High-temperature superionic phase of mixed proton conductor [Rb-0.57(NH4)(0.43)](3)H(SeO4)(2): Dynamic twinning and anomalous display of dynamically disordered hydrogen atoms

The crystal structure of the superionic phase of mixed trirubidium-ammonium hydrogen biselenate, [Rb-0.57(NH4)(0.43)](3)H(SeO4)(2), has been determine d at T = 370 K. The compound is trigonal, space group R (3) over bar m, a = 6.073(1) Angstrom, c = 22.636(3) Angstrom, V = 722.9(2) Angstrom(3), M-r = 456.34, D-x = 3.12 g . cm(-3) mu = 225.5 cm(-1). Z = 3, lambda(CuKa) = 1.5 41834 Angstrom, F(000) = 631.38, R = 0.025 for 217 unique reflections with I > 3 sigma(I). A two-dimensional sublattice of an acid hydrogen bond netwo rk linking the SeO4 tetrahedra was found to be dynamically disordered. Desp ite the low occupancies of the hydrogen atoms in their crystallographic pos itions all hydrogen atoms are clearly observed on corresponding electron de nsity maps. This "anomalous" display of the dynamically disordered hydrogen atoms allows the mechanism of the superprotonic transport in M3H(AO(4))(2) compounds to be elucidated in detail. Dynamic twinning has been observed i n our study.