Pseudo-symmetry in the crystal structure of An(2)X(5) compounds (An = Th, U, Np; X = S, Se)

We report on the synthesis and thermal decomposition of Th2X5 (X = S, Se) c ompounds and the structure refinement of Th2Se5 by Rietveld procedures. The latter compound crystallizes in the space group Pcnb with the lattice para meters: a = 792.23(4)pm, b = 793.75(4) pm, c = 1071.55(3) pm (Z = 4). There is no homogeneity range in such compounds. Under high vacuum conditions Th 2S5 and Th2Se5 decompose between 873 K and 1073 K into ThS2 and ThSe2, resp ectively, which crystallize in a PbCl2 type structure. The crystal chemistr y of An(2)X(5) compounds (An = Th, U, Np; X = S, Se) is discussed and an ex planation is given for contradictory descriptions of their structures in th e literature. The common orthorhombic structure type may be explained as a distortion of a higher symmetric tetragonal arrangement by the formation of X-2(2-) dumb-bells, i.e. alternating short and long X-X distances in a lin ear chalcogenide chain. It may be shown by an analysis of Cheshire groups t hat seemingly different sets of positional parameters are equivalent descri ptions of the same structure.