We report studies of the Seebeck coefficient $S$ of the quasi-one-dimensional compound $\mathrm{Nb}{\mathrm{S}}_3$, together with the temperature dependence of its specific conductivity ${\sigma }_{\mathrm{s}}$ and heat capacity $c_p$. The monoclinic phase $(\mathrm{Nb}{\mathrm{S}}_3$-II) is studied over the temperature range $T=80–400$ K, which covers two charge density wave (CDW) transitions at $T_{\mathrm{P}1}=360$ K (CDW-1) and $T_{\mathrm{P}2}=150$ K (CDW-2). The $S\left(T\right)$ curves show features in the vicinities of both CDW transitions and appear to be correlated with the value of ${\sigma }_{\mathrm{s}}\left(300\mathrm{K}\right)$: The increase of $S$ below $T_{\mathrm{P}1}$ in the high-Ohmic samples reveals a complete dielectrization of the electronic spectrum, while in the low-Ohmic samples $S$ decreases below $T_{\mathrm{P}1}$ and even becomes negative below $T_{\mathrm{P}2}$. The magnitude of $S$ in low-Ohmic samples at $T<T_{\mathrm{P}2}$ is well below $k_{\mathrm{B}}/e\approx 86\mu \mathrm{V}/\mathrm{K},k_B$ being the Boltzmann constant and $e$ the elementary charge, which is surprisingly low for a usual CDW semiconducting state. Our results suggest that at $T_{\mathrm{P}1}$ the main electronic band with $p$-type carriers becomes gapped, while some $n$-type carriers can remain in a separate band with low density of states. These carriers, whose concentration is defined by the compositional doping, are gapped at $T_{\mathrm{P}2}$. We also report $S$ for the triclinic dielectric phase of $\mathrm{Nb}{\mathrm{S}}_3\left(\mathrm{Nb}{\mathrm{S}}_3\text{−}\mathrm{I}\right)$. Its low absolute value, $\sim k_{\mathrm{B}}/e$, and the anomalous temperature dependence demonstrate that $\mathrm{Nb}{\mathrm{S}}_3\text{−}\mathrm{I}$ is neither a semiconductor nor a CDW conductor in the usual sense.

• Received 19 September 2017

DOI:https://doi.org/10.1103/PhysRevB.99.235155