Crystals of a new compound with a superprotonic phase transition Cs 2 (HSeO 4 )(H 2 AsO 4 ), have been synthesized for the first time by a slow evaporation method at room temperature. The structure was solved by a three-dimensional Patterson function and refined to R 1 = 0.0309 and WR 2 = 0.0482 on the basis of 2062 unique observed reflections using 67 parameters. The structure contains zigzag chains of hydrogen bonded anion tetrahedra that extend in the [010] direction. Each tetrahedron is

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... tionally linked to a tetrahedron neighbouring chain to give a planar structure with hydrogen-bonded sheets lying parallel to (1 0⎯1). Thermal-differential analysis of the superprotonic transition in Cs 2 (HSeO 4 )(H 2 AsO 4 ) showed that the transformation to hightemperature phase occurs at 515 K by one-step process. Thermal decomposition of the product takes place at much higher temperatures, with an onset of approximately 760 K. The superprotonic transition was also studied by impedance and modulus spectroscopy techniques. The conductivity in the high temperature phase at 523 K is 2.91 × 10 -4 Ω -1 cm -1 , and the activation energy for the proton transport is 0.16 eV. The conductivity relaxation parameters associated with the high disorder protonic conduction have been examined from analysis of the M"/M" max spectrum measured in a wide temperature range. Transport properties in this material appear to be due to proton hopping mechanism.