Lattice dynamics and effects of anharmonicity in different phases of caesium hydrogen sulphate

The inelastic neutron scattering spectra of caesium hydrogen sulphate (CsHSO₄) in three different low-temperature crystallographic phases have been obtained. One of the phases was obtained by applying a pressure of 1.7 GPa at ambient temperature. A comparative analysis of the vibrational spectra is presented. It is shown that the proton dynamics in the hydrogen bonds of the two ambient pressure phases are almost totally decoupled from the dynamics of the other atoms. Anharmonicity plays an important role in the dynamical properties of the ambient pressure phases. It is observed that the effects of anharmonicity in one of the ambient pressure phases can be explained within the framework of the simple anharmonic oscillator model. For the other ambient pressure phase the model of bound two-phonon (biphonon) states is required to explain the data. On the other hand in the high-pressure crystalline phase the proton dynamics exhibit an almost purely harmonic behaviour coupled with a strong interaction between the hydrogen-bond bending modes and SO ₄^2- ion internal vibrations.