Ytterbium valence ordering in the low-temperature superstructure of Yb₂Pd₂Cd

The intermetallic ytterbium compound Yb2Pd2Cd shows a structural phase transition at similar to 150 K. The structures of the room-temperature (Mo2B2Fe type, P4/mbm, a = 757.07(7), c = 371.99(4) pm, wR2 = 0.0620, 272 F-2 values, 12 variables) and low-temperature (new type, P4/mbm, a = 747.26(4), c = 741.46(4) pm, wR2 = 0.0384, 511 F-2 values, 19 variables) modifications were refined from single crystal X-ray diffractometer data. The superstructure formation corresponds to an isomorphic transition of index 2. The driving force for the structural phase transition is an ytterbium charge ordering (the superstructure exhibits two crystallographically independent ytterbium sites) with a much higher degree of divalent ytterbium in the low-temperature modification. The striking structural feature concerns the ytterbium-palladium coordination with different Yb-Pd distances: longer ones for predominantly divalent Yb1 (2 x 284.7 and 4 x 296.1 pm, & empty; = 292.3 pm) and shorter ones for trivalent Yb2 (2 x 277.5 and 4 x 288.4 pm, & empty; = 284.8 pm).