Coexistence of Uniform and Dimerized Chains in Low-Dimensional NbS ₃ : Metallic vs Semiconducting Character

A recently reported polymorph of the low-dimensional material NbS₃ remarkably exhibits both uniform and dimerized Nb chains. This is surprising because known polymorphs exclusively contain either uniform (NbS₃-V) or dimerized (NbS₃-I) Nb chains which are associated with metallic or semiconducting behavior, respectively. A detailed first-principles study of the new phase (NbS₃-VI) has been carried out to discuss the implications of this peculiar structure for the transport and low-temperature structural properties. The new polymorph is found to be metallic. The electronic structure around the Fermi level is unexpectedly found to result from the hybridization of two one-dimensional (1D) sets of bands along perpendicular directions. This is at the basis of the stability of the new polymorph which cannot collapse into the more stable NbS₃-I polymorph. Several model NbS₃ phases containing both uniform and dimerized Nb chains have been explored. The stability of the new phase under pressure has also been considered. Under moderate pressures, the new polymorph is more stable than NbS₃-I, the most stable phase at ambient pressure. However, under pressures up to 15 GPa, NbS₃-V', another polymorph with only uniform chains, is predicted to be always slightly more stable.