Spinodal decomposition of NbS$_3$ Whiskers with charge density waves

The grazing X-ray diffraction study of phase-II NbS$_3$ whiskers with charge density waves has shown that they exhibit pairs of broadened reflections from two close phases with the monoclinic angles $\beta_1$ and $\beta_2$ instead of single $(h0l)$ peaks. The phase with the smaller monoclinic angle $\beta_1=95.9^\circ$–$97.2^\circ$ is close in lattice parameters to phase I without charge density waves. Phase II with $\beta_2=99.9$–$100.0^\circ$ has a reduced b lattice parameter along the strongest bond, which implies an increased concentration of sulfur vacancies in this phase compared to phase I. It has been concluded that the studied whiskers have underwent a spinodal decomposition with the redistribution of sulfur vacancies resulting in the alternation of phases in the (001) direction perpendicular to the layers. A satellite structure indicating the formation of a superlattice with a period of 35.2 nm in this direction is observed in the most structurally perfect whisker of phase II. The spinodal decomposition is combined with high internal stresses present in whiskers with a monoclinic lattice. The partial spinodal decomposition is also observed in some phase-I whiskers without charge density waves with the appearance of thin layers of phase IV and phase II with the monoclinic angle $\beta=104^\circ$. The existence of thin layers of phase II in the bulk of phase I in such samples exposed to a high-frequency voltage has been confirmed by the presence of Shapiro steps on current–voltage characteristics. Spinodal decomposition should be taken into account to describe the properties of NbS$_3$ whiskers nominally belonging to a certain polytype.