Emergence of A-Site Cation Order in the Small Rare-Earth Melilites SrREGa3O7 (RE = Dy-Lu, Y)

La Substitution into the Melilite Derivative Ca5Ga6O14: Prediction, Synthesis and Ionic Conductivity

Melilite-type gallates of general formula RE1+xAE1-xGa3O7+x/2 are of interest for their ability to host mobile interstitial oxide ions in [Ga3O7+x] layers. The crystal structure of Ca5Ga6O14 is closely related to melilite, with [Ga3O7] layers stacked in a more complex way to accommodate an additional 0.5 interlayer cations per formula unit, suggesting the potential for similar oxide ion conduction behavior. We used a computational approach to identify the most promising routes to interstitial oxide incorporation into Ca5Ga6O14, leading to an experimental investigation of the system Ca5-xLaxGa6O14+x/2. Single-phase materials were obtained in the range 0 ≤ x ≤ 0.25 by solid state reactions, producing an ∼40× increase in ionic conductivity at 800 °C. This limited compositional range presents a challenge for characterization of the charge-compensating defects. The La substituents were observed directly by X-ray diffraction and STEM-EDX, and a combination of different structural characterization techniques and DFT calculations indicated the presence of interstitial oxide ions indirectly, explaining the conductivity response. As higher carrier concentrations (x > 0.25) are apparently inaccessible in this system, we conclude that its potential as a useful oxide ion conductor is more limited than that of established melilite materials such as La1+xCa1-xGa3O7+x/2.

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

This Review presents an overview from the perspective of tetrahedral chemistry on various oxide ion-conducting materials containing tetrahedral moieties which have received continuous growing attention as candidates for key components of various devices, including solid oxide fuel cells and oxygen sensors, due to the deformation and rotation flexibility of tetrahedral units facilitating oxide ion transport. Emphasis is placed on the structural and mechanistic features of various systems ranging from crystalline to amorphous materials, which include a variety of gallates, silicates, germanates, molybdates, tungstates, vanadates, aluminates, niobate, titanates, indium oxides, and the newly reported borates. They contain tetrahedral units in either isolated or linked manners forming different polyhedral dimensionality (0 to 3) with various defect properties and transport mechanisms. The development of oxide ion conductors containing tetrahedral moieties and the elucidation of the roles of tetrahedral units in oxide ion migration have demonstrated diverse opportunities for discovering superior electrolytes for solid oxide fuel cells and other related devices and provided useful clues for uncovering the key factors directing fast oxide ion conduction.

Emergent Transitions: Discord between Electronic and Chemical Pressure Effects in the REAl3 (RE = Sc, Y, Lanthanides) Series

Atomic packing and electronic structure are key factors underlying the crystal structures adopted by solid-state compounds. In cases where these factors conflict, structural complexity often arises. Such is born in the series of REAl₃ (RE = Sc, Y, lanthanides), which adopt structures with varied stacking patterns of face-centered cubic close packed (FCC, AuCu₃ type) and hexagonal close packed (HCP, Ni₃Sn type) layers. The percentage of the hexagonal stacking in the structures is correlated with the size of the rare earth atom, but the mechanism by which changes in atomic size drive these large-scale shifts is unclear. In this Article, we reveal this mechanism through DFT-Chemical Pressure (CP) and reversed approximation Molecular Orbital (raMO) analyses. CP analysis illustrates that the Ni₃Sn structure type is preferable from the viewpoint of atomic packing as it offers relief to packing issues in the AuCu₃ type by consolidating Al octahedra into columns, which shortens Al-Al contacts while simultaneously expanding the RE atom's coordination environment. On the other hand, the AuCu₃ type offers more electronic stability with an 18-n closed-shell configuration that is not available in the Ni₃Sn type (due to electron transfer from the RE d_z² atomic orbitals into Al-based states). Based on these results, we then turn to a schematic analysis of how the energetic contributions from atomic packing and the electronic structure vary as a function of the ratio of FCC and HCP stacking configurations within the structure and the RE atomic radius. The minima on the atomic packing and electronic surfaces are non-overlapping, creating frustration. However, when their contributions are added, new minima can emerge from their combination for specific RE radii representing intergrowth structures in the REAl₃ series. Based on this picture, we propose the concept of emergent transitions, within the framework of the Frustrated and Allowed Structural Transitions principle, for tracing the connection between competing energetic factors and complexity in intermetallic structures.