Inelastic neutron scattering study of proton dynamics in Cs$_{3}$H(SeO$_{4}$)$_{2}$ and Rb$_{3}$H(SeO$_{4}$)$_{2}$

The proton dynamics of the Cs$_{3}$H(SeO$_{4}$)$_{2}$ and Rb$_{3}$H(SeO$_{4}$)$_{2}$ have been studied by inelastic neutron scattering in a temperature range from 5 K to 60 K. The results are discussed in comparison with the isostructural acid sulphates, however the dynamics of the selenates is quite different. The splitting of selenate bands at low temperatures for both Cs and Rb salts indicate a phase transition to low symmetry. The anomalous temperature dependence of hydrogen bond bending modes in the Cs salt is discussed in the framework of interactions between strongly anharmonic stretching modes and bending vibrations