Characteristic features of the molecular dynamics and phase transitions of mixed [(NH4)1−xRbx]3H(SO4)2 (x = 0, 0.8, and 1) crystals using 1H and 87Rb NMR

► (NH4)1−xRbx3H(SO4)2 (x = 0, 0.8, and 1) mixed crystals. ► The molecular dynamics and phase transition temperatures. ► Superprotonic phase transition. ► The transition temperature by NH4-Rb substitution. ► The roles of NH4+ ions and the hydrogen-bonded protons.