# Infrared dielectric dispersion and lattice dynamics of uranium dioxide and thorium dioxide

## Abstract

Measurements at room temperature of the infrared reflectivity of single crystals of ThO2 and UO2 have been carried out and analyzed by Kramers-Kronig relations. A single strong resonance is found in each case with ωTO=279(±2) cm-1, ωLO=568(±4) cm-1, ε0-n2=15.5 for ThO2; ωTO=278(±2) cm-1, ωLO=556(±4) cm-1, ε0-n2=17.7 for UO2 (TO=transverse optic; LO=longitudinal optic). The best classical dispersionformula fit to the reflectivity yields the following parameters: for ThO2, ω0=282.7 cm-1, ε0-n2=14.85, γ̄=16.2 cm-1; for UO2, ω0=283.2 cm-1, ε0-n2=15.80, γ̄=18.5 cm-1. The low-frequency (0.3-Mc/sec) dielectric constant of ThO2 was measured to be 18.9±0.4. For UO2 the mode frequencies are in excellent agreement with those obtained by neutron spectroscopy, and can be reconciled with existing infrared absorption data by taking into account particle-size corrections which can shift the polar mode frequency by a factor ∼(ε0+2n2+2)12∼1.8 for small spherical samples. The above data are used to evaluate (Szigeti-type) effective charges (Z′Th+4=2.33, Z′U+4=2.42) and to discuss several approximate relations between the long-wave elastic and optical properties of the fluorite (CaF2-type) lattice. Absorption bands were measured in the two-phonon-combination region in UO2 and their assignment is discussed. © 1966 The American Physical Society.