LATTICE-DYNAMICS AND EFFECTS OF ANHARMONICITY IN DIFFERENT PHASES OF CESIUM HYDROGEN SULFATE

The inelastic neutron scattering spectra of caesium hydrogen sulphate (CsHSO4) in three different low-temperature crystallographic phases ha ve been obtained. One of the phases was obtained by applying a pressur e of 1.7 GPa at ambient temperature. A comparative analysis of the vib rational spectra is presented. It is shown that the proton dynamics in the hydrogen bonds of the two ambient pressure phases are almost tota lly decoupled from the dynamics of the other atoms. Anharmonicity play s an important role in the dynamical properties of the ambient pressur e phases. It is observed that the effects of anharmonicity in one of t he ambient pressure phases can be explained within the framework of th e simple anharmonic oscillator model. For the other ambient pressure p hase the model of bound two-phonon (biphonon) states is required to ex plain the data. On the other hand in the high-pressure crystalline pha se the proton dynamics exhibit an almost purely harmonic behaviour cou pled with a strong interaction between the hydrogen-bond bending modes and SO42- ion internal vibrations.