Tailoring Chemical Expansion in Zirconate-Cerate Proton Conductors
Authors: Chen Ting , Leonard Kwati , Sasaki Kazunari , Matsumoto Hiroshige , Perry Nicola H. 
Published: February 2022

Stoichiometric chemical expansion is the lattice expansion accompanying non-integer changes in stoichiometry, such as oxygen loss in mixed ionic and electronic conducting oxides, Li intercalation in battery electrodes, H uptake in hydrogen storage materials, or hydration in proton conductors. The coefficient of chemical expansion (CCE) normalizes this chemical strain (εC) to the compositional change, so for the case of hydration in proton conductors it can be defined as CCE = εC/Δ[(OH)•
O]. The chemical stresses that develop from such compositional changes can be large enough to cause mechanical failure, such as cracking or delamination, with implications for component processing, in situ characterization, and device lifetime. One way to lower the magnitude of chemical stress is to engineer materials with lower CCEs, but relatively few design principles exist to guide such engineering. In the present work we investigated the hypothesis that lower crystal symmetry could lead to lower macroscopic CCEs in polycrystalline ceramics. There are indications that such a correlation may exist among mixed ionic and electronic conducting perovskites that expand upon losing oxygen due to enlargement of multivalent cations upon gaining electrons for charge compensation. To examine whether a similar effect may be present also for the case of hydration-induced expansion, which has a different mechanism involving filling of oxygen vacancies and introduction of interstitial hydrogen, we fabricated a series of perovskite-structured proton conductors, BaY0.1Ce0.9-xZrxO3 (x=0, 0.3, 0.6, 0.9), having tailored tolerance factors – an indicator of symmetry.

Bar-shaped samples were prepared by a sol-gel route and sintering to 1500 °C, and their crystal structures and lattice parameters were analyzed by X-ray diffraction with Rietveld analysis. For each composition, the isothermal expansion upon increasing H2O content in the gas atmosphere by a fixed amount was measured by dilatometry at various temperatures up to 680 °C. The corresponding changes in (OH)•
O content were determined by thermogravimetric analysis under the same conditions. By normalizing the chemical strains to the changes in proton content, the CCEs at each temperature were determined for each composition and compared.

X-ray diffraction confirmed that the crystal structure became more cubic and the symmetry increased with increasing x, as expected on the basis of the calculated tolerance factors. At the same time the unit cell volume increased, which could also in principle contribute to modifying the chemical expansion behavior. Proton uptake (for a given steam content) was smaller for higher x values, but the lower proton uptake did not correspond with the smallest chemical strains. In fact, the normalized CCEs monotonically increased with increasing x for all but the highest temperatures, consistent with the hypothesis of higher CCEs for higher symmetry, for these randomly oriented, polycrystalline samples. These results suggest that lowering symmetry may be a promising approach for minimizing chemical expansion behavior across multiple classes of materials, with the potential to improve material and device durability. At the same time, future studies should aim to separate the effects of unit cell size vs. crystal symmetry.