Disorder in La_1−x Ba_1+x GaO_4−x/2 ionic conductor: resolving the pair distribution function through insight from first-principles modeling

Abstract

Ionic conduction in dry LaBaGaO 4 occurs through the vacant oxygen sites formed by the substitution of Ba for La. The resulting La 1Àx Ba 1+x GaO 4Àx/2 solid solution shows significant disorder characteristics. The local structure of compositions x = 0, 0.20 and 0.30 was studied using the pair distribution function (PDF). Unfortunately, increasing peak overlap and the number of independent structural parameters make PDF modeling challenging when dealing with low-symmetry phases. To overcome this problem, density functional theory (DFT) was employed to create different structural models, each one with a different relative position for the substitutional Ba ion with respect to the oxygen vacancy. The atomic distributions generated by DFT were used as a starting point to refine experimental PDF data. All models result in the formation of Ga 2 O 7 dimers, with their major axis oriented along the c axis. At the local scale, the most stable DFT model also provides the best fit of the PDF. This accounts for the dopant as first and second neighbors of the vacancy and of the O bridge in the dimer, suggesting that substitutional barium ions act as pinning centers for oxygen vacancies. Above 6 Å the average orthorhombic structure fits the PDF better than the DFT models, thus indicating that Ga 2 O 7 dimers are not correlated with each other to form extended ordered structures. The combination of DFT simulations and X-ray diffraction/PDF refinements was used successfully to model the local atomic structure in La 1Àx Ba 1+x GaO 4Àx/2 , thus suggesting that this approach could be positively applied in general to disordered systems.

Oxide Ion-Conducting Materials Containing Tetrahedral Moieties: Structures and Conduction Mechanisms

“…However, unlike the scheelite structure, the O int defects cannot be produced successfully via La 3+ substitution on Ba 2+ sites. 66,185 Multiple complementary characterizations, including NPD, static lattice simulations based on interatomic potentials method, DFT simulations, and PDF analysis, 27,40,186 have been employed to elucidate that the oxygen vacancies in La 1−x Ba 1+x GaO 4−0.5x are accommodated through forming Ga 2 O 7 units (Figure 19b), maintaining the tetrahedral Ga geometry instead of producing the unstable 3-coordinated Ga. The lowest-energy positions of oxygen vacancies are located close to the Ba La ′ cation site, as indicated by DFT calculations in 2019 by Coduri et al 40 The PDF analysis revealed a significant structural disorder nature for Ga 2 O 7 dimers because of the larger distribution of interatomic distances upon Ba 2+ substitution for the smaller La 3+ .…”

“…66,185 Multiple complementary characterizations, including NPD, static lattice simulations based on interatomic potentials method, DFT simulations, and PDF analysis, 27,40,186 have been employed to elucidate that the oxygen vacancies in La 1−x Ba 1+x GaO 4−0.5x are accommodated through forming Ga 2 O 7 units (Figure 19b), maintaining the tetrahedral Ga geometry instead of producing the unstable 3-coordinated Ga. The lowest-energy positions of oxygen vacancies are located close to the Ba La ′ cation site, as indicated by DFT calculations in 2019 by Coduri et al 40 The PDF analysis revealed a significant structural disorder nature for Ga 2 O 7 dimers because of the larger distribution of interatomic distances upon Ba 2+ substitution for the smaller La 3+ . The proton defects in La 1−x Ba 1+x GaO 4−0.5x were produced through incorporating water molecules into the oxygen vacancies, producing hydroxyl anions in the structure according to the equation H 185 The incorporation of water molecules further induces the breakup of the Ga 2 O 7 dimers into two isolated GaO 4 units, forming hydrogen bonding between them (Figure 19c).…”

“…Thus, the structural relaxation to accommodate the oxygen defects is much more complex in the tetrahedron-based structures than in the structural prototypes consisting of the 6- or 8-coordinated polyhedral units. Normally, it is hard to determine precisely the sites and occupancies of the oxygen defects and get insight into the defect structures through diffraction techniques including synchrotron powder diffraction (SPD) and neutron powder diffraction (NPD), which probe the average structures. − Multiple complementary methods have been employed to solve the defect structures and elucidate the mechanisms of oxide ion migration, including extended X-ray absorption fine structure (EXAFS), high-resolution scanning transmission electron microscopy (HR-STEM), annular bright-field scanning transmission electron microscopy (ABF-STEM), solid-state nuclear magnetic resonance spectroscopy (SS-NMR), pair distribution function (PDF) analysis, and Raman spectroscopy for the local structures − and density functional theory (DFT) calculations − as well as interatomic potentials-based atomistic modeling and molecular dynamics (MD) simulations ,− for the defect formation energies and oxide ion migration. Thanks to these advanced techniques, deep insights into defect stabilization and migration in tetrahedron-based oxide ion conductors have been achieved, correcting initial statements on the phase formations and compositions, structures, defect types, and migration mechanisms and demonstrating how technological progress over the past 40 years has advanced the development of various oxide ion conductors based on tetrahedral moieties.…”

Solid oxide proton conductors beyond perovskites

“…The oxygen vacancies in La0.8Ba1.2GaO3.9 are accommodated through the condensation of two adjacent GaO4 units to form a Ga2O7 group (in a way analogous to acceptor doped LaNbO4) 169 . On the local scale, the oxygen vacancies are pinned to the nearest-neighbour cation site by the Ba La ' defects and there is no ordering of the Ga2O7 units 170 . Water incorporation induces the break-up of the Ga2O7 units 169 , according to the reaction Ga2O7 + H2O → 2 GaO4H.…”