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

Abstract

Abstract The 131/2 a 0 x 131/2 a 0 superstructure of 1T-TaSe2 at room temperature has been studied by singel-crystal X-ray diffraction. The basic structure has symmetry P3̅m1 with a 0 = 3.4733 Å and c 0 = 6.2715 Å. The superstructure has a pseudo hexagonal lattice, but it is triclinic with a 169-fold, 131/2 a 0 x 131/2 a 0 x 13c 0 hexagonal supercell with 13 centering vectors. Structure refinements have been performed using space group P1̅ and the primitive unit cell with a basal plane 131/2 a 0 x 131/2 a 0 supercell, and with C = 2a 0 + c 0. Refinements converged to R F = 0.086 for 2327 observed reflections. Hexagram shaped clusters of 13 Ta atoms are the principle feature of this 13-fold superstructure, in accordance with the study of Brouwer and Jellinek (Physica B 99 (1980) 51). In the present refinement, significant deviations are found 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 coupling between Se atoms of consecutive layers is identified as the driving force for the observed inter-layer shift vector 2a 0. The small deviations from trigonal symmetry of the clusters are responsible for the observation of an ordered superstructure, as it is opposed to a stacking of layers according a random selection of shift vectors (2a 0, 2a 0 + b 0, and -a 0 - 3b 0) that are equivalent by the trigonal symmetry of the basic structure.