Superprotonic high temperature phase and refinement of the low temperaturestructure of CsHSeO4

The crystal structure of the high temperature superprotonic phase of CsHSeO 4-I (tI28) has been investigated by single crystal X-ray diffraction at 408 K. It crystallizes tetragonal, a=5.880(1), c=14.415(3)Angstrom, V= 498.4(2 ) Angstrom (3), Z= 4, space group I4(l)/amd. In the SeO4 tetrahedron, the S e-O distance is 1.66 Angstrom after correction for libration. A layer-like hydrogen bonding system is suggested. The low temperature structure, CsHSeO 4-II (mP28), was refined from X-ray diffraction data with an accuracy which made it possible to localize the H atom. Crystallographic relations betwee n the two phases during the phase transition are discussed.