Tb₃Pd₂, Er₃Pd₂ and Er₆Co_5-x: structural variations and bonding in rare-earth-richer binary intermetallics

The three binary Tb/Er-rich transition metal compounds Tb₃Pd₂ (triterbium dipalladium), Er₃Pd₂ (trierbium dipalladium) and Er₆Co_5-x (hexaerbium pentacobalt) crystallize in the space groups Pbam (Pearson symbol oP20), P4/mbm (tP10) and P6₃/m (hP22), respectively. Single crystals of Tb₃Pd₂ and Er₆Co_5-x suitable for X-ray structure analysis were obtained using rare-earth halides as a flux. Tb₃Pd₂ adopts its own structure type, which can be described as a superstructural derivative of the U₃Si₂ type, which is the type adopted by Er₃Pd₂. Compound Er₆Co_5-x belongs to the Ce₆Co_2-xSi₃ family. All three compounds feature fused tricapped {TR₆} (R = rare-earth metal and T = transition metal) trigonal prismatic heterometallic clusters. R₃Pd₂ is reported to crystallize in the U₃Si₂ type; however, our more detailed structure analysis reveals that deviations occur with heavier R elements. Similarly, Er₆Co_5-x was assumed to be stoichiometric Er₄Co₃ = Er₆Co_4.5. Our studies reveal that it has a single defective transition-metal site leading to the composition Er₆Co_4.72(2). LMTO (linear muffin-tin orbital)-based electronic structure calculations suggest the strong domination of heteroatomic bonding in all three structures.