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, in contrast to ionic conductors of the type Na₂UX₆ for X = Cl, Br which crystallize in the space group Pml. Atomic positions of sodium and iodine have been refined using the Rietveld profile fitting procedure. It was found that in space group R 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.