Characterization of the Dynamics in the Protonic Conductor CsH_2PO_4 by ^(17)O Solid-State NMR Spectroscopy and First-Principles Calculations: Correlating Phosphate and Protonic Motion
^(17)O 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 CsH_2PO_4. 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 ≥10^7 Hz. Variable-temperature ^(17)O 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 C_3 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 activation energy of the second process is most likely associated with the cost of breaking an O1–H1 bond. The activation energy of this process is slightly lower than that obtained from the ^1H exchange process (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 motion of the protons. The relationship between proton jumps and phosphate rotation was analyzed in detail by considering uncorrelated motion, motion of individual PO_4 ions and the four connected/H-bonded protons, and concerted motions of adjacent phosphate units, mediated by proton hops. We conclude that, while phosphate rotations aid proton motion, not all phosphate rotations result in proton jumps.
© 2015 American Chemical Society. ACS AuthorChoice - This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited. Received: January 9, 2015; Published: March 3, 2015. C.P.G. and G.K. thank the European Research Council for an Advanced Fellowship. F.B. thanks the EU Marie Curie actions FP7 for an International Incoming fellowship (Grant No. 275212) for financial support. J.M.G. also thanks the European Research Council for funding. We thank Prof. Robert Vold for helpful discussions regarding the EXPRESS simulation software. G.K. thanks Dr. Song-Yul Oh (Toyota Central R&D Laboratories. Inc., Japan) for valuable discussions about the synthesis of ^(17)O-enriched CDP. This work made use of the facilities of HECToR, the UK's national high-performance computing service, which is provided by UoE HPCx Ltd at the University of Edinburgh, Cray Inc., and NAG Ltd, and funded by the Office of Science and Technology through EPSRC's High End Computing Programme. The UK 850 MHz solidstate NMR Facility used in this research was funded by EPSRC and BBSRC, as well as the University of Warwick including via part funding through Birmingham Science City Advanced Materials Projects 1 and 2 supported by Advantage West Midlands (AWM) and the European Regional Development Fund (ERDF).
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