The stannides RE2Ni2Sn (RE = Pr, Ho, Er, Tm) - Structural transition from the W2B2Co to Mo2B2Fe type as a function of the rare earth size

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