Proton conduction mechanism in M3H(XO4)(2) crystals: the trigonal asymmetric hydrogen bond model

Localized diffusive proton motions in the solid-state protonic conductor Rb 3H(Seo(4))(2) Were studied at 500 K with quasielastic incoherent neutron sc attering on the 10(-11)-10(-10) s time scale. The experimentally determined model-independent 'apparent' EISF is used as a guidance to deduce an appro ximate proton density distribution for this time interval, as the starting point for developing the trigonal-asymmetric hydrogen bond (TAHB) model. Th e incoherent scattering function of this model is derived and the resulting theoretical EISF compared to the experiment. The obtained values of the mo del parameters, the order parameter eta and the jump distance R-21, indicat e that-in spite of its very fast relaxational motions in the neighbourhood of the oxygen-the proton essentially remains bonded to a selenate top oxyge n for a period of the order of at least 10(-10) s. Nevertheless, but with r ather low probability, it is also making-in the 10(-11)-10(-10) s time inte rval-brief steps into the hydrogen bridge. We interpret these observations as the consequence of dynamic disorder in the form of an intracrystalline c hemical equilibrium reaction: alternation between the association of the mo nomers [HSeO4](1-) and [SeO4](2-), resulting in the dimer [H(SeO4)(2)](3-) (H-bond formation) and the dissociation of the latter into the two monomers (H-bond breaking), At 500 K, this reaction has a rather asymmetric charact er: The average life-time of the dimer is about 20 times shorter than that of the monomers. (C) 2001 Elsevier Science B.V. All rights reserved.