Aging of lithium zirconate (Li2ZrO3) at room temperature: Effect of iron as a dopant

The chemical and structural evolution of Li2ZrO3 (LZO) and Fe3+-doped LZO stored for one year at room temperature under ambient conditions was studied. X-ray diffraction showed that the reaction of both samples with water and CO2 present in the air after long storage resulted in the evolution of an amorphous phase, Li2CO3 and ZrO2. Mössbauer spectroscopy indicated that Fe3+ may interact with H2O and CO2 although no changes in its oxidation state were observed, nor were new Fe-containing phases observed in XRD. The presence of the dopant accelerated the reaction rate at early stages of storage, but limited the absorption at longer storage times, as was observed by thermal analyses. We suggest that water vapour reacts with pre-existing Li and O vacancies in grain boundaries, forming OH− in positive O vacancies and H+ in Li vacancies or interstices. In a following step, LiOH would react with the atmospheric CO2 to form Li2CO3. The dopant acts in LZFO by increasing the number of O vacancies, which initially increases the H2O and CO2 absorption rates. Once the grain boundaries are covered by a LiOH-Li2CO3 layer, the absorption process starts to be limited by diffusion. SEM images showed that LZO presented a more porous microstructure and smaller grains than LZFO, therefore it had a larger surface area and continued to absorb H2O and CO2 without diffusion limitation, surpassing LZFO after 180 days. After heat treating the stored samples, the original materials were recovered, demonstrating the reversibility of the process.