Basic and charge density wave modulated structures of NbS3-II

The basic and the charge density wave (CDW) structures of the monoclinic NbS3-II polymorph were studied by synchrotron x-ray diffraction, ab initio calculations, simulation of electron diffraction patterns, and by atomic-resolution transmission electron and low-temperature scanning tunneling microscopies. It is confirmed that the basic structure belongs to the space group P2(1)/ m and is described with a unit cell, formed of four pairs of symmetry-related trigonal prismatic (TP) columns [a(0) = 0.96509(8) nm, b(0) = 0.33459(2) nm, c(0) = 1.9850(1) nm, and beta(0) = 110.695(4)degrees]. The incommensurate components of the two CDWs, (q) over bar (1) = (0, 0.298, 0) and (q) over bar (2) = (0, 0.352, 0), are related as (q) over bar (1b) + 2q(2b) approximate to 1. Both CDWs form their own modulation patterns with unit cells (a(m) = 2a(0), b(m) = b(0)/q(jb), c(m) = c(0), beta(m) = beta(0)) and are ordered along adjacent isosceles TP columns either pairwise or with both columns modulated by either the (q) over bar (1) or (q) over bar (2) CDW only. The CDWs are ordered according to one of the two possible modulation pattern space groups, Cm or C2/ m. If considered as long-period commensurate, the entire modulated structure with both CDWs included is described within experimental error with an enlarged unit cell (a = 2a(0), b = 37b(0), c = c(0), and beta = beta(0)) and with all atoms displaced from their average positions in accord with the specified modulation pattern.