Self-Intercalation and Vacancy-Ordering in 6R-CuxTa1+yS2 (x ∼ 0.23, y=0, 0.06)

The eightfold 2a0x2b0x2c0 superstructures of 6R-CuxTa1+yS2 are reported for crystal A with x = 0.237 and y = 0, and crystal B with x = 0.23 and y = 0.06. Compelling evidence is presented for the self-intercalation of tantalum onto octahedral sites, based on the diffraction data and crystal chemical arguments. The eightfold superstructure is formed by partial vacancy ordering within planes of intercalated atoms, while the stacking of TaS2 layers remains that of the 6R polytype. Temperature-dependent X-ray diffraction experiments show that the superstructure exists between T = 14 K and at least 370 K. Intercalated atoms enter the compound at octahedral and tetrahedral sites in alternating Van der Waals gaps. Intercalation is inhomogeneous, with average occupancies varying between 0.11 for octahedral sites within one Van der Waals gap and 0.23 for the tetrahedral sites within another Van der Waals gap. It is proposed that partial vacancy ordering is governed by the principle of maximum separation between intercalated atoms.