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Hydrogen Bond Networks in Cs2(HSO4)(H2PO4) As Studied by Solid-State NMR

Hayashi, Shigenobu, Jimura, Keiko
The Journal of Physical Chemistry C 2017 v.121 no.23 pp. 12643-12651
ambient temperature, cesium, hydrogen bonding, models, nuclear magnetic resonance spectroscopy, phosphorus, stable isotopes, sulfates
The inorganic solid acid salt Cs₂(HSO₄)(H₂PO₄) showed high proton conductivity in the superprotonic phase above 370 K, and the phase was retained on cooling even at room temperature for a long period. These characteristic properties should correlate to the hydrogen bond network connecting the SO₄ and PO₄ groups. In the present work, the structures of the hydrogen bond network were studied by solid-state NMR. We present ³¹P, ¹H, and ¹³³Cs magic-angle-spinning (MAS) NMR spectra, ³¹P static NMR spectra to derive chemical shift anisotropy, a ³¹P{¹H} dipolar dephasing experiment, and ¹H{³¹P} rotational-echo double resonance (REDOR) experiments. The ³¹P NMR spectra indicate that all the P sites are crystallographically equivalent, and the negative value of the chemical shift anisotropy suggests that four hydrogen bonds are formed around the PO₄ tetrahedron. The ³¹P{¹H} dipolar dephasing experiment strongly supports that the number of hydrogen bonds around the PO₄ tetrahedron is four. The ¹H and ¹³³Cs MAS NMR spectra show that a part of the hydrogen bond network is disordered and that the disordered structure relaxes slowly with time. On the basis of those NMR results, we propose a model structure of the hydrogen bond network.