Pseudomorphic 2A→ 2M→ 2H phase transitions in lanthanumstrontium germanate electrolyte apatites

Abstract

Apatite-like materials are of considerable interest as potential solid oxide fuel cell electrolytes, although their structural vagaries continue to attract significant discussion. Understanding these features is crucial both to explain the oxide ion conduction process and to optimise it. As the composition of putative P6₃/m apatites with ideal formula [A^I₄][A^II₆][(BO₄)₆][X]₂ is varied the [A^I₄(BO₄)₆] framework will flex to better accommodate the [A^II₆X₂] tunnel component through adjustment of the A^IO₆ metaprism twist angle (ϕ). The space group theory prescribes that framework adaptation during phase changes must lead to one of the maximal non-isomorphic subgroups of P6₃/m (P2₁, P2₁/m, P). These adaptations correlate with oxygen ion conduction, and become crucial especially when the tunnels are filled by relatively small ions and/or partially occupied, and if interstitial oxygens are located in the framework. Detecting and completely describing these lower symmetry structures can be challenging, as it is difficult to precisely control apatite stoichiometry and small departures from the hexagonal metric may be near the limits of detection. Using a combination of diffraction and spectroscopic techniques it is shown that lanthanum strontium germanate oxide electrolytes crystallise as triclinic (A), monoclinic (M) and hexagonal (H) bi-layer pseudomorphs with the composition ranges:

[La_10−xSr_x][(GeO₄)_5+x/2(GeO₅)_1−x/2][O₂] (0 ≤x≤ 1) apatite-2A

[La_10−xSr_x][(GeO₄)_5+x/2(GeO₅)_1−x/2][O₂] (1 ≤x≤ 2) apatite-2M

[La_10−xSr_x][(GeO₄)₆][O₂][H_δ] (2 ≤x≤ 2.96) apatite-2M

[La_10−xSr_x][(GeO₄)₆][O₂][H_δ] (2.96 ≤x≤ 5.32) apatite-2H

Furthermore, at typical fuel cell operating temperatures apatite-2A and apatite-2M will transform to apatite-2H, with the latter showing the highest conduction. The results show that small twist angles and high symmetry enhance oxygen mobility with these properties tailored by adjusting the relative size of the framework to tunnel. This information can hence aid in the design of new materials with improved oxide ion conductivity.