Y:BaZrO₃ Perovskite Compounds I: DFT Study on the Unprotonated and Protonated Local Structures

Abstract

Y-doped BaZrO(3) derivatives were studied by density functional theory (DFT) to investigate the local arrangements of the octahedral sites in Pm3m cubic frameworks. Single- and double substitution of zirconium by yttrium were considered, including in the presence of a nearby oxygen vacancy. Although the structural symmetry of undoped barium zirconate was not modified after yttrium doping, the presence of yttrium induced several differences in the oxygen sites around it, according to the local geometrical arrangement of yttrium in the host matrix. As an example, the differences between such oxygen sites were shown in the presence of a proton. In this case, different stabilization energies characterized the protonated fragments. Only in those structures, in which two yttrium atoms were neighbors (i.e., formed Y-O-Y moieties), were the relative energy differences between the corresponding proton stable sites in agreement with the order of magnitude of the experimental proton-hopping activation energies. The distribution of such energy differences suggested a grouping of the oxygen atoms into three sets, which had peculiar structural features that weren't easily deducible from their topologies. The existence of proton traps was also discussed on the basis of the energy-difference distributions.

First-principles molecular dynamics simulations of proton diffusion in cubic BaZrO $$_3$$ 3 perovskite under strain conditions

“…The valence band maximum lies below E F and conduction band minimum lies above E F proton mobility in hydrated conditions, where a BaZr 0:9 Y 0:1 O 3 shows larger lattice parameter due to the hydrostatic pressure induced by the syntheses route [36]. The discrepancies may be due to several differences in the system and the environmental conditions, such as the hydrated conditions, under which the experiment has been performed, which means that water is present and hydroxyls can form, do not resemble our simulated conditions [37,38]. However, Ottochan et al analysed proton conduction in Yttrium-doped BaZrO 3 under biaxial compressive conditions through the use of reactive molecular dynamics simulations.…”

Protonic defects in yttria stabilized zirconia: incorporation, trapping and migration

“…3(a) where, as previously discussed, the reduced Coulombic repulsion between the proton and the Y ion allows the proton to move further into the centre of the cell where repulsion with surrounding cations is at a minimum. DFT calculations 23,24 have also confirmed that the lowest energy proton migration pathway is around the Y ion in Y-doped BaZrO 3 . The hydration energies for YSZ are high which suggests a low concentration of protons in the system.…”

Alkali metals as efficient A-site acceptor dopants in proton conducting BaZrO3

“…This has been attributed to a plethora of effects including large grain boundary resistances, 6 Ba loss and partial Y dissolution on the A-site, 7,8 and complex formation and trapping. [9][10][11][12][13] While undoped bulk BaZrO 3 exhibits an activation energy of proton mobility of merely 0.16-0.20 eV, 9,11,12 nearly all B-site acceptor dopants (e.g. Sc, Y, Gd, Nd) increase this considerably due to complex formation between the acceptors and the mobile protons, thus lowering the proton conductivity.…”