Heat Capacity of Vanadium Oxides at Low Temperature

The electronic contribution to the heat capacity of ${\mathrm{V}}_{1.97}$ ${\mathrm{O}}_3$, ${\mathrm{V}}_7$ ${\mathrm{O}}_{13}$, and ${\mathrm{V}}_{\mathrm{O}.\mathrm{}86\mathrm{}}$ ${\mathrm{W}}_{0.14}$ ${\mathrm{O}}_2$ is very large, with $\gamma =(130\mathrm{}\pm 3), (80\mathrm{}\pm 5), \mathrm{and} (80\mathrm{}\pm 5)\times {10}^{-4\mathrm{}}$ cal ${\mathrm{K}}^{-2\mathrm{}}$/mole V, respectively. Comparison of effective masses calculated from the heat capacity, magnetic susceptibility, and optical properties suggests that the mass enhancement results mainly from spin fluctuations in a strongly correlated electron gas. The magnetic contribution to the entropy of ${\mathrm{V}}_4$ ${\mathrm{O}}_7$ at 53 K is 0.32 cal ${\mathrm{K}}^{-1\mathrm{}}$/mole V.