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We report on the synthesis and thermal decomposition of Th 2 X 5  (X = S, Se) compounds and the structure refinement of Th 2 Se 5  by Rietveld procedures. The latter compound crystallizes in the space group  Pcnb  with the lattice parameters:  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 2 S 5  and Th 2 Se 5  decompose between 873 K and 1073 K into ThS 2  and ThSe 2 , respectively, which crystallize in a PbCl 2  type structure. The crystal chemistry of An 2 X 5  compounds (An = Th, U, Np; X = S, Se) is discussed and an explanation is given for contradictory descriptions of their structures in the 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 linear chalcogenide chain. It may be shown by an analysis of Cheshire groups that seemingly different sets of positional parameters are equivalent descriptions of the same structure.

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