The structures of ScAuSi, YAuSi and LuAuSi were determined by single-crystal diffractometry. ScAuSi, hP6, hexagonal P6m2 (no. 187), a=4.212(1) A ̊, c=7.546(3) A ̊, Z=2, R=0.039 using 66 reflections, represents a new ternary ordered type. YAuSi, hP6, hexagonal P63mc (no. 186),a=4.288(2) A ̊, c=7.546(3) A ̊, Z=2, R=0.039 using 66 reflections, is isotypic with LiGaGe (ternary ordered derivative of the CaIn2 structure). LuAuSi, isotypic with ScAuSi, a=4.267(2) A ̊, c=7.151(3) A ̊, was refined to R=0.052 using 90 reflections. In both ScAuSi and YAuSi structures trigonal prisms of scandium or yttrium sharing their lateral faces are stacked along the c axis. The gold and silicon atoms forming rumpled layers centre the prisms in an ordered way. The difference between the two structures consists in the different stacking of these layers along the c axis. In YAuSi atoms of gold in a layer alternate with atoms of silicon in the subsequent layer, and vice versa; in ScAuSi the layers become equally oriented so that Au-Au and Si-Si pairs are formed.
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