Phase diagrams and chemical expansion upon hydration of proton conductors BaZrxCe0.8-xY0.1Yb0.1O2.9(H2O)n (0 ≤ x ≤ 0.8; 0 ≤ n ≤ 0.1)

Proton-conducting ceramics are promising candidates for applications in sustainable energy technologies, with BaZrxCe0.8-xY0.1Yb0.1O2.9 (BZCYYb) perovskites standing out as excellent proton conductors. However, comprehensive structural studies of these materials, particularly concerning their hydrated and dehydrated states, as well as the effects of composition (x) and temperature, are still limited. This study explores the crystal structures of BaZrxCe0.8-xY0.1Yb0.1O2.9(H2O)n and BaZrxCe0.8-xY0.1Yb0.1O2.9 (0 ≤ x ≤ 0.8) using neutron and X-ray diffraction techniques. The chemical expansion due to water incorporation is quantified by measuring unit cell volumes via X-ray diffraction, showing strong agreement with mass loss data from thermogravimetric analysis (TGA). Phase diagrams for both hydrated and dehydrated phases are developed, revealing a decrease in symmetry and a reduction in phase transition temperatures as x increases. Hydration notably affects octahedral tilting, as evidenced by comparisons of hydrated and dehydrated structures at room temperature (RT). Furthermore, a bond valence sum (BVS)-based approach is proposed, offering improved predictions of octahedral tilting and structural stability compared to the traditional Goldschmidt tolerance factor. These structural insights, particularly the influence of hydration, are essential for advancing our understanding of these materials and providing a solid foundation for linking their structure to their properties.