The structure of Na₂UCl₆ and Li₂UCl₆ from multiphase powder neutron profile refinement

The structures of Na₂UCl₆ and Li₂UCl₆ have been determined by powder neutron diffraction at room temperature in the presence of NaCl and LiCl respectively as impurity. Na₂UCl₆crystallizes in space group Pm1, with a = 11 8062(9), c = 6.3243(2) Å, and Z = 3. The best description of Li₂UCl₆ was found in space group P6₃/mmc, a = 11.1915(5), c = 6.0356(1) Å, and Z = 3, with uranium and lithium ions disordered over the available octahedral sites. The results give the first illustration of the relative ease of analysing a two-phase mixture using a Rietveld profile program adapted to refine simultaneously more than one phase.