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

Local dynamics and hydrogen bonding in CsH2PO4 have been investigated by H-1, H-2, and P-31 solid-state NMR spectroscopy to help provide a detailed understanding of proton conduction in the paraelectric phase. Two distinct environments are observed by H-1 and H-2 NMR, and their chemical shifts (H-1) and quadrupolar coupling constants (H-2) are consistent with one strong and one slightly weaker H-bonding environment. Two different protonic motions are detected by variable-temperature H-1 MAS NMR and T-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. P-31 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 P-31 line shape is sensitive to the protonic motion, the reorientation of the phosphate ions does not lead to a significant change in the P-31 CSA tensor. Rapid protonic motion and rotation of the phosphate ions is seen in the superprotonic phase, as probed by the T-1 measurements along with considerable line narrowing of both the H-1 and the P-31 NMR signals.