  Charge density wave (CDW) ordering in NbSe3 and the structurally related
quasi one-dimensional compounds is reconsidered. Since the modulated ground
state is characterized by unstable nano-domains, the structural information
obtained from diffraction experiments is to be supplemented by some additional
information from a method, able to reveal details on a unit cell level.
Low-temperature (LT) scanning tunneling microscopy (STM) can resolve both, the
local atomic structure and the superimposed charge density modulation. It is
shown that the established model for NbSe3 with two incommensurate (IC) modes,
q1 = (0,0.241,0) and q2 = (0.5,0.260,0.5), locked in at T1=144K and T2=59K and
separately confined to two of the three available types of bi-capped trigonal
prismatic (BCTP) columns, must be modified. The alternative explanation is
based on the existence of modulated layered nano-domains and is in good accord
with the available LT STM results. These confirm i.a. the presence of both IC
modes above the lower CDW transition temperature. Two BCTP columns, belonging
to a symmetry-related pair, are as a rule alternatively modulated by the two
modes. Such pairs of columns are ordered into unstable layered nano-domains,
whose q1 and q2 sub-layers are easily interchanged. The mutually
interchangeable sections of the two unstable IC modes keep a temperature
dependent long-range ordering. Both modes can formally be replaced by a single
highly inharmonic long-period commensurate CDW.
