Optical properties of CrO2and MoO2from 0.1 to 6 eV

The reflectivities of single crystals of Mo${\mathrm{O}}_2$ and oriented films of Cr${\mathrm{O}}_2$ grown on rutile substrates have been measured from 0.1 to 6 eV. A Kramers-Kronig analysis has been performed to obtain the dielectric functions and optical conductivities. A rise in the absorption coefficient above 1.7 eV for Cr${\mathrm{O}}_2$ and 3 eV for Mo${\mathrm{O}}_2$ suggests that these energies separate the highest oxygen $2p$ levels from the Fermi level in the cation $d$ bands. The conduction-electron response is fit by $\frac{m^{*}}{m_e}\sim 10$ for Cr${\mathrm{O}}_2$ and ∼5 for Mo${\mathrm{O}}_2$, assuming one carrier per cation. A peak in the optical conductivity at 0.8 eV in both materials is compared with similar features in V${\mathrm{O}}_2$ and ${\mathrm{V}}_2$ ${\mathrm{O}}_3$. A possible interpretation of these peaks is discussed based on a recent calculation of the optical conductivity using the Hubbard model. If this interpretation is valid, an estimate may be made of the intraion Coulomb energy, $U_c\sim 0.8$ eV.