The stannides RE2Ni2Sn (RE=Pr, Ho, Er, Tm) were synthesized by arc-melting of the elements and characterized by powder X-ray diffraction. Pr2Ni2Sn crystallizes with the orthorhombic W2B2Co-type structure, Immm, a=443.8(1), b=572.1(1), c=855.1(2) pm, wR2=0.0693, 293 F2 values, 13 variables. A structural transition to the tetragonal Mo2B2Fe type occurs for the heavier rare earth elements. The structures of Ho2Ni2Sn (a=729.26(9), c=366.66(7) pm, wR2=0.0504, 250 F2 values, 12 variables), Er2Ni2Sn (a=727.2(2), c=364.3(1) pm, wR2=0.0397, 262 F2 values, 12 variables), and Tm2Ni2Sn (a=725.2(1), c=362.8(1) pm, wR2=0.0545, 258 F2 values, 12 variables) were refined from single-crystal diffractometer data. The switch in structure type is driven by the size of the rare earth element. The [Ni2Sn] substructures are composed of Ni2Sn2 squares and Ni4Sn2 hexagons in Pr2Ni2Sn, and of Ni3Sn2 pentagons in Er2Ni2Sn. The Ni4Sn2 hexagons and Ni3Sn2 pentagons exhibit Ni2 pairs with Ni-Ni distances of 247 pm in Pr2Ni2Sn, and of 250 pm in Er2Ni2Sn.
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