Optical properties of Cr${\mathrm{O}}_2$, Mo${\mathrm{O}}_2$, and W${\mathrm{O}}_2$ in the range 0.2-6 eV

The dielectric functions and optical conductivities of Cr${\mathrm{O}}_2$, Mo${\mathrm{O}}_2$, and W${\mathrm{O}}_2$ have been obtained from reflectivity measurements, and the results are interpreted on the basis of qualitative models proposed for the band structures of the transition-metal dioxide series. Cr${\mathrm{O}}_2$ has been studied as a function of temperature above and below its ferromagnetic transition at $T_c=392\mathrm{}°$K. Three conductivity peaks are observed to shift substantially near $T_c$, and a band near 0.8 eV in the ferromagnetic phase is assigned to transitions between exchange-split Cr $d$ bands. An increase in $3d$ bandwidth and $2p-3d$ separation is observed in the sequence Cr${\mathrm{O}}_2$, V${\mathrm{O}}_2$, Mo${\mathrm{O}}_2$, and W${\mathrm{O}}_2$, and these effects are correlated with an increasing cation radius. No evidence is found for greatly differing bandwidths for $3d$ orbitals of different symmetries, as has been proposed on empirical grounds.