periodic supercells, we find that ∼3% Ba-doping stabilizes a proton at a distance 2.7 A from the dopant by up to 0.2 eV relative to positions far from the dopant. The Ba-dopant creates a narrow potential energy well and only changes the proton's potential energy surface by ±0.05 eV when the proton is farther than 2.7 A from the dopant. Electrostatic interactions between the proton and dopant account for the majority of the binding energy of proton sites and are associated with a complex redistribution of the charge induced by the dopant on the neighboring oxygen ions. In contrast, the strain field created by the Ba-dopant gives rise to a relatively small contribution to the interaction energy.