The structures of GdAsSe and NdAsSe were determined from single crystal data. GdAsSe crystallizes in the monoclinic CeAsS-type structure. The deviations from the parent tetragonal ZrSiS-type structure are smaller than in the corresponding sulphides. The As-As distances in the infinite arsenic chains appear to be too large to warrant non-metallic properties. The structure of NdAsSe is isopuntal with the CeAsS-type; however, only As pairs can be identified. The CeAsS-type structure was also found in the LnAsSe compounds with Ln = Pr, Sm, Tb, . . ., Tm, Lu, Y. The distortions of the ZrSiS-type substructure become more pronounced on going to smaller cations. The stoichiometry of the samples depends strongly upon the preparation.
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