NA3AS REVISITED .2. SPACE GROUP CONFIRMATION AND STRUCTURE REFINEMENTAT 150, 210 AND 240K

The crystal structure of Na3As at 150 K, 210 K and 240 K was refined i n space group P6(3)cm to R1(F) = 0.0197, 0.0182, 0.0171 for F-o> 4 sig ma F-o), R1(F) = 0.0322, 0.0318, 0.0309 and wR2(F-2) = 0.0518, 0.0954, 0.0978 for all data respectively. It has been found that refinement o n F-2 gives much better results than refinement on F, particularly wit h respect to the U-ij, and U-eq values, which are by far better balanc ed after using the F-2 refinement strategy. There is a gradual decreas e of the U-ij values towards lower temperatures, as expected. In addit ion, the refinement in P6(3)cm was straightforward, regarding R-values and convergence. In contrast, refinement in space group P321, propose d by Cardoso Gil (Dissertation, University of Stuttgart, 1994) for Na3 As, was definitely worse, not converging at all and resulting in highe r R-values. It could also be shown that there is no need to lower the symmetry from hexagonal to trigonal, i.e. to change from space group P 6(3)cm to its maximal subgroup P(3)c1. It can thus be concluded with g ood confidence that space group P6(3)cm, proposed earlier by us, is in deed the correct one for Na3As.