Binary Intermetallics in the 70 atom % R Region of Two R–Pd Systems (R = Tb and Er): Hidden, Obscured, or Nonexistent?

Although rare-earth-metal-transition-metal (R/T) phase diagrams have been explored extensively, our recent studies have uncovered new previously nonexistent binary intermetallics. These compounds belong to a narrow region between 70 and 71.4 atom % of the rare-earth metal but represent four different structure types. The binaries Tb 7 Pd 3 and Er 17 Pd 7 are compositionally approaching (less than 1 atom % difference) the previously reported R 2.16 Pd 0.89 (R = Tb and Er), and apparently form by peritectoid transformation, thus, being hard to detect by fast cooling. Tb 7 Pd 3 ( 1 ) crystallizes in the Th 7 Fe 3 structure type ( hP 20, P 6 3 mc , a = 9.8846(4) Å, c = 6.2316(3) Å, Z = 2) while Er 17 Pd 7 ( 2 ) belongs to the Pr 17 Co 7 type being its second reported representative ( cP 96, P 2 1 3, a = 13.365(2) Å, Z = 4). Er 17 Pd 7 ( 2 ) is overlapping with the cubic F -centered Er 2.11 Pd 0.89 ( 3b , Fd 3̅ m , a = 13.361(1) Å, Z = 32) with practically identical unit cell parameters but a significantly different structure. Electronic structure calculations confirm that heteroatomic R-T bonding strongly dominates in all structures; T-T bonding interactions are individually strong but do not play a significant role in the total bonding.