Basic and charge density wave modulated structures of NbS3 -II

Abstract

The basic and the charge density wave (CDW) structures of the monoclinic NbS3-II polymorph were studied by synchrotron x-ray diffraction, ab-initio calculation, simulation of electron diffraction patterns and by atomicresolution transmission electron and low-temperature scanning tunneling microscopies. It is shown that the basic structure belongs to the space group P 21/m and is described with a unit cell, formed of four pairs of symmetry-related trigonal prismatic (TP) columns (a0 = 0.96509(8) nm, b0 = 0.33459(2) nm, c0 = 1.9850(1) nm, β0 = 110.695(4) • ), with all Nb and S atoms in 2e special positions. The two CDWs, with q1 = (0, 0.298,0) and q2 = (0, 0.352, 0), form their own modulation unit cells (am = 2a0, bm = b0/qjb, cm = c0, βm = β0) and are ordered pairwise along adjacent isosceles TP columns. The symmetries of both qj modes belong to the space group Cm and are related according to the 2a special positions. If considered as long-period commensurate, the entire modulated structure with both CDWs included is best described with an enlarged unit cell (a = 2a0, b = 37b0, c = c0, β = β0), with all Nb and S atoms in 1a positions of the space group P 1.

Rich Polymorphism of Layered NbS3

“…The calculated cell parameters, relative energy, and band gaps are reported in Table 1. One of them is the wellknown NbS 3 -I phase, 20 three correspond to the recently characterized NbS 3 -II, 19 NbS 3 -IV, 27 and NbS 3 -V 27 phases, and the remaining three are so-far unreported polymorphs with the structures of NbSe 3 , TiS 3 , and TaSe 3 . The agreement between experimental and theoretical structures for the four presently characterized compounds is excellent, thus lending reliability to these calculations.…”

“…The unit cell of NbS 3 -II contains eight chains of Nb atoms trigonally coordinated with S atoms running along the b-direction (Figure 1d). There are four different types of NbS 3 chains which in order to simplify the discussion have been highlighted with different colors in Figure 1d (to help the comparison with experimental results, these colors are the same as those in the recent structural determination of the average structure 19 ). The purple and yellow chains are paired in the respective dimer units, whereas the red and orange chains form mixed red-orange pairs.…”

“…1 Although different superstructures of the basic NbS 3 -I structure were proposed for this polymorph, a clear-cut structural determination remained elusive until recently (Figure 1d). 19 NbS 3 -II is metallic 23 and has been reported to exhibit three CDWs at around 450−475, 360, and 150 K, respectively. 24 This behavior is reminiscent of the NbSe 3 and m-TaS 3 systems, but now with an additional CDW transition.…”

Synthesis and self-assembly of one-dimensional nanostructures of a transition metal trichalcogenide

“…In a single batch of the crystal growth product, materials with different phases and compositions can coexist, which lead to challenges in studying and understanding both the growth mechanism and the physical properties of the material (4). Moreover, the charge density wave (CDW) transitions and therefore the associated periodic lattice distortions of NbS3 have been shown to depend on the material's crystal phase (5).…”

Anisotropic quasi-one-dimensional layered transition-metal trichalcogenides: synthesis, properties and applications

“…[58][59][60][61][62] The NbS 3-II polymorph was rst acknowledged in 1978 following electron diffraction experiments, with weak pair satellite diffraction streaks that resolve into rows of spots upon modications in temperature. 63,64 It is proposed that NbS 3-II is an elevated structure of NbS 3-I with different types of chain accretion, and the rows of spots arise at random positions with analogous separation to NbS 3-I (Table 2). NbS 3-II is testied to have three CDWs at 150 K, 330-370 K, and 620-650 K. 61 In 1982, Kikkawa and co-workers obtained monoclinic NbS 3-HP via the extraordinary high-pressure modi-cation of NbS 3 synthesized at 700 C with 2 GPa pressure.…”