Behavior of H<sub>2</sub>O surrounding NH<sub>4</sub><sup>+</sup> and Al<sup>3+</sup> in NH<sub>4</sub>Al(SO<sub>4</sub>)<sub>2</sub>·12H<sub>2</sub>O by <sup>1</sup>H MAS NMR, <sup>14</sup>N NMR, and <sup>27</sup>Al NMR
RSC advances