fig3

Figure 3. ABO₃ perovskite structure: (A) B-site cation at the center of the unit cell, (B) A-site cation at the center of the unit cell, (C) emphasizes the BO₆ octahedra formation and (D) unit cell of fluorite CeO₂. Reproduced under the terms of the CC BY 3.0 License Copyright 2020, Book Chapter Published by Intechopen^[51], (E) MgAl₂O₄ normal spinel structure, (F) MgGa₂O₄ inverse spinel structure. Unit cells, along with different coordination environments, are Reproduced under the terms of the CC BY 4.0 License Copyright 2020, Published by Springer Nature^[53]. (G) Proton insertion in layered Li compound where Li⁺ is the red-colored sphere, and H⁺ is the blue-colored sphere. Reproduced with permission^[54] copyright 2014 John Wiley and Sons.