Proton NMR relaxation study of the CsHSO 4 solid acid system

Abstract

At 141 °C the solid acid CsHSO 4 is known to undergo transition to a superprotonic phase that is characterized by dramatic (several-order-of-magnitude) increases in hydrogen ion conductivity. Proton NMR spin–spin relaxation time T 2 measurements reported here for CsHSO 4 also reveal substantial increases (factors of 20–30) in the vicinity of the transition temperature. In the temperature range just below the transition (70–136 °C), T 2 increases by a factor of order 10 relative to the rigid-lattice regime, suggesting motional narrowing of the NMR resonance line. In the regime of motional narrowing, the activation energy barrier to diffusion is 0.40 eV, as determined from the present T 2 results. NMR spin–lattice relaxation T 1 measurements also show behavior consistent with transition to a regime of rapid hydrogen motion. In particular, proton T 1's decrease with temperature (from 80 to 120 °C), and then drop sharply near the transition temperature. Above the transition temperature, T 1 exhibits a minimum in which the correlation time is found to be ∼2 ns.