The low-temperature superstructures of 1T-TaSe2 and 2H-TaSe2

The structure of the low-temperature form of 1T-TaSe2 has been studied by X-ray diffraction. The symmetry of this form is triclinic (a = b = 12.54 Å; c = 9.36Å; α = 101.9°; β = 43.8°; γ = 120°), but twinning suggests trigonal symmetry. The content of the triclinic cell is Ta13Se26, the 13 Ta atoms forming a star-of-David-shaped cluster. It is proposed that the structure of the low-temperature form of 2H-TaSe2 contains clusters of 7 Ta atoms.