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 Tb7Pd3 and Er17Pd7 are compositionally approaching (less than 1 atom % difference) the previously reported R2.16Pd0.89 (R = Tb and Er), and apparently form by peritectoid transformation, thus, being hard to detect by fast cooling. Tb7Pd3 (1) crystallizes in the Th7Fe3 structure type (hP20, P63mc, a = 9.8846(4) Å, c = 6.2316(3) Å, Z = 2) while Er17Pd7 (2) belongs to the Pr17Co7 type being its second reported representative (cP96, P213, a = 13.365(2) Å, Z = 4). Er17Pd7 (2) is overlapping with the cubic F-centered Er2.11Pd0.89 (3b, Fd3̅m, 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.