Commensurately modulated structure of4Hb−TaSe2determined by x-ray crystal-structure refinement

Abstract

The (ͱ13aϫͱ13aϫc) superstructure of 4H b -TaSe 2 at room temperature has been determined by x-ray crystal-structure refinement using ͑3ϩ2͒-dimensional superspace. The resulting modulation has its largest amplitudes for a longitudinal displacement of the Ta atoms in the octahedrally coordinated ͑T͒ 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 layers͒, 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 ͑STM͒: the superstructure features observed by STM on the H layers is a direct consequence of the atomic displacements in this layer.

Femtosecond streaking of electron diffraction patterns to study structural dynamics in crystalline matter

“…Here the bright peaks are the Bragg peaks of the underlying host lattice, while the ð ffiffiffiffiffi 13 p  ffiffiffiffiffi 13 p ÞR13.9 CDW superstructure appearing below 410 K gives rise to 6 faint peaks (referred to as CDW peaks) surrounding each of the bright lattice reflections. 16 To extract the transients from the streaked diffraction patterns as seen in Fig. 2(b), all streaks were superimposed for improved SNR (see Fig.…”

Structure of the two-dimensional incommensurate charge-density wave in(PO2)4(WOLüdecke1, Jobst2, Geupel3 et al. 2001Phys. Rev. BSelf Cite7060View full textAdd to dashboardCite

“…As reported earlier 8 the lattice remained orthorhombic at Tϭ20 K with lattice parameters aϭ5.294(1) Å, b ϭ6.591(2) Å, and cϭ17.415 (16) Å. The components of the modulation wave vectors corresponding to the first CDW were obtained from the positions of the satellite reflections as determined by Q scans along the three lattice directions.…”

Photoinduced Dynamics in the Charge Density Wave Compound 4HB-TaSe2

“…The metal atoms are arranged in a planar hexagonal configuration with 3 closest chalcogenide atoms above and below. In TaSe 2 as one example of this class, the selenite atoms can arrange in different geometries within the layers such as octahedral or trigonal prismatic, representing different polytypes 1T-TaSe 2 and 2H-TaSe 2 , respectively [1]. Due to strong electron coupling, commensurate or incommensurate charge density waves (CDW) are developed below corresponding phase transition temperatures, accompanied by periodic lattice displacements (PLD).…”

“…The pure 2H polytype develops an incommensurate CDW only below 122 K. The compound under consideration here, 4H B -TaSe 2 , is a combination of the two: it consists of alternating layers of octahedral (T) and trigonal prismatic (H). Below 410 K the octahedral layers lock into the √13 x √13 order [1], and display the corresponding diffraction patter n at room temperature, see Fig. 1, whereas the intermediate H layers act as a shield between the twodimensional CDWs.…”