Commensurately modulated structure of 4Hb-TaSe2 determined by x-ray crystal-structure refinement

The (root 13ax root 13axc) superstructure of 4H(b)-TaSe2 at room temperature has been determined by x-ray crystal-structure refinement using (3+2)-dimensional superspace. The resulting modulation has its largest amplitudes for a longitudinal displacement of the Ta atoms in the octahedrally coordinated (T) layers, thus leading to a superstructure of hexagram-shaped clusters containing thirteen Ta atoms each. The atoms in the corresponding planes of Se atoms show a transversal modulation, which can be interpreted as the relaxation of these layers to the modulation of the Ta layers. Forming a similar pattern, modulations were found of the atoms in the layers with Ta in trigonal prismatic coordination (H layers), with amplitudes of about 1/10 of the sizes of the modulations in the T layers. These results confirm the presence of a charge-density wave in the T layers, while the modulations of the H layer can be assigned to elastic coupling between the two types of layers. The observation of nonzero displacements of the atoms of the H layers provides an interpretation for recent observations by scanning tunneling microscope (STM): the superstructure features observed by STM on the H layers is a direct consequence of the atomic displacements in this layer. [S0163-1829(99)03306-8].