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Single crystals of Cu xTa1+ ySe2 were grown by chemical vapor transport. Single crystals of different compositions were obtained at slightly different reaction conditions from mixtures of the reactants of the same nominal composition. It is suggested that different diameters of the ampoules imply different contributions of convection and diffusion to the mass transport, and thus are responsible for different ratios of the amount of Cu, Ta, and Se transported. 2H-Cu0.52TaSe2 ( x = 0.52, y = 0) is formed in the narrower ampoule (diameter 15 mm). The crystal structure is based on the MoS2 type of stacking of TaSe2 layers. Partial ordering of Cu over the tetrahedral sites is responsible for a 2 a0 × 2 b0 × c0 superstructure with hexagonal P $\bar{6}$ m2 symmetry [ a0 = 3.468 (1) Å, c0 = 13.568 (3) Å]. 2H-Cu0.16Ta1.08Se2 ( x = 0.16, y = 0.08) is formed in the wider ampoule (diameter 18 mm). It possesses a NbS2-type of stacking. A superstructure is not formed, but the presence of Cu and intercalated Ta in alternating van der Waals gaps is responsible for the reduction of symmetry from P63/ mmc to P $\bar{6}$ m1 [ a0 = 3.439 (2) Å, c0 = 12.870 (2) Å]. Single crystals are formed towards the hotter side of the ampoules up to a temperature of 1168 K in both reactions. [ABSTRACT FROM AUTHOR]

The eightfold 2a0×2b0×2c0 superstructures of 6R-Cu xTa1+ 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. [ABSTRACT FROM AUTHOR]