Domain structure and refinement of the triclinic superstructure of 1T-TaSe2 by single crystal X-ray diffraction

The 13(1/2) a(o) x 13(1/2) a(o) superstructure of IT-TaSe2 at room temperat ure has been studied by single crystal X-ray diffraction. The basic structu re has symmetry P (3) over bar m1 with a(o) = 3.4733 Angstrom and c(o) = 6. 2715 Angstrom. The superstructure has a pseudo hexagonal lattice, but it is triclinic with a 169-fold, 13(1/2) a(o) x 13(1/2) a(o) x 13c(o) hexagonal supercell with 13 centering vectors. Structure refinements have been perfor med using space group P (1) over bar and the primitive unit cell with a bas al plane 13(1/2) a(o) x 13(1/2) a(o) supercell, and with C = 2a(o) + c(o). Refinements converged to R-F = 0.086 for 2327 observed reflections. Hexagra m shaped clusters of 13 Ta atoms are the principle feature of this 13-fold superstructure, in accordance with the study of Brouwer and Jellinek (Physi ca B 99 (1980) 51). In the present refinement, significant deviations are f ound from local 6/m symmetry as it was assumed by Brouwer and Jellinek. The relative phase of the superstructure on neighbouring layers represents the most favourable packing of the buckled planes of Se atoms, and elastic cou pling between Se atoms of consecutive layers is identified as the driving f orce for the observed inter-layer shift vector 2ao. The small deviations fr om trigonal symmetry of the clusters are responsible for the observation of an ordered superstructure, as it is opposed to a stacking of layers accord ing a random selection of shift vectors (2a(o), 2a(o) + b(o), and -a(o) - 3 b(o)) that are equivalent by the trigonal symmetry of the basic structure.