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The location of protons in crystals and their dynamics are successfully studied by the methods of neutron scattering. In contrast to X-rays, the cross-section of the neutron–nucleus interaction does not have regular dependence on the atomic weight. Light atoms, like hydrogen, can scatter neutrons more effectively than heavy ones and therefore are more easily located in the structure with the help of neutron diffraction. The absence of selection rules in inelastic neutron scattering allows to observe all vibrational modes of the crystal. The anomalously large incoherent neutron scattering cross section of hydrogen in comparison to other elements considerably simplifies the interpretation of inelastic and quasielastic scattering results. In the present review examples of the complex use of different neutron scattering methods for the study of structure and dynamics of some superprotonic (with proton conductivity higher than 10 −4 Ohm −1 cm −1 ) conductors with hydrogen bonds is given.

The sequence of phase transitions at temperatures from 290 to 420 K in the crystals of Cs5H3(SO4)4·0.5H2O (CTSM) and Cs5D3(SO4)4·0.5D2O (DCTSM) was studied by Brillouin light scattering and neutron diffraction. Anomalies in the temperature dependences of the velocity and damping of hypersonic acoustic phonons in the vicinity of 360 K were observed in the CTSM crystal. In contrast, no anomalies were found in the DCTSM crystal. Preliminary analysis of neutron diffraction data does not show symmetry changes in the temperature range studied. Thus it has been found that deuteration of the CTSM crystals leads to a change in the sequence of phase transitions.

Caesium hydrogen sulphate is one of the most extensively studied superprotonic conductors with hydrogen bonds. A first order phase transition, from the low conductivity to the high conductivity phase, takes place at 414 K. The crystallographic structure of both phases has been established using high-resolution neutron powder diffraction. Quasielastic neutron scattering has provided information about the spatial and temporal characteristics of proton transport. In the present paper we report the results of reverse Monte Carlo modelling of neutron diffraction (total scattering) data which makes it possible to obtain the diffusion pathways for protons. The results are in good agreement with the qualitative model for proton transport proposed previously; in particular we show clearly that the proton motion is highly correlated with the rotation of the sulphate groups. In the low temperature phase we identify a weak correlation between protons and oxygens on the next-nearest-neighbour sulphate group which increases with temperature and probably drives the phase transition.