Understanding the Conduction Mechanism of the Protonic Conductor CsH₂PO₄ by Solid-State NMR Spectroscopy

Abstract

Local dynamics and hydrogen bonding in CsH₂PO₄ have been investigated by ¹H, ²H, and ³¹P solid-state NMR spectroscopy to help provide a detailed understanding of proton conduction in the paraelectric phase. Two distinct environments are observed by ¹H and ²H NMR, and their chemical shifts (¹H) and quadrupolar coupling constants (²H) are consistent with one strong and one slightly weaker H-bonding environment. Two different protonic motions are detected by variable-temperature ¹H MAS NMR and <i>T</i>₁ spin–lattice relaxation time measurements in the paraelectric phase, which we assign to librational and long-range translational motions. An activation energy of 0.70 ± 0.07 eV is extracted for the latter motion; that of the librational motion is much lower. ³¹P NMR line shapes are measured under MAS and static conditions, and spin–lattice relaxation time measurements have been performed as a function of temperature. Although the ³¹P line shape is sensitive to the protonic motion, the reorientation of the phosphate ions does not lead to a significant change in the ³¹P CSA tensor. Rapid protonic motion and rotation of the phosphate ions is seen in the superprotonic phase, as probed by the <i>T</i>₁ measurements along with considerable line narrowing of both the ¹H and the ³¹P NMR signals