  We present a systematic study on the stability and the structural and
electronic properties of mixed molybdenum-niobium disulphides. Using density
functional theory we investigate bilateral doping with up to 25 % of MoS2
(NbS2) by Nb (Mo) atoms, focusing on the precise arrangement of dopants within
the host lattices. We find that over the whole range of considered
concentrations, Nb doping of MoS2 occurs through a substitutional mechanism.
For Mo in NbS2 both interstitial and substitutional doping can co-exist,
depending upon the particular synthesis conditions. The analysis of the
structural and electronic modifications of the perfect bulk systems due to the
doping is presented. We show that substitutional Nb atoms introduce electron
holes to the MoS2, leading to a semiconductor-metal transition. On the other
hand, the Mo doping of Nb2, does not alter the metallic behavior of the initial
system. The results of the present study are compared with available
experimental data on mixed MoS2-NbS2 (bulk and nanoparticles).
