Energetics of Protonic Species in Yttrium-doped Barium Zirconate: A Density Functional Theory Study
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Energetics of Protonic Species in Yttrium-doped Barium Zirconate: A Density Functional Theory Study Massimo Malagoli1 , M.L. Liu2 , Hyeon Cheol Park3 , and Angelo Bongiorno1 1
School of Chemistry & Biochemistry, Georgia Institute of Technology, Atlanta, GA 303320400, U.S.A. 2
School of Material Science & Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, U.S.A. 3
Advanced Materials Research Center Samsung Advanced Institute of Technology (SAIT), San 14-1, Nongseo-dong, Yongin-si 446-712, Republic of Korea
ABSTRACT Density functional theory calculations are used to study the equilibrium energetics of protons on the surface and in the bulk of Y-doped BaZrO3 . It is shown that protonic species in direct contact with Y dopants have energies lower than in perfect BaZrO3 by up to 0.4 eV. This energetic stabilization is achieved when the protonic species is in direct contact with two Y dopants. On the (001) surface of BaZrO3, protonic species are found to be energetically more stable than in the bulk by 1.1 eV and 1.6 eV on the BaO and ZrO2 surface terminations, respectively. At these terminations, the energy of protons recover the bulk value after penetrating three surface layers, and the energy cost associated with bulk incorporation is larger than 1 eV. INTRODUCTION Several perovskite-type oxides with the general formula ABO3 exhibit significant proton conductivity at elevated temperatures and are potential candidates as electrolyte materials in various electrochemical applications [1, 2]. One of such oxides is BaZrO3 , a perovskite with the ideal cubic structure over a wide range of temperatures, high chemical stability, and a large electronic band gap, and thus a material suited to be used as electrolyte membrane in proton ceramic fuel cells [3, 4]. Proton migration in BaZrO3 (henceforth abbreviated BZ) has been extensively investigated by both theory [5–8] and experiment [9]. These studies have shown that the long-range proton migration in BaZrO3 occurs as a sequence of hydrogenbond mediated proton transfer steps between neighboring lattice oxygens and reorientations around the same oxygen site [9]. Open issues about the proton conductivity of BaZrO3 , and more in general of ABO3 materials, are the role played by cation doping on both the solubility and diffusivity of protons, and the formation and bulk incorporation processes of protons at the surface of these materials [10]. The proton conductivity of barium zirconate is tied to the presence of Y3+ substitutional dopants at Zr4+ sites, or equivalently, to the occurrence of oxygen vacancies in the perovskite oxide [9]. On the surface of Y-doped BaZrO3 (henceforth abbreviated BZY), water molecules from the gas phase indeed react with oxygen ion vacancies yielding superficial hydroxide ions
and protons, and hence protonic species diffusing into the bulk [9]. Doping and ambient conditions influence the proton conductivity of barium zirconate, and these effects have been quantified experimentally [9]. Notwithstanding, only a few computational st
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