Preparation, crystal structure, crystal morphology and ionic conductivity of two-dimensional layered M<sub>2</sub>ZrP<sub>2</sub>O<sub>8</sub>·<i>n</i>H<sub>2</sub>O (M: Li, Na, K, Rb, and C

In two-dimensional protonated layered Zr phosphates, H + ion exchange with monovalent alkali metal ions (M + ) of smaller atomic radii (e.g., Li + , Na + ) in aqueous solution proceeds much more easily than with larger ions (e.g., K + , Rb + , and Cs + ).Therefore, this study investigated the preparation of M2ZrP2O8nH2O (M: Li, Na) and M2ZrP2O8nH2O (M: K, Rb, Cs) powders via ion substitution-from H + to Li + and Na + in -Zr(PO4)(H2PO4)-2H2O in aqueous solution-and solid-state reactions, respectively.The prepared M2ZrP2O8nH2O (M: Li, Na) was monoclinic with a thin plate-like crystal morphology, whereas M2ZrP2O8nH2O (M: K, Rb, Cs) was tetragonal with a thick plate-like crystal morphology.Further analyses revealed (002) or (001) peaks, indicating an interlayer distance of approximately 5-10.Moreover, the interlayer distance of M2ZrP2O8nH2O (M: Li, Na) exceeded that of M2ZrP2O8 nH2O (M: K, Rb, Cs).Finally, the ionic conductivity of M2ZrP2O8nH2O (M: K, Rb, Cs) at 30-80 C exceeded that of M2ZrP2O8 nH2O (M: Li, Na) by approximately 1.5 orders of magnitude, while the apparent activation energy for ionic conduction was lower.The ionic conductivities of M: K, Rb, and Cs at room temperature were found to be 1.210 -3 , 9.110 -4 , and 1.310 -3 Scm -1 ,