Interaction between Charge Density Waves in the Monoclinic Phase of NbS3

In the monoclinic polytype NbS₃-II, the monoclinic b-axis of which coincides with the direction of greatest conductivity, two charge density waves (CDWs) have been observed at room temperature: CDW-0 with the wave vector q₀ = 0.5a* + 0.352b* + 0.5c* and CDW-1 with the wave vector q₁ = 0.5a* + 0.298b* + 0.5c*. X‑ray diffraction patterns obtained in the temperature range of 90–500 K have shown that the vectors q₀ and q₁ depend on the temperature, but their b* coordinates satisfy the relation \(2q_{0}^{{{\mathbf{b}}\text{*}}}(T) + q_{1}^{{{\mathbf{b}}\text{*}}}(T) = {\text{const}} = 1\). It has been concluded that the wave vector q₁ of CDW-1 is determined not only by the nesting of Fermi surfaces, but also by the vector q₀, because CDW-1 is adjusted to the second harmonic of CDW-0, which indicates the interaction between charge density waves. This conclusion has been confirmed by structural studies of NbS₃-II samples, the lattice of which is strongly deformed due to a high concentration of twins. In these samples, CDW-0 is conserved, and the wave vector of CDW-1 has the b* component not related to q₀.