Revealing Local Dynamics of the Protonic Conductor CsH(PO3H) by Solid-State NMR Spectroscopy and First-Principles Calculations

Abstract

A joint study incorporating multinuclear solid-state NMR spectroscopy and first-principles calculations has been performed to investigate the local structure and dynamics of the protonic conductor CsH(PO3H) in the paraelectric phase. The existence of the superprotonic phase (>137 °C) is clearly confirmed by NMR, in good agreement with the literature. The variable-temperature 1H, 2H, and 31P NMR data further reveal a distribution of motional correlation times, with isotropic rotation of the phosphite ion being observed below the superprotonic phase transition for a small but gradually increasing subset of anions. This isotropic rotation is associated with fast local protonic motion, with the distribution of correlation times being tentatively assigned to internal defects or surface adsorbed H2O. The phosphite ion dynamics of the majority slower subset of phosphite ions is quantified through analysis of variable-temperature 17O spectra recorded from 34 to 150 °C, by considering a model for the pseudo C3 rot...