Characterizationof the Dynamics in the Protonic ConductorCsH2PO4 by 17O Solid-State NMR Spectroscopyand First-Principles Calculations: Correlating Phosphate and ProtonicMotion

摘要

¹⁷O NMR spectroscopy combined with first-principles calculations was employed to understand the local structure and dynamics of the phosphate ions and protons in the paraelectric phase of the proton conductor CsH2PO4. For the room-temperature structure, the results confirm that one proton (H1) is localized in an asymmetric H-bond (between O1 donor and O2 acceptor oxygen atoms), whereas the H2 proton undergoes rapid exchange between two sites in a hydrogen bond with a symmetric double potential well at a rate ≥10⁷ Hz. Variable-temperature ¹⁷O NMR spectra recorded from 22 to 214 °C were interpreted by considering different models for the rotation of the phosphate anions. At least two distinct rate constants for rotations about four pseudo C3 axes of the phosphate ion were required in order to achieve good agreement with the experimental data. An activation energy of 0.21 ± 0.06 eV was observed for rotation about the P–O1 axis, with a higher activation energy of 0.50 ± 0.07 eV being obtained for rotation about the P–O2, P–O3^d, and P–O3^a axes, with the superscripts denoting, respectively, dynamic donor and acceptor oxygen atoms of the H-bond. The higher activationenergy of the second process is most likely associated with the costof breaking an O1–H1 bond. The activation energy of this processis slightly lower than that obtained from the ¹H exchangeprocess (0.70 ± 0.07 eV) (Kim,G.; Blanc, F.; Hu, Y.-Y.; Grey, C. P. J. Phys. Chem. C 2013, 117, 6504−6515 []) associated with the translational motionof the protons. The relationship between proton jumps and phosphaterotation was analyzed in detail by considering uncorrelated motion,motion of individual PO4 ions and the four connected/H-bondedprotons, and concerted motions of adjacent phosphate units, mediatedby proton hops. We conclude that, while phosphate rotations aid protonmotion, not all phosphate rotations result in proton jumps.