Commensurately modulated structure of 4H(b)-TaSe2 determined by x-ray crystal-structure refinement

The (root 13ax root 13axc) superstructure of 4H(b)-TaSe2 at room temperatur e has been determined by x-ray crystal-structure refinement using (3+2)-dim ensional superspace. The resulting modulation has its largest amplitudes fo r a longitudinal displacement of the Ta atoms in the octahedrally coordinat ed (T) layers, thus leading to a superstructure of hexagram-shaped clusters containing thirteen Ta atoms each. The atoms in the corresponding planes o f Se atoms show a transversal modulation, which can be interpreted as the r elaxation of these layers to the modulation of the Ta layers. Forming a sim ilar pattern, modulations were found of the atoms in the layers with Ta in trigonal prismatic coordination (H layers), with amplitudes of about 1/10 o f the sizes of the modulations in the T layers. These results confirm the p resence 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 la yers. The observation of nonzero displacements of the atoms of the H layers provides an interpretation for recent observations by scanning tunneling m icroscope (STM): the superstructure features observed by STM on the H layer s is a direct consequence of the atomic displacements in this layer. [S0163 -1829(99)03306-8].