Preparation and chemical and physical properties of the new layered phases Li x Ti 1 − y M y S 2 with M = V , C r , o r F e

We have prepared the layered compounds $\mathrm{Li}{\mathrm{Ti}}_{1\ensuremath{-}y}{M}_{y}{\mathrm{S}}_{2}$ ($M=\mathrm{V},\phantom{\rule{0ex}{0ex}}\mathrm{C}\mathrm{r},\phantom{\rule{0ex}{0ex}}\mathrm{F}\mathrm{e}$) by direct synthesis from a mixture of ${\mathrm{Li}}_{2}$S and the appropriate elements. The structure of these compounds is $1T$ for $M=\mathrm{V}\mathrm{and}\mathrm{Cr}$, and $3R$ for $M=\mathrm{Fe}$. In the Fe case two phases are obtained for $yg0.66$. Magnetic and M\"ossbauer data indicate that Fe is divalent and in the high-spin state for $yl0.5$ (effective magnetic moment \ensuremath{\sim}4.6 Bohr magnetons), that Cr is trivalent (effective magnetic moment \ensuremath{\sim}3.4 Bohr magnetons), and that V is nonmagnetic. The lithium can be removed by reaction with ${\mathrm{I}}_{2}$ in an acetonitrile solution at room temperature, producing layered compounds containing oxidized sulfur if $M=\mathrm{Cr}\mathrm{or}\mathrm{Fe}$. The Cr and Fe compounds are metastable, decomposing above 300\ifmmode^\circ\else\textdegree\fi{}C into Ti${\mathrm{S}}_{2}$ and ${\mathrm{Cr}}_{2}$${\mathrm{S}}_{3}$ or Fe${\mathrm{S}}_{2}$. In ${\mathrm{Ti}}_{1\ensuremath{-}y}{\mathrm{V}}_{y}{\mathrm{S}}_{2}$ the V is in the 4+ state and has a local magnetic moment at low concentrations. Cr and Fe are trivalent, and the latter is in low-spin state. Consequently, the valences are assigned as follows: ${{\mathrm{Ti}}^{4+}}_{1\ensuremath{-}y}{{M}^{3+}}_{y}{({\mathrm{S}}^{(\frac{2\ensuremath{-}y}{2})\ensuremath{-}})}_{2}$. In the Cr case the compounds are poor electrical conductors, indicating that the $y$ holes are localized, either by the cation disorder or by forming stable monovalent and divalent sulfur atoms (the monovalent species might form ${\mathrm{S}}_{2}$ pairs in this case, but they dissociate when lithium is reintercalated). Since the Fe compounds remain conducting, the holes are not localized and a band picture with the Fermi level moved well into the valence band to produce $y$ holes applies. Electrochemical measurements in nonaqueous lithium cells showed behavior similar to that of Ti${\mathrm{S}}_{2}$. Those compounds containing large amounts of Fe could be intercalated with more than one lithium per transition metal, but the reaction beyond $x=1$ was irreversible.