Superprotonic conductor CsH2PO4 studied by 1H, 31P NMR and X-ray diffraction

Paraelectric to superionic phase transition of CsH2PO4 was studied by means of X-ray diffraction and solid-state NMR. 1H NMR powder spectrum showed the characteristic high field shift (8.0 ppm relative to H2O) as is expected for the proton forming a strong hydrogen bonding. The temperature dependence of the 1H and 31P NMR suggests a reorientational jump of the H2PO4− anion above 420 K. The reorientational jump of the H2PO4− anion was also confirmed by the 31P NMR using a single crystal and the activation energy was estimated to be 0.74(1) eV by the spectrum simulation. Above the superionic phase transition at 502 K, a dynamically disordered PO4 unit was confirmed by the Rietveld analysis and also by the isotropic sharp spectra of the 1H, 31P NMR. These observations suggest that the reorientation of the H2PO4− anion plays an important role for the proton conduction even in the paraelectric phase.