Structure analysis of Na2UI6 by neutron powder diffraction

Based on investigations at 9 and 295 K, Na2UI6 has been shown to crystallize in a new structure type corresponding to space group R{\bar 3}, in contrast to ionic conductors of the type Na2UX6 for X = Cl, Br which crystallize in the space group P{\bar 3}ml. Atomic positions of sodium and iodine have been refined using the Rietveld profile fitting procedure. It was found that in space group R{\bar 3} the U, Na and I atoms occupy positions 3(a), 6(c) and 18(f), respectively. The relation between the Na2UCl6-type structure and the structure of Na2UI6 is discussed.