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First-principles prediction of high oxygen-ion conductivity in trilanthanide gallates Ln₃GaO₆.

ABSTRACT: We systematically investigated trilanthanide gallates (Ln₃GaO₆) with the space group Cmc2₁ as oxygen-ion conductors using first-principles calculations. Six Ln₃GaO₆ (Ln = Nd, Gd, Tb, Ho, Dy, or Er) are both energetically and dynamically stable among 15 Ln₃GaO₆ compounds, which is consistent with previous experimental studies reporting successful syntheses of single phases. La₃GaO₆ and Lu₃GaO₆ may be metastable despite a slightly higher energy than those of competing reference states, as phonon calculations predict them to be dynamically stable. The formation and the migration barrier energies of an oxygen vacancy (V _O) suggest that eight Ln₃GaO₆ (Ln = La, Nd, Gd, Tb, Ho, Dy, Er, or Lu) can act as oxygen-ion conductors based on V _O. Ga plays a role of decreasing the distances between the oxygen sites of Ln₃GaO₆ compared with those of Ln₂O₃ so that a V _O migrates easier with a reduced migration barrier energy. Larger oxygen-ion diffusivities and lower migration barrier energies of V _O for the eight Ln₃GaO₆ are obtained for smaller atomic numbers of Ln having larger radii of Ln³⁺. Their oxygen-ion conductivities at 1000 K are predicted to have a similar order of magnitude to that of yttria-stabilized zirconia.

SUBMITTER: Lee J

PROVIDER: S-EPMC6407603 | biostudies-literature | 2019

REPOSITORIES: biostudies-literature

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First-principles prediction of high oxygen-ion conductivity in trilanthanide gallates Ln3GaO6.

Lee Joohwi J Ohba Nobuko N Asahi Ryoji R

Science and technology of advanced materials 20190206 1

We systematically investigated trilanthanide gallates (Ln3GaO6) with the space group Cmc21 as oxygen-ion conductors using first-principles calculations. Six Ln3GaO6 (Ln = Nd, Gd, Tb, Ho, Dy, or Er) are both energetically and dynamically stable among 15 Ln3GaO6 compounds, which is consistent with previous experimental studies reporting successful syntheses of single phases. La3GaO6 and Lu<sub ...[more]

PMID: 30863467

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Project description:Sm2-xCaxZr2O7-x/2 (x = 0, 0.05, 0.1) and Gd2-xCaxZr2O7-x/2 (x = 0.05, 0.1) mixed oxides in a pyrochlore-fluorite morphotropic phase region were prepared via the mechanical activation of oxide mixtures, followed by annealing at 1600 °C. The structure of the solid solutions was studied by X-ray diffraction and refined by the Rietveld method, water content was determined by thermogravimetry (TG), their bulk and grain-boundary conductivity was determined by impedance spectroscopy in dry and wet air (100-900 °C), and their total conductivity was measured as a function of oxygen partial pressure in the temperature range: 700-950 °C. The Sm2-xCaxZr2O7-x/2 (x = 0.05, 0.1) pyrochlore solid solutions, lying near the morphotropic phase boundary, have proton conductivity contribution both in the grain bulk and on grain boundaries below 600 °C, and pure oxygen-ion conductivity above 700 °C. The 500 °C proton conductivity contribution of Sm2-xCaxZr2O7-x/2 (x = 0.05, 0.1) is ~ 1 × 10-4 S/cm. The fluorite-like Gd2-xCaxZr2O7-x/2 (x = 0.1) solid solution has oxygen-ion bulk conductivity in entire temperature range studied, whereas proton transport contributes to its grain-boundary conductivity below 700 °C. As a result, of the morphotropic phase transition from pyrochlore Sm2-xCaxZr2O7-x/2 (x = 0.05, 0.1) to fluorite-like Gd2-xCaxZr2O7-x/2 (x = 0.05, 0.1), the bulk proton conductivity disappears and oxygen-ion conductivity decreases. The loss of bulk proton conductivity of Gd2-xCaxZr2O7-x/2 (x = 0.05, 0.1) can be associated with the fluorite structure formation. It is important to note that the degree of Ca substitution in such solid solutions (Ln2-xCax)Zr2O7-δ (Ln = Sm, Gd) is low, x < 0.1. In both series, grain-boundary conductivity usually exceeds bulk conductivity. The high grain-boundary proton conductivity of Ln2-xCaxZr2O7-x/2 (Ln = Sm, Gd; x = 0.1) is attributable to the formation of an intergranular CaZrO3-based cubic perovskite phase doped with Sm or Gd in Zr sublattice.

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First-principles prediction of high oxygen-ion conductivity in trilanthanide gallates Ln₃GaO₆.

Publications

First-principles prediction of high oxygen-ion conductivity in trilanthanide gallates Ln<sub>3</sub>GaO<sub>6</sub>.

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