Optical, electrical-transport, and heat-capacity studies of the solid solutions ${\mathrm{Ti}}_x{\mathrm{Ta}}_{1-x}{\mathrm{S}}_2$, ${\mathrm{Zr}}_x{\mathrm{Ta}}_{1-x}{\mathrm{S}}_2$, and ${\mathrm{Ti}}_x{\mathrm{Nb}}_{1-x}{\mathrm{Se}}_2$

The electrical resistivity and optical absorption of the layered dichalcogenides ${\mathrm{Ti}}_x{\mathrm{Ta}}_{1-x}{\mathrm{S}}_2$, ${\mathrm{Zr}}_x{\mathrm{Ta}}_{1-x}{\mathrm{S}}_2$, and ${\mathrm{Ti}}_x{\mathrm{Nb}}_{1-x}{\mathrm{Se}}_2$ ($0\leqq x\leqq 1$), all of the Cd${\mathrm{I}}_2$ structure, are reported together with heat capacities of the solid solutions ${\mathrm{Ti}}_x{\mathrm{Ta}}_{1-x}{\mathrm{S}}_2$ ($1<T<\mathrm{}5\mathrm{}$ °K). Free-carrier absorption edges of the ${\mathrm{Ti}}_x{\mathrm{Ta}}_{1-x}{\mathrm{S}}_2$ series were analyzed using Drude's equation for the dielectric susceptibility, showing that, as $x$ decreases, the optical scattering time decreases and ${\omega }_p$ increases. Appreciable linear heat-capacity coefficients were found in Ti${\mathrm{S}}_2$, Ta${\mathrm{S}}_2$, and two of their solid solutions. The electronic structure of Ti${\mathrm{S}}_2$ and Ta${\mathrm{S}}_2$ are briefly discussed in light of these results.