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At high temperatures the cesium hydrogen sulfate compound, cs2hpo4, undergoes a phase transition to a superprotonic conductor. The structural properties of this material have been studied by total scattering neutron diffraction and inelastic neutron scattering (INS) spectroscopies, as well as periodic density functional theory calculations. The average structure of cs2hpo4 is tetragonal (space group I41/amd) with an atomic radius of 1.32 A . However, the local structure is unknown. Existing structures based on experimental INS data have either partial occupancies of oxygen and/or hydrogen or non-physical O-H distances, and they are unable to explain the observed increase in conductivity with temperature.

This paper presents the INS spectra of cs2hpo4 in phase III and in the disordered (1-x)CsH2PO4-xCsH5(PO4)2 composite electrolyte at 430 K. The spectra have been measured using the TOSCA instrument and compared to INS calculations for the complete Brillouin zone based on the structures of Itoh et al. for phase III and Belushkin et al. for phase II.

The INS spectra reveal the existence of a broad range of modes, including translational vibrations below 200 cm-1, sulfate bending modes in the range 200-700 cm-1 and out-of-plane S-O-H bends at >1500 cm-1. The calculated spectra agree with the experimental INS spectra in all respects except for the amplitudes of the out-of-plane S-O-H peaks, which differ by up to 10 A-1. This discrepancy is due to the limited spatial resolution of the INS experiment. However, the measured and calculated INS spectra have comparable peak heights at the G-point and similar binding energies. This is the first evidence that minor deviations from a stoichiometric composition of cs2hpo4 have a significant effect on its conductivity at LT phase and may be associated with defects in the crystal structure.