Pressure-induced reentrant transition in $\mathrm{Nb}{\mathrm{S}}_3$ phases: Combined Raman scattering and x-ray diffraction study

We report the evolution of charge density wave states under pressure for two $\mathrm{Nb}{\mathrm{S}}_3$ phases: triclinic (phase I) and monoclinic (phase II) at room temperature. Raman and x-ray diffraction (XRD) techniques are applied. The x-ray studies on the monoclinic phase under pressure show a compression of the lattice at different rates below and above ∼7 GPa but without a change in space group symmetry. The Raman spectra of the two phases evolve similarly with pressure; all peaks almost disappear in the ∼6–8 GPa range, indicating a transition from an insulating to a metallic state, and peaks at new positions appear above 8 GPa. The results suggest suppression of the ambient charge-density waves and their subsequent recovery with new orderings above 8 GPa.

Figure 1

(a) XRD pattern of $\mathrm{Nb}{\mathrm{S}}_3-\mathrm{II}$. (b) Raman scattering spectrum for $\mathrm{Nb}{\mathrm{S}}_3-\mathrm{II}$ with peak and respective vibrational modes (black: observed data; red: peak fitting; and green: individual fitted peaks). (c) Raman scattering spectrum for $\mathrm{Nb}{\mathrm{S}}_3-\mathrm{I}$ sample.

Figure 2

(a) Synchrotron XRD patterns of $\mathrm{Nb}{\mathrm{S}}_3-\mathrm{II}$ under various high pressures (the dashed line is the guide for an eye) and (b) GSAS refinement on XRD patterns of $\mathrm{Nb}{\mathrm{S}}_3-\mathrm{II}$ under the pressures of 5 and 13 GPa.

Figure 3

(a) Pressure dependence of lattice parameters (left axis: $a$, first right axis: $b$, and second right axis: $c$). (Inset) Pressure dependence of β. (b) Experimental compression curve of $\mathrm{Nb}{\mathrm{S}}_3-\mathrm{II}$ and EoS fitting in separate regions below and above 7.66 GPa, the estimated bulk modulus.

Figure 4

(a) Raman scattering spectra of $\mathrm{Nb}{\mathrm{S}}_3-\mathrm{II}$ under selected pressures up to 15 GPa. (b) Pressure evolution of Raman shifts. Gray color region: the metallic region where is no observance of Raman peaks.

Figure 5

(a) Raman scattering spectra of $\mathrm{Nb}{\mathrm{S}}_3-\mathrm{I}$ under selected pressures up to 9.5 GPa; (b) Pressure evolution of Raman shifts. Gray color region: the metallic region where is no observance of Raman peaks.

Figure 6

Electronic band structure of $\mathrm{Nb}{\mathrm{S}}_3-\mathrm{I}$ at (a) 0 and (b) 6 GPa.

Figure 7

Fermi surface of $\mathrm{Nb}{\mathrm{S}}_3-\mathrm{I}$ at (a) 5 and (b) 6 GPa.