NdSe1,9 has been prepared as grey, strongly reflecting platelets by reaction of elemental neodymium with selenium in the ratio 1 :2 and subsequent chemical vapour phase transport with iodine. It crystallizes in the tetragonal space group P42/n with a = 925.36(7), c = 1679.2(3) pm and Z = 20. The structure determination results in a formulation of NdSe1,9 as Nd 20(Se(I)2)8Se(I)2⃞2Se(II)20, where the defect layer built from the Se(I)22- and Se(I)2- ions corresponds to the F layer in the PbFCl substructure.
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