Lattice distortions in layered type arsenides LnTAs2 (Ln=La–Nd, Sm, Gd, Tb; T=Ag, Au): Crystal structures, electronic and magnetic properties

The lanthanide coinage-metal diarsenides LnTAs2 (Ln=La, Ce–Nd, Sm; T=Ag, Au) have been reinvestigated and their structures have been refined from single crystal X-ray data. Two different distortion variants of the HfCuSi2 type are found: PrAgAs2, NdAgAs2, SmAgAs2, GdAgAs2, TbAgAs2, NdAuAs2 and SmAuAs2 crystallize as twofold superstructures in space group Pmcn with the As atoms of their planar layers forming zigzag chains, whereas LaAgAs2, CeAgAs2 and PrAuAs2 adopt a fourfold superstructure (space group Pmca) with cis-trans chains of As atoms. The respective atomic positions can be derived from the HfCuSi2 type by group-subgroup relations. The compounds with zigzag chains of As atoms exhibit metallic behaviour while those with cis-trans chains are semiconducting as measured on powder pellets. The majority of the compounds including 4f elements show antiferromagnetic ordering at TN<20 K.