Chemical and Structural Evolution of Rb_{2–2<i>x</i>}K_2<i>x</i>Ti₂O₅ Layered Titanates When Exposed to Ambient Air

In this report, we combine thermogravimetric analysis, X-ray diffraction, and infrared spectroscopy to investigate the chemical and structural evolution upon exposure of alkali titanates Rb₂Ti₂O₅ and K₂Ti₂O₅ and mixed compound Rb_2-2<i>x</i>K_2<i>x</i>Ti₂O₅, for which 0 ≤ <i>x</i> ≤ 1, to the ambient atmosphere. We show that a complete solid solution exists between the potassium and rubidium end-members and that mixed compounds exhibit an intermediate behavior. When exposed to the ambient atmosphere, all compounds degrade, with the progressive formation of first hydrates and then bicarbonates and a much quicker degradation of the rubidium titanate than of the potassium titanate. On the contrary, we show that upon hydration in the absence of CO₂, the crystal structure of K₂Ti₂O₅ is more quickly impacted than that of Rb₂Ti₂O₅. These observations should be useful for understanding the evolution of the functional properties of M₂Ti₂O₅ alkali titanates upon aging in the ambient atmosphere, as well as for the optimization of their hydration-dependent properties.