Seven nearly stoichiometric compounds with general formulas RESn 1+x Ge 1-x (RE ) Y, Gd-Tm; x ≈ (0.15) have been synthesized from the corresponding elements using high-temperature reactions and molten Sn as a metal flux. They crystallize with the centrosymmetric space group Cmcm (No. 63) and their structures can be viewed as built up of two distinct polyanionic moieties: infinite ∞ 1 [Ge 2 ] zigzag chains and square sheets of tin atoms, ∞ 2 [Sn], with rare-earth cations enclosed between them. Within this formalism, such a bonding arrangement can also be described as a ternary derivative of the ZrSi 2 structure (Pearson's symbol oC12), obtained by "coloring" the anionic sites with two different elements, Ge and Sn in the present case. This is a unique aspect of the crystal chemistry of the intermetallic compounds in general, because elements from the same group typically do not show a tendency for site preferences. It is discussed in detail, along with an analysis of the trends across the whole series. The temperature dependence of the magnetic susceptibility, resistivity, and specific heat for all members of this family (some measured down to 400 mK and in applied fields up to 70 kOe) are reported as well. All lanthanide compounds are metallic and order antiferromagnetically at low temperatures.

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