We present band-structure calculations of ${\mathrm{SnTaS}}_2$, an intercalation compound in which the Sn atoms have a very uncommon linear coordination by two S atoms and an unusual formal valency ${\mathrm{Sn}}^{1+}$. The Sn 5$p_x$ and 5$p_y$ states show strong metal-metal bonding in the intercalant planes and the Fermi level is crossed by a very wide band composed of these Sn 5$p_x$$p_y$ states. Angle-resolved photoelectron spectra are in agreement with the calculated band structure. ${\mathrm{SnTaS}}_2$ is metallic as is reflected in the measured transport properties (resistivity, Hall effect, thermoelectric power). Low-temperature specific-heat measurements show an anomaly due to superconductivity below 2.8 K. The Sn 4d core levels, measured with photoelectron spectroscopy, show a splitting of 1 eV, indicating the presence of two different types of Sn atoms. This could be due to a valence disproportionation ${2\mathrm{S}\mathrm{n}}^{+}$→${\mathrm{Sn}}^0$+${\mathrm{Sn}}^{2+}$ and rapid valence fluctuations between ${\mathrm{Sn}}^0$ and ${\mathrm{Sn}}^{2+}$.

• Received 1 September 1989

DOI:https://doi.org/10.1103/PhysRevB.40.12111