H-bond network dynamics of the solid-state protonic conductor Rb3H(SeO4)2 from FTIR and IINS measurements

Abstract

Fourier transform infrared (FTIR) studies on small single crystals have been performed with the aim to analyse structural and dynamical aspects of the H-bond network in Rb3H(SeO4)2. A comparative factor group analysis for both the monoclinic and the trigonal phase of Rb3H(SeO4)2, based on centrosymmetric (C2/c and R3¯m) and non-centrosymmetric (Cc, C2 and R3m) space group symmetries, has been carried out. Hypothetical structure models based on non-centrosymmetric space group symmetries are presented both for the monoclinic and the trigonal phase. They are shown to be of invaluable use in the interpretation of certain aspects of the observed vibrational spectra. In fact, our FTIR measurements and the comparative factor group analysis made clear that the centrosymmetric (time-averaged) structures are not sufficient for the description of the instantaneous and short-time/short-range order of the protons and for the related vibrational modes. Inelastic incoherent neutron scattering (IINS) spectra at 60, 100, 167 and 297 K are also reported. The OH bending mode (?OH) gives large IINS intensities. A detailed infrared (IR) band analysis between 800 and 1100 cm?1, together with the IINS results, showed that the identified modes (?1[SeO4]2?, ?1[HSeO4]1?, ?3[SeO4]2? and ?3[HSeO4]1?) can be classified in two groups differing by the strength of the monomer coupling. Proton conductivity in Rb3H(SeO4)2 crystals is shown to be due to a dynamic disorder in the form of an intracrystalline chemical equilibrium reaction: alternation between the association of the monomers [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).