Infrared Dielectric Dispersion and Lattice Dynamics of Uranium Dioxide and Thorium Dioxide

Measurements at room temperature of the infrared reflectivity of single crystals of Th${\mathrm{O}}_2$ and U${\mathrm{O}}_2$ have been carried out and analyzed by Kramers-Kronig relations. A single strong resonance is found in each case with ${\omega }_{\mathrm{TO}}=279(\pm 2)$ ${\mathrm{cm}}^{-1\mathrm{}}$, ${\omega }_{\mathrm{LO}}=568(\pm 4)$ ${\mathrm{cm}}^{-1\mathrm{}}$, ${\epsilon }_0-n^2=15.5\mathrm{}$ for Th${\mathrm{O}}_2$; ${\omega }_{\mathrm{TO}}=278(\pm 2)$ ${\mathrm{cm}}^{-1\mathrm{}}$, ${\omega }_{\mathrm{LO}}=556(\pm 4)$ ${\mathrm{cm}}^{-1\mathrm{}}$, ${\epsilon }_0-n^2=17.7\mathrm{}$ for U${\mathrm{O}}_2$ (TO=transverse optic; LO=longitudinal optic). The best classical dispersionformula fit to the reflectivity yields the following parameters: for Th${\mathrm{O}}_2$, ${\omega }_0=282.7\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$, ${\epsilon }_0-n^2=14.85\mathrm{}$, $\overline{\gamma }=16.2\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$; for U${\mathrm{O}}_2$, ${\omega }_0=283.2\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$, ${\epsilon }_0-n^2=15.80\mathrm{}$, $\overline{\gamma }=18.5\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$. The low-frequency (0.3-Mc/sec) dielectric constant of Th${\mathrm{O}}_2$ was measured to be 18.9±0.4. For U