  The crystal structures of ANb$_3$Br$_7$S (A = Rb and Cs) have been refined by
single crystal X-ray diffraction, and are found to form highly anisotropic
materials based on chains of the triangular Nb$_3$ cluster core. The Nb$_3$
cluster core contains seven valence electrons, six of them being assigned to
Nb-Nb bonds within the Nb$_3$ triangle and one unpaired d electron. The
presence of this surplus electron gives rise to the formation of correlated
electronic states. The connectivity in the structures is represented by
one-dimensional [Nb$_3$Br$_7$S]$^-$ chains, containing a sulphur atom capping
one face ($\mu_3$) of the triangular niobium cluster, which is believed to
induce an important electronic feature. Several types of studies are undertaken
to obtain deeper insight into the understanding of this unusual type of
material: the crystal structure, morphology and elastic properties are
analysed, as well the (photo-) electrical properties and NMR relaxation.
Electronic structure (DFT) calculations are performed in order to understand
the electronic structure and transport in these compounds, and, based on the
experimental and theoretical results, we propose that the electronic
interactions along the Nb chains are sufficiently one-dimensional to give rise
to Luttinger liquid (rather than Fermi liquid) behaviour of the metallic
electrons.
